Loss of comm. with ECM and tcm?

[Dmax87]

Hey guys, workin on an 08 chevy topkick/c series ambulance with an lmm. Showed up customer said truck would not go over 45, speedometer doesn't work and check engine light. I pull it in, hook up my Mac scanner and find a u1000, u0101 and u0106. 1 was ECM lost communication to tcm, next was ECM lost communication to glow plug module and the other was stored in the tcm saying lost communication with ECM. I busted out the wiring schematic and found all 3 modules are on the GMLAN line wondering if anyone has ever had this issue, maybe a terminating resistor issue? Or a failed tcm? Gonna start tomorrow with ohming data link wires from each module but if I find nothing wrong with wires what would be the best way to rule out and pinpoint individual modules. Thanks for any help.

 

[Dmax87]

Nothing?

 

[mike diesel]

I have seen this or very similar to it on an lbz, it was a blown fuse. Check them all before digging to deep. I can't remember which fuse exactly.

 

[c20elephant]

Had the same lost communications (ECM and TCM) on my LBZ several months ago, the large plug on the rear of the transmission was not "snapped" in all the way. The CEL never had a chance to set as something did not feel right so I scanned it. It had not gone through several cycles to set the CEL yet. The codes were TCM U1300 and ECM U1301 on my V2

 

[onebaddmaxxx]

they have an issue for harness rub where the harness, ac lines and turbo inlet run by the air cleaner housing.

They also have a bulletin for a false u0106 getting set, and there is an updated calibration to flash in the truck. ignore the u1000 as its because your getting the u0106. check the harness and get back with us!

 

[Dmax87]

Thought I'd give you guys an update and a fix. Traced and ohmed all wires and pins for the GMLAN data link found no broken or chaffed wires, checked all powers and grounds at tcm, replaced tcm, FIXED!!!!!!

 

[Yellow Jacket]

Let me know if something changes, I have a 2008 LMM c5500 with a loss of communication with TCM code, and when I hook up my Tech 2 I can't communicate with the ECM or TCM through the DLC??? It had a loss of communication with Glowplug Control Module a week or 2 before that and it comes and goes also.

 

[onebaddmaxxx]

Yellowjacket.

Take a meter and measure ohms between terminal 6 and 14 on your dlc connector.

 

[Dmax87]

Correct, I believe the reading I got from these pins was 30ish ohms, this would be for the terminating resistor. Sounds like that may be your issue yellowchevy. Also just another heads up on the same chassis, i worked on this truck a few months ago for an airbag fault and found on the passenger side frame rail right behind the fender well there is a black box that hold 2 60pin connectors and also likes to hold water. If anyone has weird issues with other components this is a great place to start as pins like to rott off in the plugs.:thumb:

 

[Dmax87]

Yellowjacket, how did you retrieve codes then? Just wondering

 

[onebaddmaxxx]

30ohms is not correct. Should be 60~ish if there's a problem with any of the resisters inline it will read higher. Like 120

 

[Dmax87]

Ok, I wasn't positive on this number but at least I know I didn't have a problem there I also didn't know exact pins I was in cause I couldn't get a breakdown of the datalink. The 30ish was me just goin from pin to pin. How many terminating resistors are there?

 

[Yellow Jacket]

59.7 ohms resistance

I can get codes when it does communicate, they are not always active...

 

[Dmax87]

My glow plug fault came out of the blue while i was test driving before I put the tcm on. You very well may have the same issue that I had.

 

[onebaddmaxxx]

Yellow, no resisters are bad. Do u have the codes from the last time you pulled them? Like ALL of them?

 

[Yellow Jacket]

As of right now my Tech 2 won't communicate with the ECU, resistance is still 59.7 ohms and the TCM has a U0100 in it, that's it... But I can't see ECU faults or data... If I move the truck to a more level place in the driveway I will probably get communication back...

 

[onebaddmaxxx]

Do a class 2 message monitor test. See if it picks up the ecm on class 2

 

[Yellow Jacket]

Never done that, I will try it.

Currently I moved it, I now have a check engine light, and cannot communicate with the ecm or TCM... I ohmed out pins 6 & 14, with positive lead on pin 6 and negative lead on pin 14 I get OFL... But if I switch the leads negative to #6 & positive to #14 I get 0.0 ohms!!!! Why would lead location make a difference on reading? The first ever fault was loss of communication with GPCM, I'm starting to wonder if that module is bad or a wiring problem...

 

[onebaddmaxxx]

You have to check with the key off.

 

[Yellow Jacket]

I just used DVOM and no voltage at pin 2 class 2 data... Pins 4 & 5 have good grounds. And I'm getting 13.8 volts on pin 8... I triple checked pins 7 & 14 again. Positive lead on pin #7 and negative lead on pin #14 equals 18-28 ohms of varying resistance (bounces around a lot). If I switch leads it goes to 0.0 ohms and shows good continuity, switch leads back and shows no continuity!

I definitely need to find some damaged wires or harness or something obvious, right???