multiple electrical issues all at once

Jelco

Original poster
Member
Dec 8, 2011
8
Hey everyone, bit of a problem here. I was loading some things in the TB this morning when I heard the ding from the dash as if the low fuel light had come on. I looked up from the rear and noticed several warning lights on. Check Gauge, SES, Service 4x4, Battery, Brake, ABS and Airbag to be exact. Also the gauges on the right side of the cluster are all off. Upon further inspection I find The DIC is not recieving info other than ODO and Trip. It reads unknown driver. The TACH and Speedo are working. the gear indicator letters are there, but the line under them is gone. Other than the dash, the radio has power, but wont turn on. the red security light flashes, and if you push the button, it will show the time. the 4x4 selector doesnt light up and won't make any changes to drivetrain. No funtion at Driver/Pass door switches. Steering wheel buttons work for DIC, but not Therm or Radio. Heat seams to work fine from unit itself. rear windows will roll down with rear switches. Key fob has no function. none of theese problems have occured on their own in the past, and they all happend together. the engine was running at the time. upon searching i did find that jump starting can lead to problems, but ive never jumped it. The TB is a 2003 LT, non Bose. I would think it would have to be a fuse or relay, but im not sure where to start checking.If the info is already here and i missed it, sorry. please help
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Those exact symptoms are unique, so you won't find them in a search. But the common thread to all those is the data communication bus. Some module is sending jibberish messages on the bus that connects ALL intelligent modules in the vehicle, and there are about a dozen. Or the data wire is shorted to ground somewhere. It will take an experienced auto electrical person to troubleshoot this using knowledge of how the systems communicate, plus disconnecting a module at a time from the bus using the two data comm splice packs.

Or if you're very lucky, it's just a loose master ground bolt (G201) at the bottom of the center stack in the dash.

http://gmtnation.com/f25/electrical-problems-1560/
 

Jelco

Original poster
Member
Dec 8, 2011
8
Thanks. So it sounds like I can disconnect each module one at a time and when Ive got the faulty one disconnected, most functions will return. then i will know which module to inspect. The only other thing I didnt mention was the issue, i believe, happened just after i folded down the rear passenger side seat. I did check under the seat and didnt see anything out of place. Is there any wiring under there that could be directly related the this? Thanks
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Thanks for the detail. I often neglect to ask if posters shared everything even marginally relevant to the change in behavior.

The passenger side rear seat hides the OnStar module, which if it was ever installed may be still there even if you don't pay for it now. And a HUGE amount of wiring including the rear data comm splice pack under a carpet flap with about a 3" 3-sided cut for access. So do a very detailed visual search on the wiring in the carpet and hinge areas before pulling modules.
 

GM GUY

Member
Dec 23, 2012
5
Jelco said:
Hey everyone, bit of a problem here. I was loading some things in the TB this morning when I heard the ding from the dash as if the low fuel light had come on. I looked up from the rear and noticed several warning lights on. Check Gauge, SES, Service 4x4, Battery, Brake, ABS and Airbag to be exact. Also the gauges on the right side of the cluster are all off. Upon further inspection I find The DIC is not recieving info other than ODO and Trip. It reads unknown driver. The TACH and Speedo are working. the gear indicator letters are there, but the line under them is gone. Other than the dash, the radio has power, but wont turn on. the red security light flashes, and if you push the button, it will show the time. the 4x4 selector doesnt light up and won't make any changes to drivetrain. No funtion at Driver/Pass door switches. Steering wheel buttons work for DIC, but not Therm or Radio. Heat seams to work fine from unit itself. rear windows will roll down with rear switches. Key fob has no function. none of theese problems have occured on their own in the past, and they all happend together. the engine was running at the time. upon searching i did find that jump starting can lead to problems, but ive never jumped it. The TB is a 2003 LT, non Bose. I would think it would have to be a fuse or relay, but im not sure where to start checking.If the info is already here and i missed it, sorry. please help

Check the BCM (body control module) you will need a tech 2 scanner to run codes.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
GM GUY said:
Check the BCM (body control module) you will need a tech 2 scanner to run codes.
Instrument cluster data messages come from the PCM. And in my experience, only 0.01% of members here have their own Tech IIs, 0.03% have easy access to one at work or a buddy, and most folks are looking for a free or cheap alternative to paying a mechanic who owns one (or heaven forbid - a dealer). So I hardly ever lead off with suggesting a Tech II session. Do you have your own? If so, cool! :thumbsup: It's all about the membership demographics.
 

