By on September 27, 2019

TTAC Commentator mr_mike59 writes:


I’ve been following you and Sanjeev for many years on TTAC. Until now, I have mostly been interested in your advice and input related to other folks, and their sometimes obscure issues. At this time, however, I feel the need to reach out to look for some much needed advice.

My father-in-law is a Suburban man. Has been for over 35 years now. Unfortunately, the one he has at this point is suffering from some hard to track down electrical gremlins. He currently has a 2008 Suburban with the 5.3-liter and close to 150k on the odometer. For the past 3 winters, on their way to their snow birding location (an on the way home), they have been beset with charging issues that several shops have been unable to properly track down. The car will run fine for a good portion of the trip, with the only indication of a problem being a charging gauge that doesn’t sit still. However, when they are a good 6 or 8 hours into day one of their travels (pulling a 3,500 lb trailer), the voltmeter starts to sag, and all of the electrical systems start to shut down. This has caused them to either spend a fair amount of time on the side of the road, or cost into dealers up and down the east coast.

Many items have been replaced so far, and the car does not exhibit any of these issues, either in Florida during the winter, nor in the North during the summer season. (The needle does move around on the dash, but never far enough that the car shuts itself down).

Items replaced so far:
Alternator (3 or 4 times)
Positive Battery Cable
Negative Battery Cable

While these might be items that were ready to be replaced, none of them has shown to be the magic charm. None of the dealers have been able to recommend further places to look, beyond the many replaced parts, and we’re running out of ideas and patience. Prices have gotten insane on the newer ones, so as a retired minister, he isn’t replacing it any time soon, so we’re hoping you and the B&B can help make their next trip south just a little more relaxing!!!

Sajeev asks:

Has anyone tested the replaced alternators to see if they are running below 13.7 volts? Sounds like it might just be that re-manufactured alternators are sometimes poor quality and he needs to pack a spare with his luggage.

That’s what I do with two of my vehicles: a total of four alternators, and two of them have lifetime warranties from parts stores. (The other two are rebuilt locally and seem to last 4-8 years before dying)  They certainly come in handy when you keep cars for decades, not years!

mr_mike59 responds

I don’t believe that they have been checked. However, a little more to add… In all cases, once parts have been changed, or it has sat long enough to cool down, it will start right up. So the battery is not drained by the low voltage condition, nor is it contributing to keeping things going when the alternator isn’t.

While his recent luck doesn’t preclude substandard replacement parts in several states, it just doesn’t seem to be the answer.

Sajeev concludes:

It doesn’t sound like the answer to me, either.

And newer vehicles (in general) are harder on charging systems too, especially to the ol’ 12-volt battery.

More to the point, your father-in-law needs to keep a plug-in voltmeter in the Suburban’s console, plugging it into the cigarette lighter whenever he goes on a long trip.  Actually just plug it in every time the Suburban leaves the lot, we need to see long term data on when voltage drops, when the battery charge drops, etc.

If no valid data shows up, perhaps it’s time to check every large ground wire (engine to body, body grounds) and install a marine (deep cycle) battery with the same amount of cold cranking amps instead. Or not…Best and Brightest?

[Image: General Motors]

Send your queries to [email protected]com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice. 

Get the latest TTAC e-Newsletter!

49 Comments on “Piston Slap: The Undiagnosed, Fatal Discharge?...”

  • avatar

    How about something like an overheating relay or fuse box connection? Something that would expand and lose contact after a long period of time, but would contact after a while? Or maybe a solid state component that would heat soak and shut down? Good idea on the plug in voltmeter. I’d look at the wiring diagram to see where the charging gauge is fed from, might not be directly to the battery. It could be that the ECM/BMC/etc. isn’t getting voltage, and the charging system itself is fine. Good luck, I’m sure it’ll be easier/cheaper than a mid-2000’s VAG!

    • 0 avatar

      Based on the info provided I agree with smallblock that the root cause may be an overheating part. If it starts right up every time its had time to cool off. The trailer wiring could be a contributor to the issue if it is pulling enough current off of the part causing the issue. If it does it regardless of the trailer then at most it is only aggravating the issue. One old school example I can give were Lucas electronic distributor’s cmponents that will start to fail but do not exhibit the symptoms until the engine compartment fully heats up. Several time I had to pull over in my Rover to let it cool down to get home until I could work on it that weekend. Once cooled down it would start right up like nothing was wrong. Unfortunately I suspect this will be hard to diagnose in the Suburban unless you can get it to fail while at the shop. You might find an issue if you inspect all of the electrical wiring and components for signs of fatigue or cracking. Be sure to check all connections, etc. Given my example above I would first look closely at anything attached to the engine such as the ignition coils. Not sure of the setup on an 08 Chevy but many readers may remember the VW recalls on bad ignition coils.

