98 GMC K2500 four wheel drive, reaching out for help?

CP

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Make
GMC
Model
K2500
Year
1998
Miles
?
Engine
5.7
Customer brought me his pickup that 4 wheel drive won’t engage. Hooked up scanner and can’t get any communication. Checked power and grounds to transfer case module, checked wiring to actuator, transfer case , and other places. Lights on the dash switches don’t light up either. No fuses blown. Then all at once the dash switch lights came on and shift motor worked, then dead again. Put in a different module, same thing , no communication, no lights on the switch, then all at once again the things came to life for about 2 seconds and went dead again. No amount of wiggling wires and unplugging things will change anything, it stays dead. Alldata which I have ,I really haven’t found there any troubleshooting??
Anyone have any step by step troubleshooting? Or any suggestions ?
thanks
 

billr

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"Hooked up scanner and can’t get any communication"

Does that mean no com with the PCM at all, or just to the transfer module; and did it come back at the same time the switch lights came back? Likewise, are the "dash switches" you are talking about the switches for the transfer, or all dash switches?
 

CP

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"Hooked up scanner and can’t get any communication"

Does that mean no com with the PCM at all, or just to the transfer module; and did it come back at the same time the switch lights came back? Likewise, are the "dash switches" you are talking about the switches for the transfer, or all dash switches?
Yes, only no communication to the transfer case module. I can communicate with the PCM. And is the transfer case switches that I mentioned. The 2 hi, 4 hi, 4 low Switches. Tried unplugging the actuator, the shift motor, in case something is shorting, but the same thing. Everything is dead having to do with the transfer case module. One thing I now noticed this morning starting it up is the brake and ABS light’s stay on. They possibly have something to do with it?
thanks
 

grcauto

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Does the scan tool allow for special tests so you can do things like command the actuator?
 

billr

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I'll look for an electrical schematic. We need that to track this down.
 

billr

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Well, you will have to be patient with me. There are four pages to the schematic that I need to study.

You checked for power at the module C1 pin E10? And ground at C1 E16? How about the serial data, C1 E9, any AC or DC signal there?
 
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CP

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Hope this diagram comes thru clear? I have power at c6 , c8 ,d14, d15. Ground at d12, and d13
B+ at d2 in park only
ground @ d16 in park and neutral
continuity from D3 to ALCL connector, orange wire
B+ at the actuator with key on
I’ve tried two scanners , neither one could communicate
I’ve run the transfer case motor with power and ground, then reversed the wires to run the other way, seemed fine going into 4wd and 2 wd
i didn’t check for any serial data, like you asked?
Had to run it outside to work on a different vehicle today, so the last thing I did was order another transfer case module.
thanks for your help!!
 

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billr

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The attachment quality is great, but it doesn't correspond to what I have at all!

How many connectors do you have on the t'case module? How many pins on each? Do the wire colors and cavity letters correspond to your schematic? The schematic I have shows just two connectors. C1 is 32 pins, with an ORG housing. C2 is 4 pins with a BLK housing.
 

billr

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I did not lay them side-by-side and compare every detail, but it looks like the schematics posted by CP and grcauto are the same. The one I found is bogus. Do we all agree on that?

CP, do the wire colors on the schematic(s) correspond to what you actually have?

There are several ground and power pins to that module, have you checked them all? And, I mean back-probing with the harness connected to the module, so there is load on those circuits.

How about the DLC signal, have you checked there yet? I am focused on that "no com" symptom. It should be easy to resolve, and there is no point looking at any other stuff until it is resolved.
 

CP

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Thanks for the help and suggestions. Seems the wire colors on the schematic and the vehicle are the same, the ones I’ve checked. I do wish they would have given more wire length on the harness to the connector of the module. Way up under the dash, almost impossible to unplug, and plugging it back in you have to push the module up into the connector. So no way to backprobe. You’re right, I need communication with the module. Waiting for the replacement one to come, so see if plugging another one in will make it come to life. Maybe I mentioned before it’s a rusty ole snowplow truck? Finally got all the snow and ice to melt off over the last weekend. Now it’s parked outside with a neg 12 windchill. So hope in a couple days to get it back in to warm up and work on it again
Thanks again!
 

billr

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I thought you already tried another module? Was it not "known good"? Have you checked for continuity on the wire from the module connector to the DLC connector?
 
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