Fried plug at fan blower switch? What causes it? "REP"

ifixedit · October 30, 2014

Outstanding! Hope you got it licked.

Problems like this are the WORST, but the feeling of elation when you finally nail it is awesome.

I had a somewhat similar issue years ago with an '89 LeBaron (don't laugh, 40 MPG, 5 speed manual, and got 250K out of it before my ex went over a speed bump at 50 and collapsed the subframe). Anyway, came back from a trip, car was parked at the airport, and it would run for a few seconds then quit. Couldn't do anything in the lot so had it towed to a nearby shop. Guy had it 3 months and couldn't figure it out. Finally told me it was done, problem was the alternator (hah). Drove it home and the lights kept dimming. Noticed the alternator wire wasn't plugged in - I did so and it immediately died.

I dicked with it for a month myself and finally had an idea - it would die when the battery voltage got up to about 13. Tried a new coil and it was even worse - then I pulled the plugs and saw they were absolutely shot.

What was happening is because the plugs were so bad, the wires were arcing when the battery voltage was up, and creating electrical noise in the computer which got confused and shut down the engine. 4 new plugs and a set of wires, i was good to go. The feeling of accomplishment was pretty overwhelming

Glad to hear you got yours figured out as well! It does indeed feel good especially when it's one of these really frustrating problems. Let's hope I finally did as well as it's certainly been one SOB to figure out and I've read countless stories of those that have just given up, perhaps like your mechanic that decided to just leave the alternator wire off and get it out of his shop and be done with it. I agree, my problem had to have been what you call an electrical noise issue due to what ultimately displayed itself as very questionable connections at the BCM due to the results I got after I applied dielectric grease to the connections, that combined with a weakened TB, a badly shorted fan switch, all of which was going back to the PCM and contributors to it doing the REP thing. Since I've tried everthing else it's the only plausible explanation I can come up with at this stage of the game.

A buddy of mine who's been an electrician a long time rode along with me when I drove the thing up north this past weekend to check out our deer hunting spot, it had ran fine for 2 days prior to this then did the ol' REP on us about 6 times that day on the drive up, but ran perfect all the way back, go figure. After I told him about all the things I had already done and replaced under the hood all to no avail but that I had been noticing as of late and that occasionally when I messed with the fan switch while I was driving that this was one of a few things that seemed to set it off from time to time. I also explained it always also seemed to do it either when it was cold out and did it once the truck's interior warmed up, or when it was very humid or damp out, or when I was running the A/C, but almost never did it when it was warm and dry. He figured since I had inspected every plug under the hood along with everything else I had already done in parts replacement, that it had to be something under the dash, like a plug or connection that once warmed up things were expanding just enough, or if it was damp or very humid was causing what was already a poor connection to get even worse which creates additional resistance and could send static spikes through the system which he figured could ultimately reach and effect the TB and or the PCM since everything is all connected to the same power source. The only problem was, what connections?

Your BCM idea provided a very strong clue the other night as once I removed the plugs, packed them with dielectric grease and proceeded to see what would happen when I reconnected everything, before I even started the truck it totally freaked out, but I figured afterwards that once I was able to scan it again I would finally get the true story and I had finally found THE major contributor to the problem. Sure enough, the next day 3 new codes that had never shown their ugly heads ever before popped up. So, it seems a well connected BCM is critical even to the drive by wire side of things and unfortuneately I never caught this until now and likely wrecked 3 throttle bodies in the process. I've read how delicate the TB's can be and how careful you are supposed to be with them or they can easily be damaged. Electrical ups and downs, static noise or whatever you want to call it makes sense and I can see how that could certainly over time render a good TB junk in a fairly short period of time.

As I mentioned last night, another clue I had been noticing since last winter when all this trouble started is that the entry and interior lights were acting weird and not turning on when they should using the FOB, rear door locks that would not respond one day and would the next, the weird 2 beeps thing when I locked it with the FOB one day and normal 1 beep the next time, things all controlled by the BCM.

So, now that I know what I know and to do things in the most cost effective way, if you've first tried the basics like cleaning the throttle body and have scanned it for codes and have replaced any parts the codes indicate and it's still going into REP and you can't figure it out: Id first start with runnng the truck and do the wiggle and tug test on each wire at the TB. If none of that results in anything out of the norm and it continues to idle fine I would then move on to wiggling and tugging on the wires to the TAC mounted on the drivers side of the firewall, I would also apply dieletric grease to the TB and TAC plugs to see if that makes any difference. I would next perform the voltage and ground tests of the wires at the TB plug.http://easyautodiagnostics.com/gm/4.8L-5.3L-6.0L/p0220-tp-sensor-tests-1 If any of the readings are way off you've found your problem which is either a bad ground or a broken wire somewhere in the harness. At this point I'd first move to the three grounds in question, the two at the back of the engine and the two under the drivers side door on the frame. Then go back to the voltage and ground test to the plug at the TB, if you still get bad results I'd then replace the plug with the pigtail plug and harness that can be purchased through a number of sources, if you look around you should be able to get one for under $50. If everything with the voltage and ground tests now checks out fine at the TB connection and this still does not resolve the problem I would then move to the BCM, disconnect the battery, remove the plugs, pack the plug sockets with dielectric grease, plug them back in, connect the battery and try to start it. it's either going to now run fine or get very weird on you like mine did, I'd be willing to bet it will get weird on you. At that point I could safely say your TB is shot and needs to be replaced and should solve your problem. While you're at it and one of the first reasons I started this thread: Check your fan switch too and get it replaced if it needs it!

You can shortcut any of this if you like, but then you're just asking for more headaches and guessing so you might as well do it all and be sure you are not missing anything, by doing it all you are also assuring yourself that you've covered all of the bases and everything should be correct.

Also while you are doing all of this, confirm your battery connections on both sides are good and that the negative cable is tightly attached and is getting good contact at the engine block. Also be sure you have a battery that is in good condition reading around 12.5 + volts with the engine off and reads around 14.7 volts when the engine is running.

Two other possibilities could be a bad TAC (highy unlikely even if you get the P1516 code, mine was not bad and I got the P1516 code). Or, the pedal position switch mounted to the pedal could also be bad, mine with 203,000 miles on it was not bad.

Just drove it again tonight to go get the kid at the high school, it ran great so I'm feeling pretty confident at this point but we need a bit more time with it yet, I've been fooled by it one too many times so I've gotten a bit leery of it. The next few days it is supposed to cool down considerably and maybe a little rain as well, will be a real good test for it.

Thanks everyone for all the input and ideas here, it's much appreciated!