      Also, was the old alternator tested when replaced? Was it actually bad? My opinion is the shops that have worked on the vehicle were simply replacing the “obvious stuff”. Even if the alts. were bad why had they failed. I would say because it is a symptom of the issue not the cause.

  • avatar

    Um, does this only occur when pulling a trailer?

    If so, I’d check the trailer wiring, and also make sure there there’s a good ground path between the truck and trailer.

    I’ve heard people say that putting part of a steel wool pad on top of the trailer ball will often fix trailer grounding issues.

    • 0 avatar

      If this is only an issue when pulling that trailer, there may be some unusual current drain coming from that trailer.

      Check the current draw (amps, not volts) between the Suburban and the trailer.

      (Hypothesis: The alternator is trying to keep up, but ultimately fails on the 6-8 hour trip. This causes enough wear/fatigue on the components that they act a little squirrelly the rest of the time.)

      If this is correct, no one is going to be able to diagnose it looking at the tow vehicle alone.

  • avatar

    Ditto on the trailer. My (bought used) T100 had a bad trailer wiring job that caused a bunch of electrical issues, even without a pulling a trailer. Eventually there was a short that melted the connection to the fuel pump, stranding me on the side of the road.

  • avatar

    Did they replace the voltage regulator? I agree with putting a voltmeter on it see if it drops out at some point in a drive.

  • avatar

    I’ve had only two cars with a battery gauge, and I’ve never seen the needle bounce around on either one. They were older cars that didn’t have ECUs, or even air conditioning. Is that normal for a modern car with all its sensors?

    Another thing that puzzles me: the statement that after replacement parts are installed, the vehicle starts right up. Does that mean the electronics are shutting down, but the battery is still being charged? That sounds like the charging system isn’t the culprit at all, but sensors, or even the ECU, that are overheating on a long trip.

  • avatar

    Two contradictory statements:

    “the car does not exhibit any of these issues, either in Florida during the winter, nor in the North during the summer season.”

    “In all cases, once parts have been changed, or it has sat long enough to cool down, it will start right up.”

    I am no car diagnostician, but those two must combine into some sort of clue.

  • avatar

    If my memory is correct, heat expands and cold contracts and this is a cold weather problem.

    In addition to checking the trailer wiring I’d check all of the chassis grounds. A cold ground connection might be loosening up enough to lose contact. As Sajeev and I know, Ford electrical systems will act possessed with a bad/intermittent ground and I bet the General is no different.

  • avatar

    I second the idea to connect a plug-in voltmeter, so he can watch the actual voltage. The cluster is going to be muffled a bit.

    Also, when the issue happens, he should be able to disconnect the signal wire to the voltage regulator on the alternator. This should allow the alternator to go into a maximum charge default mode.

  • avatar

    This thing looks like a giant bread box. Why would anyone want to drive this thing. Driving is supposed to be pleasurable.

    • 0 avatar

      Some people like to go in straight lines in maximum comfort, nothing wrong with that. I had some relatives whose “last ride” was a 1990 square Suburban with 454 and up level interior. The most miles it got on it was going from Ohio to Texas every winter to avoid the snow.

      I prefer to be able to corner and some handling but to each their own.

    • 0 avatar


      What you are seeing is the partial Lutzification of the Suburban. Further Lutzification would come in 2013 (or maybe you like that one better).

  • avatar

    Repair shops like to throw parts at a problem, starting with the most expensive. Has anyone done the wiggle test on all connectors and plugs involved? Especially when it’s hot? It sounds like it has to be very hot.

  • avatar

    I’d find a good mom n’ pop auto electrical shop near their home, and let them take a look at it. I have alternators and starters rebuilt by a local place here. It’s relatively cheap, and they work better than the reman units from the chain stores.

    • 0 avatar

      I will warn that my local independent repair shop charges an additional $25 per hour on the normal labor rate to chase down electrical problems. (Still cheaper than the dealer.)

      • 0 avatar

        @PrincipalDan – Did you ask him “why” the upcharge? It sounds like they don’t want the work.

        If it’s more complicated than swapping parts, they’re not interested. Or the owner has to leave his air conditioned recliner, since he has a shop full of drones that are only good for R&R’ing parts.

