2005 AVY 1500-Z71 Electrical Gremlins - holy crap!

[Indy Gman]

Where are locations of all computer/control modules on this beast? I think my problem is one of them or the harness plugging into them with the datallink comms. I already know where the BCM and 4WD modules are. And I see one near the battery I assume is engine control.
Symptoms: first it was rad. cooling fans came on and ran battery down (mid day 22F outside just sitting in driveway) even though the battery could be saved I replaced it, I frequently loose locks, windows, radio, all dash inst. fail except speedo and tack, the security and airbag lights comes on. Even when failing it has started every time but one weekend it warmed up. I tried the next day, nope. Two days later it was <30F it started right up and everything worked. Driving home (20min), 5 min from home, it still running, failure of the mentioned items again.
One observation that might help you help me track it down...when I am fighting the Gremlins...if you press the driver info/setting button on the str whl to scroll through settings, it won't get past english as the language, it can't retrieve those settings wherever they are.
I have not had the truck long, it has a clean carfax. The only mod I found was a gps recovery tracking module some jackwagon installed under the dash right by the BCM. I removed it.
When it is cold out (like <25F) and has set all night everything usually works, driving to work, or after it has sit out and temp is >30 = Gremlins are back.
I checked for power at fuses for things like locks and windows...they were hot,
I am confident it is not a battery or ign switch,
I already tried putting in a different BCM from an (04 silverado),
The only thing I found wrong is a yellow and a green wire running in a harness under the main fuse block on the fender well have been chewed on by a mouse. Those wires are not shorted together. But I removed his mouse nest from the fusebox cradle and searched for more mouse damage, found none. I actually think the mouse nest is old, like it has been there a year or two. I plan to patch up those wires in the next few days. I tugged and checked connections and wires under the drivers side dash and to the kick panel fusebox, all is well. I did the same under the hood with visible harnesses and connectors on the drivers side. Nothing found.

[Indy Gman]

Does anyone know details about the datalink comms? is it like a serial 2 or 4 wire protocol with tx/rx?, what voltages may be present?
anyone know where the theft deterrent module, elec brake control module,
HVAC module so I can check the connections?

My problem may be behind the dash or in the engine compartment because all my systems work when it is<25 outside when I start the truck, then after driving for 15min the cluster, pwr win, locks, radio, dome light, all take a dump. On warm days >35 those systems fail before I turn the key.

Took it to the dealer today, all they could determine without chasing down every harness to each module connected to the datalink is datalink comms are not working. They wanted to take up to 4 hours to take the carpet and seats up and dash apart to trace wires. They say since there is no comms connecting to the ecm or bcm would tell them nothing.

[j cAT]

I WOULD first replace the cigar fuse. the reason I say replace is fuses in these vehicles can be intermittent when the amps approach the blow point. this does occur and had me going for a while.

next repair the chewed up wire insulation with liquid tape and be very gentile with those fragile wires. the fuse box is a great place for rodent nests. inpect all those small wires and the connectors may be damaged from the rodent crap. allow time for the tape material to cure.

with the battery ground connection removed clean all the grounds in the engine and frame areas. the block ground and the under driver seat frame , the ground at the engine block rear esp. make sure the fuses are set into the fuse sockets securely . if they pull out easy this is bad.

see what happens.

the problem with the dealership you went to is there are checks to be made to properly do this troubleshooting. since the comm is down they must do what I just explained first.

if this does nothing then you must monitor the ignition voltage feed when driving to determine if this is a ignition switch intermittent. using voltmeter no test lights. if the voltage drops below 9volts the comm system will fail.

as far as what data comm is used this is top secret GM stuff. even the dealerships have no idea whats going on exactly.

since this is a 2005 you will need to find a dealership that has experience with this issue and it has a tech with this knowledge. good luck ......

the dealerships are great to rip your vehicle apart, say they fixed , give you a 600.oo bill with no warrantees. if they start replacing wiring harnesses you screwed.

