Fix heated seats

[NipRing]

I have the classic "turns off in a few seconds" on the driver seat. Pass is fine except when i try the driver heat, then both turn off. I took my driver seat out tonight and with a 9v battery, some alligator clips and wire, i got the upper and lower heating elements to work. So, i'd "like" to assume they are both good. But the power seat module (flat black box) under the seat clicks every time i try the heat. It got too dark so i just put it all back in the truck. As i search the web, this is a hugely common problem for gmt800 vehicles. I found this post on another site. And while it doesn't really help me, maybe it'll help others.

 

This applies to everything built on the full-size GMT800 truck platform -- Silverado, Sierra, Suburban, Yukon, Escalade, Tahoe.

The seat system on these trucks is complex and is not covered in depth in the Hayes book or anywhere else on teh internets so here is the GM Truck Club exxclusive writeup. I will add some photos another day.

I'll explain basic troubleshooting steps, common problems, and repair methods with the heated seats, which fail frequently. Much of the same info applies to problems with the power seat adjuster.

Before you complain tl;dr well these things are complicated and so I'm going to have to use a lot of words to explain it so hold on and pay attention, you want simple either get a 1997 or complain to The General.

== System design ==

There's microcontroller module under the driver's seat that runs the whole show. The General calls this the "memory seat module." It connects to a wiring harness in the rails under the seat using four large multi-pin connectors. This wiring harness, in turn, has connections to each of three seat motors, the chassis, the seat bottom, and the seat back.

There's just one "memory seat module" controlling both seats, and also controlling the adjustable brake and accelerator pedals.

The control switches for the heated seats in each door don't have a physical circuit to the "memory seat module" but rather communicate using the chassis serial bus.

The reason we didn't really see heated seats very often before the late 1990s is that they tend to fail in ways that cause them to overheat and start the seat foam on fire. To deal with this the microcontroller in the "memory seat module" provides close supervision of the whole system, with a temperature sensor in each seat, and sensing of the current draw for each of the four elements. If the microcontroller detects anything dodgy going on it shuts down the whole heated seat subsystem until the next time the ignition is switched off and on.

There are nine sensors in this system. Each seat has three position sensors and one temperature sensor, and then there's a sensor for the position of the accelerator/brake pedals.

These sensors all share a 5 volt supply from the "memory seat module." As a result, if any of them short to ground, the "memory seat module" can't sense temperature or seat position and pretty much shuts down everything.

== Common failures ==

The flexible heater pads fail most often. The temperature sensor is incorporated into the seat back heating pad, and the whole thing with connectors is serviced as an assembly. The seat bottom heating pad doesn't have a sensor. Dorman makes both these so you can get them from NAPA, Carquest, Rock Auto, etc., for around $100 each. They're a fast moving part and my local NAPA had them in stock.

Next most common is a wiring short to ground, most commonly in the wire bundle that goes up to the seat back assembly next to the seat belt latch. That's an easy fix with some tape and a little piece of wire loom. Otherwise well yes you can have a wiring short pretty much anywhere under either seat because the seats move and the wires flex, and if they're not dressed quite right they will rub.

Next up we have the power wiring to the drivers side seat. Well when you hit the memory button there's the possibility of all 7 motors running at once and so that's quite a current draw, more than for the heated seats, and it's common for the connector between teh chassis wiring and the seat frame to overheat. Now this connector has maybe 24 circuits going through it but you can just cut the power and ground wire and crimp on some bullet or spade connectors on those and call it good.

Finally though rare the position sensor for the accelerator/brake pedal assembly can short out and since it shares the same +5 volt power supply with everything else the whole thing quits working.

== Troubleshooting ==

If you are a tech at a stealership then The General gives you access to DTCs in the "memory seat module" that are supposed to help. The rest of us have to go lo-tech.

If nothing on either seat works at all, no movement, no heat, nothing -- either intermittently or all the time -- undo the grey connector near the floor under the driver's seat. On anything with a second row of seats you can get at it fairly easily from the second row, on a regular cab pickup you'll have a time of it. Look at the connector pins for signs of overheating on the orange and black wires, if so, cut those two wires and crimp on a suitable bullet or spade connector. They are hot with the key off so either be careful or disconnect the battery ground strap.

For heated seat problems the first thing to do is determine if one side or both sides are affected. The engine has to be running for the heated seats to work so start things up and check one side. See if you can turn on the seat heat, and then see whether it will stay on for at least a minute or so. Then turn the key all the way off to reset the "memory seat module," want a few seconds, fire it back up, and check the other side. Once the "memory seat module" has identified a fault in either seat it shuts them both down so you have to do this or you'll fool yourself into thinking they're both broken.

If one side works OK but the other side doesn't do anything at all, the lights don't flash even a little, or one button works when you mash it but the other one doesn't, then the switch module in the door panel is probably toast.

If one side works OK but on the other side the lights come on and then shut off either right away or after a few seconds or half a minute or whatever then chances are that you just need to replace the heating pad on the bad side. You can isolate whether it's the back or the bottom pad by shutting off the engine again, starting it up, and mashing the "back heat only" button on the bad side. If that works OK and stays on for a minute or more well then the problem is with the bottom pad, on the other hand if it shuts off on "back heat only" then it's the back pad that you have to replace.

If both sides are bad try moving the seats using the power seat controls. If the seats just move in fits and starts well then you have shorted sensor wiring somewhere. You can crawl under the dash and disconnect the connector to the brake/accelerator pedal position sensor, which is up above the brake pedal. If you hold the brake pedal down and look up there you can see it moving on the pedal assembly. Just pop the connector out. Now that you've done that you can fire up the engine and see if that fixed anything, if it did well you need a new pedal position sensor from your local stealership.

Otherwise there's a short somewhere else and you're going to have to take the seat bottoms out and look for wires rubbed off. The wire going to the seat back, near the seatbelt latch, is a known problem area, but the short could be anywhere. You remove the seat bottoms by undoing the two nuts underneath at the front of the seat, and the disconnecting *all* the connectors. It took me a half hour to figure out how to pop them all loose because they're all different. I'm not going to try to describe it so just take a deep breath, get your flashlight and a little screwdriver, and start fussing. I would suggest disconnecting the battery ground strap before you start so one of the seat motors doesn't fire up at the wrong time and squish your fingerz.

The power seat module itself can fail but that's rare and expensive to fix. The stealership has to flash it for your specific situation so there's no guarantee that a boneyard one will work. There were some flash updates that came out ages ago to fix a problem with the heated seats shutting down for no good reason -- apparently the original software was a little too paranoid -- but most vehicles will already have those applied.

Well if you try all that and still can't get the seats to work you probably should just trade in your truck because chances are at that point even if you take it to the stealership they'll charge you $500 for half a day of diagnostics and won't be able to figure it out either.

Edited October 21, 2014 by NipRing (see edit history)

[NipRing]

This conversation might be helpful as well http://www.justanswer.com/gm/2o66h-2004-gmc-heated-seats-the-driver-s-side-heat-keeps-shutting-off.html

 

And this diagnostic diagram http://ww2.justanswer.com/uploads/Bluegorilla/2009-11-12_172546_GMT_800_heated_seat_drivers_diagnose.pdf

 

But, i checked the ohms of both seats. Driver was 6.8 and 10.3 while the pass was 6.5 and 8.0. None of which fall between 1.5 and 5.5, yet the pass side works fine

Edited October 21, 2014 by NipRing (see edit history)