        Dealer Service is even worse. They don’t want to tie up a valuable bay or tech that could be busy throwing parts at cars. I’m not a mechanic but I’ve had to fix numerous “intermittent” and or “no code” problems myself, except starting from the cheapest parts (plus some googling), with surprisingly excellent results.

        • 0 avatar

          Part of it is that they work on much older cars too (like pre OBD and those with carburetors, points, distributors). When there are reproduction harnesses available he wants to encourage customers to use those and not ask him to create a custom harness.

          • 0 avatar

            Thanks, Plug-n-Play for the win. But yeah I get it. “Service” type businesses only exist to solve their own problems. Except a lot of times no one is re-popping obsolete harnesses. Most shops don’t have a problem leaving customers high and dry.

            When you find a good mechanic, you stick with him.

        • 0 avatar

          Probably because he doesn’t do it himself and calls in a mobile electrician that he has to pay his rate and mark it up to make a profit and pay for the space it is taking up in his shop.

          • 0 avatar

            Yeah my experience when I was a claim adjuster was the shop either hired out electrical or they charged more per hour because they ended with unbillable hours and that helped covered it.

  • avatar

    I assume the actual battery itself was replaced? I noticed that this wasn’t on the “items replaced so far” list. Several colleagues at work have been stranded with what they claim as an odd power issue followed by a no start condition. On one owners 2015 Taurus he would get various sensor warnings that traction control and stability control were disabled and a few minutes later the car wouldn’t start. Another person with a 2014 Impala had a similar issue but when triple A would come out to jump start the car it would be fine for weeks or even months until the sensor warnings started tripping and the no start condition returned. In both cases a new battery totally solved these issues as apparently both had an intermittent short that didn’t show up in load testing. A good rule is if the battery is more than 4-5 years old it should be swapped out these days.

    Note too that these newer cars use the computer to actually trickle charge the battery as needed when it detects a sag so I agree with the others about checking the ground path between trailer and truck and closely monitoring voltage when this happens. A good skilled tech should be able to figure this out

  • avatar

    I had this problem with my Mark VIII. Ford was unable to supply reliable replacement alternators for this car, selling only rebuilt Mitsubishi units. Every one failed, with symptoms much like these many times. Other times they didn’t work when installed. Sometimes internal failures caused the alternator to send alternating current into the car’s electrical system, causing all kinds of strange symptoms, including engine shutdown. In 1 year, Ford gave me 12 replacement alternators, but that didn’t make up for the fact that this luxury car wasn’t reliable enough to leave town. You need a voltmeter that reads AC voltage as well as DC voltage to make a true diagnosis of alternators, and I don’t believe the average dealer or shop is capable of doing this.

    I would never replace an alternator or starter with a rebuilt unit. Always get a new one. In theory an alternator can be rebuilt as good as new, but in practice I don’t believe that ever happens, unless by chance.

    In the case of the Mark VIII, regular Ford alternators would not fir this engine, so there was no new alternator available.

    So I sold the car and got a BMW. Never had an electrical problem for the first 200,000 miles.

  • avatar
    R Henry

    Two options:

    1) Trace the 10 gauge alternator hot lead from alternator, through the engine harness/fuse block, through the firewall connector, into the under-dash harness, and then back to the battery. I believe you will find corroded connectors or cables, which are contributing to this intermittentcy. In my experience, the firewall connector is your mostly likely culprit.

    2) Do what hot rodders have been doing for a long time–ditch the factory alternator and upgrade to a “One Wire” alternator, and run an 8 or 10 gauge wire from the new alternator directly to the battery–bypassing the existing charging circuit–which is likely where your problem exists.

  • avatar

    So I will throw my wild guess out there. Not much experience with modern GM stuff but some experience with VW-Volvo-Mopars, but in this case I have alot of experience with marine alternators.
    I’m thinking the alternator has a thermal overload. on the very long drive something is causing an additional draw on the system.(trailer bad wiring etc). The draw causes the alternator to output more then it normally would which causes them to over heat. On the long drive it’s more likley this will show up. The thermal overload is kicking in allowing the alternator to cool, the initial cycling of this will cause the battery gauge to flutter. If the draw is high enough and the alternator stays in cool down longer then expected it will prevent the battery from charging and the load will kill the battery.
    In boats you get guys with stock or cheap aftermarket alternators with high loads and big battery banks that never charge because the alternator keeps overloading.
    I would check the trailer first, but it might not be there. I would then by a clamp on DC multi meter that can read DC amps (DC is important some only do AC), and take a look at the system draw with and with out the trailer. If your lucky you may be able to isolate it with out driving for 8 hours.