[Indy Gman]

thanks for the advice,
I have checked voltages on all fuses, yanked them re-seated them, replaced the cig ltr fuse with next size bigger, went thru the under dash wires around the BCM etc 4 times. Did the same for the main fuse box, and IP fuse box (took them loose n looked at the plugs and wires in the back of them). I checked some grounding points under the hood, but planning on looking underneath for frame to engine or body and the under seat ground you mentioned next. And I plan to unplug one module at a time to see if the ECM/BCM comms perk up and make my stuff work. ie. ABS, 4WD, radio, cluster, HVAC, etc etc
The 3 or 4 wires that have a little insulation damage from the mouse in the harness near the main fuse box are not shorting together but I will liquid tape them soon. My power conns at battery and fusebox are good.
In the meantime I have started talking mean to my truck and calling it Christine...that has not helped either.
I took part of the dash apart and checking HVAC, radio, and other wiring around the glove box, both driver and pass. kick panels. So far I see no problems. Something is causing the datalink serial comms to stop working which talks to PCM aka ECM, BCM, 4wd module, airbag, HVAC, radio, ABS, etc.
Any other thoughts would be appreciated, thanks!

[j cAT]

Originally Posted by Indy Gman

thanks for the advice,
I have checked voltages on all fuses, yanked them re-seated them, replaced the cig ltr fuse with next size bigger, went thru the under dash wires around the BCM etc 4 times. Did the same for the main fuse box, and IP fuse box (took them loose n looked at the plugs and wires in the back of them). I checked some grounding points under the hood, but planning on looking underneath for frame to engine or body and the under seat ground you mentioned next. And I plan to unplug one module at a time to see if the ECM/BCM comms perk up and make my stuff work. ie. ABS, 4WD, radio, cluster, HVAC, etc etc
The 3 or 4 wires that have a little insulation damage from the mouse in the harness near the main fuse box are not shorting together but I will liquid tape them soon. My power conns at battery and fusebox are good.
In the meantime I have started talking mean to my truck and calling it Christine...that has not helped either.
I took part of the dash apart and checking HVAC, radio, and other wiring around the glove box, both driver and pass. kick panels. So far I see no problems. Something is causing the datalink serial comms to stop working which talks to PCM aka ECM, BCM, 4wd module, airbag, HVAC, radio, ABS, etc.
Any other thoughts would be appreciated, thanks!

all the vehicles management systems you mention connect to this comm module. so these will act up if no communication is occuring.

under the dash is a ground for this system on driverside . also there is a splice pack that could be a possible cause . if the vehicle is in good shape and not been in flood waters I doubt this is the problem but it is a possilble one since it is part of the circuit.

troubleshooting this is not easy even for a experienced dealership tech. GM is very tight on these system details. top secret diagrams and operation.

post back with what you found ..

I would drive around and monitor the ignition switched voltage for drop outs it is possible the ignition switch on the connection to the switch is the cause of a loss of voltage. grounds also need be good.

[Indy Gman]

I have been sick last few days so I have not done anything yet, but thanks for advice, "This comm module" you mention that they all talk to...I assume you mean the BCM? Which ignition wire am I suppose to monitor the voltage on? I did not see the ground near/above the drivers feet you are referring to, I will keep looking. I think I saw something that could be the splicepack but if you know its exact location please tell me how to identify it. I plan to unplug the harnesses going to the back along the floor on both sides and check under the drivers seat. And, I want to unplug other modules 1 at a time to see if I get comms working.

[j cAT]

Originally Posted by Indy Gman

I have been sick last few days so I have not done anything yet, but thanks for advice, "This comm module" you mention that they all talk to...I assume you mean the BCM? Which ignition wire am I suppose to monitor the voltage on? I did not see the ground near/above the drivers feet you are referring to, I will keep looking. I think I saw something that could be the splicepack but if you know its exact location please tell me how to identify it. I plan to unplug the harnesses going to the back along the floor on both sides and check under the drivers seat. And, I want to unplug other modules 1 at a time to see if I get comms working.

the splice pack is the component that connects all the monitored systems to the BCM/PCM. this is behind the BCM. since you lost all communications MY guess is this is not the problem.

check all wires going to the DLC and the BCM. ground G203 is on the driverside under the dash top cover. this has a nut securing the sp203 to ground. this is part of the ground for this system.has 4 wirings going into it with a ground lug to the metal support frame.

G103 is also part of this system this is on the rear of the ENGINE block driverside.

monitor the ignition voltage at pin 16 of the data link conn [DLC] plug for diagnostic reader. use pin 4 for return ground connection. cycle the ignition switch to on/off / do not start engine/crank.

check the data link connector has 11/12 wires to it. [sp205]..

if all this is good. then you must disconnect each system connected to the data link . one at a time since it is possible a short of the comm is because of a monitored system output.

your dash inst. panel is / has a very bad track record. also any aftermarket electrical installed ? dash instrument panel ever removed for repairs ?

[Indy Gman]

Thank you my friend for the info.

