p1518

[rockola1971]

So I disconnected both harnesses from the ECU. I disconnected the battery first of course and touched both ends together fora few minutes with the key in the on position for good measure. I cleaned with electronic spray, dried them to evaporating with my new TEMU hand air blower and reconnected them both. All looked super clean. i have since started it up about 7 or 8 times and so far so good. Yesterday it would happen at every turn. Today nothing so far after the cleaning mentioned here. Not sure if ECU had corrosion that cant be seen with the naked eye or there is a broken wire in that mass that just got jostled straight as I was cleaning.

Usually when the PCM or any other module under the hood or chassis gets water intrusion into a plug you will get the green/blue crusties growing on the pins and its pretty obvious.

 

[tomloans]

Yes it was very clean like new inside. And I just went back out and it is back. P1518 by itself. Don't have to start the motor to get the code. Just turn the key to the on position. Well fellas, if you can think of anything else, let me know. I have a new used PCM on the way. We will see if that is the problem. Like mentioned this was sudden. It wasn't a gradual build up over time. Got the code and kept getting it. No serious breaks in-between like in my past experiences. Often I would go 500 to 1000 miles before I would get codes and reduced engine power. At least this time it is easy to repeat and test.

 

[Fless]

What's on the way -- PCM or TAC?

 

[tomloans]

Sorry I am losing it. TCM is on the way. My ECM looks super clean. But then again so does the TCM. Car has been garaged for about 100k and then under a carport for the last 130k miles. Owned it since new. So it is pretty clean. Do you think maybe something in the ignition switch? There is an initial moment where the ECM and TCM are not communicating and afterwards all is fine. What else would cause an initial start up miscommunication and not do it while running?

 

[tomloans]

Thanks again for the support and responses. This is driving me crazy.

 

[rockola1971]

Sorry I am losing it. TCM is on the way. My ECM looks super clean. But then again so does the TCM. Car has been garaged for about 100k and then under a carport for the last 130k miles. Owned it since new. So it is pretty clean. Do you think maybe something in the ignition switch? There is an initial moment where the ECM and TCM are not communicating and afterwards all is fine. What else would cause an initial start up miscommunication and not do it while running?

You dont have a TCM. Your PCM (Powertrain Control Module) includes a ECM (Engine Control Module) and TCM (Transmission Control Module) all in one unit called the PCM. Your PCM is located under the hood on drivers side.

 

[tomloans]

You dont have a TCM. Your PCM (Powertrain Control Module) includes a ECM (Engine Control Module) and TCM (Transmission Control Module) all in one unit called the PCM. Your PCM is located under the hood on drivers side.

Water ever the acronym is, I have a throttle control module coming soon. This...

 

[rockola1971]

Water ever the acronym is, I have a throttle control module coming soon. This...

OK you mean TAC(Throttle Actuator Control) Module. Possibility yours is hosed but they do rarely fail. Our chassis is getting old so the this could be the norm in the next 1-5 years. I personally have (2) 2003 Tahoe LT's and a 2005 Yukon Denali.

Attached is the TAC Module schematic with pinout if you need further data. The grounding being clean and tight is very important in our chassis (and others). You see the 5v reference voltage. This signal is very important in many of the sensor circuits of our chassis. Get a bad ground and the needed output signal derived from the 5 volt reference is trash at best and 0 volts at worst and 0 volts is an out of tolerance output voltage and throws the PCM into hey something is major bad here mode. IE: Reduced Engine Power mode, etc.

 

[Fless]

OK you mean TAC(Throttle Actuator Control) Module. Possibility yours is hosed but they do rarely fail. Our chassis is getting old so the this could be the norm in the next 1-5 years. I personally have (2) 2003 Tahoe LT's and a 2005 Yukon Denali.

Attached is the TAC Module schematic with pinout if you need further data. The grounding being clean and tight is very important in our chassis (and others). You see the 5v reference voltage. This signal is very important in many of the sensor circuits of our chassis. Get a bad ground and the needed output signal derived from the 5 volt reference is trash at best and 0 volts at worst and 0 volts is an out of tolerance output voltage and throws the PCM into hey something is major bad here mode. IE: Reduced Engine Power mode, etc.

IMHO I sense that circuit tests, say, for example with a voltmeter, are being bypassed in favor of condemning the module. Sure, maybe it's a bad module or pin fitment/corrosion somewhere, but I'd much rather see the symptoms be diagnosed instead of introducing variables by replacing parts on a whim.

 

[tomloans]

IMHO I sense that circuit tests, say, for example with a voltmeter, are being bypassed in favor of condemning the module. Sure, maybe it's a bad module or pin fitment/corrosion somewhere, but I'd much rather see the symptoms be diagnosed instead of introducing variables by replacing parts on a whim.

I completely hear you. I am OK with electric issues. I know what I know but it is a little over my head to try to figure out something like this. I think may have found the problem though. I thought to myself, what was the last thing I did with the truck. Give me a few days and I am going to report back in here probably quite embarrassed. Thanks guys.

 

[Fless]

Please do. We're not into shaming anyone, but are interested in what you find. Might save the next person some frustration.

 

[Rocket Man]

I completely hear you. I am OK with electric issues. I know what I know but it is a little over my head to try to figure out something like this. I think may have found the problem though. I thought to myself, what was the last thing I did with the truck. Give me a few days and I am going to report back in here probably quite embarrassed. Thanks guys.

That’s what I usually mention when somebody is troubleshooting- think back to the last thing you did to the vehicle, and double check your work. You’d be surprised how many times it’s related to or a result of what we just worked on.

 

[tomloans]

Alright here is what happened. If you look at the picture below. I had directional diodes connected to my high beams, low beams, and Fogs. What is missing in this picture is the 3 relays for all three of these lights. What you see is this. When I turn on my low beams, my fogs come on as well. When I hit my high beams, my low beams and my fogs come stay on as well. Amazing flood of light when I hit my high beams. I have leds for my high beams and xenon's for the low beam and the fogs. I did this some 60K miles ago and haven't had a problem until now. What happened recently was the one low beam would turn off once in awhile. I thought it was a relay so I changed them all out and it has been good for about a month now. Yesterday, I found out the relay for my high beams was not working due to a new relay that was a lemon ( GM relay as well) and also likely one or more of the diodes, died on me. So high beams were not working at all. I took all diodes off and replaced the new relay with working relay, fuse box is in original condition and no more P1518 code. The only thing I can think of is that the relay or diodes unable to resist the current when I turned the key would absorb a lot of the energy needed for the ECM and TAC to talk and for that one instant, it would crash on me and into reduced engine mode. Whatever it was, all is back to normal. Tomorrow, I am going to take the diode assembly off of my other 2003. Ordered the ones on eBay to make it easier. The only difference is that my Fogs wont be on but I can make that happen. I ordered some heavy duty diodes as well.

Sorry for wasting your time. I was really dumbfounded.

 

[tomloans]

One last thing. This is actually quite scary. My long term fuel trim came back to normal. I kid you not.