    • 0 avatar

      Thermal cut-off makes a lot of sense. Replacement alternators would also overheat. I don’t know about accessibility to the alternator on a Suburban but is there line-of-sight, that temperature could be measured with an infra-red thermometer?

      • 0 avatar

        Not sure. I have heard the cutoffs (which depending on the brand and type may just cut the voltage down a little bit or a lot) can vary from 145 to over 200 degrees.
        This is complicated by what JagBoi brings up below in that the alternator has more inputs then old style alternators, you would need to check what possible inputs might effect the regulation. I would still say temp is high on the list. So in addition to a larger then normal draw on the battery causing the alternator to work to hard you may have other inputs like the current sensor Jagboi mentions causing an issue. Basically I think trouble shooting is similar but now the first step is going to be using a scan tool to check for codes and possibly check sensor data if that option is available (again not a GM guy). If that checks out your back to there being a load overwhelming the alternator causing it to cut back to prevent overheating.

  • avatar

    As others have mentioned. Does this only happen when pulling the trailer? If so then trailer wiring and connections might well be at fault.

  • avatar

    If a cool down solves the problem and it restarts w/o replacing any parts then the battery hasn’t been discharged because the alternator isn’t doing its job.

    When it dies is it a crank-no start, no crank or slow crank condition?

    It could be related to the trailer, however do they do any 6-8 hours of driving w/o the trailer attached?

    While I agree with Sajeev that voltages need to be monitored, the power point is not the place to do so. For $15 you can purchase a Bluetooth or WiFi OBD II dongle. With that and the right program on a smart phone, or windows machine you can monitor the voltage where it matters at the PCM, you can also look for a loss of fuel pressure or signal from a sensor and monitor the 5v reference voltage.

    For a little more money you can get a quality OBD II dongle and better software. I recommend this package. which gets you access to virtually all modules on GM cars. This is useful in that you can reach each of those modules input voltage ie the instrument cluster too.

  • avatar

    Anyone else see “piston slap” and a picture of a gm truck and think..”yeah, all the 5.3’s have piston slap”?

  • avatar

    “Rebuilt Alternators”

    So here is the facts on what you buy at your local parts store.

    The cheapest alternator they sell is usually a 100% used unit. The company gathers a batch of a particular unit from core returns and those purchased from core consolidators. The disassemble them all select serviceable components and assemble them into a unit that is briefly tested. Some may replace the brushes as a rule while others use them if they meet a min length.

    Now those left over units go one of two ways, for some they replace only what is needed and throw them in with the rest of that batch. Others form the basis for their premium line where key or all wear components and electronics are replaced.

    The other option for some vehicles are 100% new units. In the case of GM you can get factory Bosch units for many applications at your local NAPA and they are only slightly more expensive than the used/reman units.

  • avatar
    schmitt trigger

    At least the vehicle still has a gauge.

    Newer vehicles only have idiot lights. If it turns on, you are already toast. They don’t provide any clues at all for borderline failures.

    That is the reason I also bought an external voltmeter, which is permanently plugged into the cigarette lighter plug.

    ** I know, cigarette lighters are no more. Let’s call it an auxiliary power port **

    • 0 avatar

      Most gauges nowadays are nothing more than glorified idiot lights. They are often connected to a computer that tells you what the mfg wants you to see or what they think you want to see. So for example on many temp gauges the needle will be in the same place whether the engine temp is 180 or 220. Many volt meters are the same with a wide range that gives the same reading.

  • avatar

    Clearly, as others have already noted, this is a heat-related issue where long-term running under load is causing the problem.

    The first thing that comes to my mind is that a diode or regulator gradually drifts into thermal runaway due to the electrical load. This could be due to a high-resistance short. I do, however, find it interesting that it hits its extreme while towing, leading me to wonder if there may be an electrical issue in the trailer. Possible and worth checking but not necessarily the cause.

    Another issue may be the alternators or the voltage regulator (assuming they are semi-separate components.) Towing a trailer as he does, is the alternator strong enough for the task? Are the diodes in the regulator up to the load when towing…and then weakened so that normal driving shows only intermittent issues?

    And don’t forget the wiring, especially leading back to the trailer hitch. The insulation on the wires should be checked, especially if they pass through frame or body members but also anywhere they might touch metal. Without knowing when this issue originally started, it’s hard to know if maybe a grommet or seal may have dried or fallen out, allowing the wires to rub against the metal.