Jelco

Original poster
Member
Dec 8, 2011
8
So I found a buddy of a buddy whos owns a shop. Had him scan with a Tech II and it was unable to communicate with either the BCM or PCM. He says" I'm 75% sure its a bad body module". apparently every time he tried to scan it stopped at the same point. I've never used one, so I'm not sure what that means exactly. I had thought I would have it all figured out after that trip, but now I feel like know even less. Bummer. At least it didnt cost anything. So, should an unsucsessful scan lead me in any direction, or is it down to pulling module wires?
 

nmdeltaforce

Member
Dec 4, 2011
5
I
the roadie said:
Those exact symptoms are unique, so you won't find them in a search. But the common thread to all those is the data communication bus. Some module is sending jibberish messages on the bus that connects ALL intelligent modules in the vehicle, and there are about a dozen. Or the data wire is shorted to ground somewhere. It will take an experienced auto electrical person to troubleshoot this using knowledge of how the systems communicate, plus disconnecting a module at a time from the bus using the two data comm splice packs.

Or if you're very lucky, it's just a loose master ground bolt (G201) at the bottom of the center stack in the dash.

http://gmtnation.com/f25/electrical-problems-1560/

I too have had some weird electrical problems lately with my 06 Envoy XL Rear speakers were not working, Then the driver's side the front driver's side speaker stopped. All of a sudden both front speakers started working but the horn would not. I first thought it might be a loose wire somewhere in the radio or speakers, but the horn? After a couple of days the horn now works and the front speakers as well... some of the time. Any suggestions.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
nmdeltaforce said:
After a couple of days the horn now works and the front speakers as well... some of the time. Any suggestions.
Totally unrelated systems. Troubleshoot them separately. The horn has a contact inside the steering wheel I've posted pictures of and has been discussed recently. The speakers are subject to wire flexing/breaking at the hinge line, or connector/wiring problems at the head unit, or the speakers themselves can get wet and go flaky.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Jelco said:
So I found a buddy of a buddy whos owns a shop. Had him scan with a Tech II and it was unable to communicate with either the BCM or PCM. He says" I'm 75% sure its a bad body module".
Did you show him my posts? It could be ANY intelligent module hanging up the communication bus. Like somebody with a bullhorn can kill conversations at a party.
 

Sparky

Member
Dec 4, 2011
12,927
Just throwing this out there in reference to what Roadie is talking about, when my TCCM on my Trailblazer died it was evidently sending garbage on the data bus. It didn't seem to affect much of anything except for my radio. It would just randomly cut out and would not come back on unless I turned the key off. Ever since I replaced the TCCM the radio has not cut out even once. Screwy, I know, but it goes to show how one thing can cause another otherwise unrelated item to have problems.
 

Old Coot

Member
Jan 2, 2013
58
I just started watching this thread and it has me curious enough to ask. Roadie mentioned a loose ground and that was the first thing that popped to mind when seeing what appears to be a somewhat systemic issue. So, my question is: Will either poor ground or low B+ voltage cause the threashold point of the logic components in the data stream to be low enough to confuse them as to which state they are in, i.e. on or off?
 

Old Coot

Member
Jan 2, 2013
58
Ok Roadie. Hope to add some background to my limited knowledge of the Trailblazer systems. Are the individual logic components within the modules running on a 12 V. bus or a 5V. bus? Is the output from the modules the same voltage or are there level translators involved?
 

Sparky

Member
Dec 4, 2011
12,927
I'm not 100% sure but I believe it is 5V based as that is what most logic circuits are designed around. Also, vehicle electrical systems are "12V" but it it has too many variances to be used for any sort of logic. 5V would be easy to regulate from that variable 12V source and would be able to operate even in a weak battery (<12V) condition.

This is just a "logical" deduction on my part - heh heh see what I did there :tongue:
 

Old Coot

Member
Jan 2, 2013
58
Sparky said:
I'm not 100% sure but I believe it is 5V based as that is what most logic circuits are designed around. Also, vehicle electrical systems are "12V" but it it has too many variances to be used for any sort of logic. 5V would be easy to regulate from that variable 12V source and would be able to operate even in a weak battery (<12V) condition.

This is what I suspected, but didn't know definitively. If the regulated line in the module were low it might be a good indicator of whether or not the module was at fault or supply voltage.

Sparky said:
This is just a "logical" deduction on my part - heh heh see what I did there :tongue:

Yes! I did see what you did Spock, ah, er Sparky.

I think I will have to find the OBDII document Roadie mentioned and see what I can find out.:thumbsup:
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Sparky said:
I'm not 100% sure but I believe it is 5V based as that is what most logic circuits are designed around.
A bit off-topic, but 5V logic is quite last-century now. It was originally used for TTL logic starting in 1963. 3.3V CMOS was popular for a while. Then 2.5. Almost all the stuff we do nowadays is 1.8V. Until we have to talk to the outside world, where there are many standard buses available to choose from. I prefer analog-land, where nanovolts are still nanovolts and kilovolts can kill 'ya.
 