If the instr. cluster is causing all this problem I should be able to disconnect it and the datalink would work to the other modules? Do I need to disconnect the battery (or can I yank the fuse) to reset the ECM and BCM when trying to take something out of the loop?

I printed off some schematics, I work with industrial control systems all the time, but pls explain their terminology for connections. For ex.: for 1 conn. on the BCM ground shows "C2, B6, 1851, bk/wh, .35". I'm sure its a blk with a wht stripe wire or its blk or wht, and C2 is prob. connector 2, and B6 is pin 6, maybe? and .35 means?

I took the inst cluster out, unplugged it, re-installed it, saw nothing, it could be bad internally. I will check ign. voltage. I will find the grounding points now that half the dash is apart.

No other aftermarket junk has been installed, and I am not aware of the IP ever having removed or repaired before I got the truck.

[j cAT]

Originally Posted by Indy Gman

Thank you my friend for the info.

If the instr. cluster is causing all this problem I should be able to disconnect it and the datalink would work to the other modules? Do I need to disconnect the battery (or can I yank the fuse) to reset the ECM and BCM when trying to take something out of the loop?

I printed off some schematics, I work with industrial control systems all the time, but pls explain their terminology for connections. For ex.: for 1 conn. on the BCM ground shows "C2, B6, 1851, bk/wh, .35". I'm sure its a blk with a wht stripe wire or its blk or wht, and C2 is prob. connector 2, and B6 is pin 6, maybe? and .35 means?

I took the inst cluster out, unplugged it, re-installed it, saw nothing, it could be bad internally. I will check ign. voltage. I will find the grounding points now that half the dash is apart.

No other aftermarket junk has been installed, and I am not aware of the IP ever having removed or repaired before I got the truck.

the disconnecting of the monitored components like the dash inst panel and then operating may reveal if these type components are shorting out the data link signal. to be more specific with your vehicle what we need is the oem manual . thats where wire colors and pin numbers come into play.

[Indy Gman]

actually nevermind about my Q. re: pinouts and pin # ID's. I found diagram of BCM connections and wires online. I checked for ground to ground and power on the pins of the blue grey and brown connectors after unpluggin BCM, and found no problems.

I also found some diagrams with locations of grounds which I will check this weekend. I still have not found the drivers side under dash ground point G203 but that is next thing I am going to find.

I will try the ing. voltage monitoring this weekend as well. Thanks for your ideas!

[Indy Gman]

j cat,

I took the cluster out and searched high and low under the top of the dash for grounds ie G203, I don't see any, can you elaborate on its exact location?

review of the problem: All I have to do is hit my keyless remote (no unlock) or open the door (no lights but ceiling lamps work with switches) or put the key in ign. (no ding). Upon turning the key, gauges stay on zero, no radio power, drivers info display missing info.

[Indy Gman]

I got everything working! Hopefully it stays that way. Removed pcm/ecm and the bottom half of the blue connector was wet inside. I had traced the serial datalink coms to a wire there. So after cleaning/drying and hookin the battery back up she works! shew, what a lot of horsin around. It does not look like a crappy design of the plastic shield does not cover or protect the bottom or end of the ECM where the connectors plug in. I am going to try and fab up something to protect it better. Thanks for all your advice along the way. By the way, I never did find the ground in the upper dash area above the steering column.

[j cAT]

Originally Posted by Indy Gman

I got everything working! Hopefully it stays that way. Removed pcm/ecm and the bottom half of the blue connector was wet inside. I had traced the serial datalink coms to a wire there. So after cleaning/drying and hookin the battery back up she works! shew, what a lot of horsin around. It does not look like a crappy design of the plastic shield does not cover or protect the bottom or end of the ECM where the connectors plug in. I am going to try and fab up something to protect it better. Thanks for all your advice along the way. By the way, I never did find the ground in the upper dash area above the steering column.

the pcm connectors should be water resistant. some have had issues with corrosion and the oem manual does mention checking for clean connections/connector pins. you may need a shield installed in the wheel well to keep the pcm protected. I use rubber roofing. and have good success to extend the inner fender down about 3 inches to protect this and the engine/brake/fuel lines esp. on the driver side.

I did not think it was in the interior with this problem as in most cases its an external wire but I have seen internal wires shorted to the metal dash support frame.

you can also put a dielectric silicone grease on these connectors. I use this also to keep the pins from water and DE-icing chemicals.

you saved alot of money with your persistence in solving this.

good luck