    Issues like these are notoriously difficult to pinpoint but are surprisingly easy to fix–once found.

  • avatar
    87 Morgan

    As others have said/typed.

    1. Someone needs to take an 8 hour road trip in this rig WITHOUT a trailer hooked up to try and replicate the issue. If it does not occur, you have your culprit.

    2. heat soak? If the Burb has is 2LT or LTZ it will have the navigation radio. The one in my 08′ gets very hot after long periods of time; shockingly so. You can turn the display off, which I do, and this drastically reduces the heat. Summer road trips you can feel the heat radiating off the display….

    Having had multiple issues in my lifetime with trailer wiring being faulty, whether the trailer was wired wrong, the tow vehicle lights improperly wired, trailer brake wiring harness installed incorrectly etc. I am willing to wager a nominal sum the issue lies somewhere here. I am hard pressed to think that the issue, if not trailer related, would not have occurred at any point in the past at some other interval than just when driving up and down I-95 presumably from Jersey to Florida with trailer in tow.

  • avatar

    The alternator is controlled by the ECM, and is connected to both the ECM and Body processor. There is a 175A mega fuse between the alternator and the battery, I’d look at those connections, plus there is a Battery Current Sensor module between the battery negative and ground, plus this module is connected to the body processor.

    I’d be looking at the connections between the battery current sensor module and the body processor, and ground. At this age, you might have cracked/bad solder joints in the battery current sensor module.

    When the truck stops charging see if it puts out code B1516-08, which is battery current sensor performance signal invalid.

    • 0 avatar

      JagBoi, I see so GM did like every one else and start using more sophisticated charge control/voltage regulation. That’s kind of what I was implying when I said voltage regulation. Lots of older car alternators had a simple voltage regulator at a fixed voltage with a thermal overload as I discuss above. I know for instance my 2000 Durango is also controlled by the ECM, and uses batt temp outside temp and a few other paramenters to control the voltage output of the alternator. Which effects how the battery is charged. As you said now you have additional places to look for the cause of the issue.

  • avatar

    I had a heat specific problem with a Camry once, and brought it to a competent mechanic that diagnosed to a heat soaked starter, replaced it with a Denso reman and it works fine ever since.

    Since you have done the starter, battery cables, and the alt. I’d imagine it is something along the way. Try to check and see any parts along the way and maybe test with a heat gun pointing along the way for every part.

    Have you ever though about fuel pump, wiring harness, or some other weird modules?

  • avatar

    I wouldn’t be surprised to find out after all was said and done that this was a connector that was corroded or cracked somehow. I would make sure every ground on the truck was tight and if any of them looked “off” (Discolored or corroded) they be cleaned and re-connected. I’ve had two vehicles where the firewall connector was corroded and all kinds of fun occurred.

  • avatar

    I assume the alternator is also charging the trailer battery while travelling. It’s either linked by a solenoid or an isolator. If it’s a solenoid, I’d dump that puppy and get an isolator.

  • avatar

    There is a current sensor somewhere in the power path between the alternator and the battery. If the trailer wiring is connected to the wrong side of the current sensor, the battery voltage can be allowed to drop, or cause a charging error due to the ECM not being able to see the battery reach a proper charge. I believe you are not supposed to hook anything to the battery post itself, only to the terminal on the other side of the current sensor (shunt).

    I would also wonder if they are towing in a lower gear than they normally would drive on the highway – causing higher sustained RPM and higher heat.

    Also, while towing, engine compartment heat will be higher than normal driving, even if they somehow stay in top gear. There could be some other sensor going out of range causing the ECM to start depowering systems and limiting power. It is possible the coolant/oil temp is actually too hot, but not being reported by the dash idiot gauge.

Read all comments

Back to TopLeave a Reply

You must be logged in to post a comment.

Recent Comments

  • EBFlex: “According to a T-shirt I saw a young man wearing on June 19th at church celebrating the 4th of July...
  • EBFlex: “His keyboard doesn’t have spell- or IQ-check.” “Certainly no IQ check. But probably an...
  • Inside Looking Out: Happy July 4th to all!
  • DenverMike: It’s not just the parking thing. Most Americans simply can’t drive something that big, scared...
  • DenverMike: Unless you were already struggling to feed the beast, yeah a few small changes and you’re good. Yet...

New Car Research

Get a Free Dealer Quote

Who We Are

  • Adam Tonge
  • Bozi Tatarevic
  • Corey Lewis
  • Jo Borras
  • Mark Baruth
  • Ronnie Schreiber