Old Coot

Member
Jan 2, 2013
58
the roadie said:
A bit off-topic, but 5V logic is quite last-century now. It was originally used for TTL logic starting in 1963. 3.3V CMOS was popular for a while. Then 2.5. Almost all the stuff we do nowadays is 1.8V. Until we have to talk to the outside world, where there are many standard buses available to choose from. I prefer analog-land, where nanovolts are still nanovolts and kilovolts can kill 'ya.

On the link below about half way down under SAE J1850 it says High Voltage = +7V. and
the decision point is = +3.5V. This is for GM, other manufactures appear to have different specs.

Thanks to you guys I am learning something new every day.

On-board diagnostics - Wikipedia, the free encyclopedia
 

Jelco

Original poster
Member
Dec 8, 2011
8
I began the proccess of isolating the problem by disconnecting both front door controls and the onstar, with no change. I then removed the cap to disconnect the entire rear splice pack and most all functions returned, and all warning lights on the dash cleared. I believe, please correct me if I am wrong, the rear splice pack connects both front door controls, the onstar, and the lift gate module. If thats true, then the problem must be with the LGM, right? But the lift gate still latches when I close it. Is that not controled by the LGM? So,where I stand now: The keyless entry has no function, the rear wiper doesnt work and the dome lights work manually, but dont recognize when a door is opened. also the radio works fine, but shuts off with the key instead of when the door is opened. Still Working...
 

BRomanJr

Member
Dec 9, 2011
371
Jelco said:
I began the proccess of isolating the problem by disconnecting both front door controls and the onstar, with no change. I then removed the cap to disconnect the entire rear splice pack and most all functions returned, and all warning lights on the dash cleared. I believe, please correct me if I am wrong, the rear splice pack connects both front door controls, the onstar, and the lift gate module. If thats true, then the problem must be with the LGM, right? But the lift gate still latches when I close it. Is that not controled by the LGM? So,where I stand now: The keyless entry has no function, the rear wiper doesnt work and the dome lights work manually, but dont recognize when a door is opened. also the radio works fine, but shuts off with the key instead of when the door is opened. Still Working...

Keyless entry functions are contained in the LGM.

You can further diagnose by using multiple jumper wires in the splice pack, the wires are color coded to each Module, so when you find the offending wire you can fault a particular Module.
 

Jelco

Original poster
Member
Dec 8, 2011
8
Jumper test confirms. Blue/White to tan, brown, and orange- no problem, function returns to front door controls. Blue/White to Lt. Blue- radio shuts off and the dash looks like a christmas tree. So, can I assume the LGM has failed or is there any other common problems in that system I should be checking? Fuses, connection points, ect. Thanks for all your input. I would say it is invaluble, but I'm sure the dealership would have had no problem putting a value on a soultion to this.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
The power AND data communication wires to the LGM are flexed a LOT in the rubber boot at the liftgate hinge area. I'd pull the boot back and inspect them for fraying or cracked insulation. Or pull the inner liftgate panel and unolug the LGM.
 

Jelco

Original poster
Member
Dec 8, 2011
8
I pulled the inner panels and disconnected the LGM, put the rear splice pack back together and same problems. Has to be the wire itself. I traced and inspected that little blue wire until I couldn't take it anymore. So I fished a new wire through all the panels and attached it at the module and splice pack. all functions are restored. Never found out what happened to the wire, and that does concern me a bit being that its bundled in with other wires they might too end up damaged. I suppose if that happens I'll Pull the rear seat and carpet and do some hardcore troubleshooting. As it turns out, a "mechanic" with a Tech II is no match for a determined owner with the support of the GMT Nation. Thanks for all the help.
 

mrphoenix80

Member
Jan 1, 2013
251
FYI the Comm systems in these trucks use all kinds of voltages.
E&C(entertainment and comfort) bus used 12v but that was OBDI:diggrave:
UART the standard for OBDI is 7v:diggrave:
CLASS II is the OBDII standard uses 5V
and
CAN BUS or GMLAN systems use a high and a low circuit. High being 2.5 to 5v and Low being 0 to 2.5v
GMLAN also uses terminating resistors to connect to ends of the circuits (120ohm) so a quick check is to ohm check between the high and low and look for 60ohms anything else shows a problem.

And the thing to remember is that most meters are RMS and so when you measure the data bus you get about 1/2 the volts because the voltage above is a "1" and 0v is "0"
 

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