97 k1500 no start, failed PCM?

[rk396]

I have a 97 k1500, 4wd, 350 (5.7). For the past year or so i've been having problems with the fuel pump. Mainly that the connection inside the tank has been corroding and causing the pump to "fail". I seem to have that worked out finally, and now drive the truck about once a week.

last week it failed to start. So i suspected the pump was at fault again. After confirming that the pump was functioning correctly. i turned my attentiion up streem. I've checked the relay, and it was working fine. I found that i wasn't getting a prime pump signal OR a signal to the coil side of the relay (while cranking). I then checked for spark... nothing. Then i hooked a scanner (a simple code reader) to the DLC, and it failed to link. THe scanner does work, and the dlc plug shows good ground/continuity and voltage where necessary. All the obvious fuses are ok, and the cig lighter works.

I started checking voltages and grounds at the PCM, and i found several things.

1) C3-#9 doesn't show voltage. This is a white wire coming from the ICM. I suspect that this won't show anything until the engine is cranking however. any one know for sure?

2) C3-17, and 18 show good ground( continuity and low resistance) when the key is off. When the Key is on It doesn't. It shows continuity but with a Really High Resistance. When i do this test with C1 and C2 unplugged i loose the 18 ground all together, and the 17 ground is good.

Am i barking up the wrong tree here thinking it could be a grounding issue? i'm kinda at a loss as for what's going on. The fact i can't link up says that the pcm may be bad. I do appear to be getting power to all the correct pin outs as well.

any one have any thoguhts, or where to look next? any help would be MOST appreciated. thanks

[j cAT]

Originally Posted by rk396

I have a 97 k1500, 4wd, 350 (5.7). For the past year or so i've been having problems with the fuel pump. Mainly that the connection inside the tank has been corroding and causing the pump to "fail". I seem to have that worked out finally, and now drive the truck about once a week.

last week it failed to start. So i suspected the pump was at fault again. After confirming that the pump was functioning correctly. i turned my attentiion up streem. I've checked the relay, and it was working fine. I found that i wasn't getting a prime pump signal OR a signal to the coil side of the relay (while cranking). I then checked for spark... nothing. Then i hooked a scanner (a simple code reader) to the DLC, and it failed to link. THe scanner does work, and the dlc plug shows good ground/continuity and voltage where necessary. All the obvious fuses are ok, and the cig lighter works.

I started checking voltages and grounds at the PCM, and i found several things.

1) C3-#9 doesn't show voltage. This is a white wire coming from the ICM. I suspect that this won't show anything until the engine is cranking however. any one know for sure?

2) C3-17, and 18 show good ground( continuity and low resistance) when the key is off. When the Key is on It doesn't. It shows continuity but with a Really High Resistance. When i do this test with C1 and C2 unplugged i loose the 18 ground all together, and the 17 ground is good.

Am i barking up the wrong tree here thinking it could be a grounding issue? i'm kinda at a loss as for what's going on. The fact i can't link up says that the pcm may be bad. I do appear to be getting power to all the correct pin outs as well.

any one have any thoguhts, or where to look next? any help would be MOST appreciated. thanks

as you crank engine crank sensors tell computor that engine is rotating and it is ok to initiate spark etc. so this is suspect.as well as grounds electrical conn . rodent damage etc........

[777stickman]

Originally Posted by rk396

I have a 97 k1500, 4wd, 350 (5.7). For the past year or so i've been having problems with the fuel pump. Mainly that the connection inside the tank has been corroding and causing the pump to "fail". I seem to have that worked out finally, and now drive the truck about once a week.

last week it failed to start. So i suspected the pump was at fault again. After confirming that the pump was functioning correctly. i turned my attentiion up streem. I've checked the relay, and it was working fine. I found that i wasn't getting a prime pump signal OR a signal to the coil side of the relay (while cranking). I then checked for spark... nothing. Then i hooked a scanner (a simple code reader) to the DLC, and it failed to link. THe scanner does work, and the dlc plug shows good ground/continuity and voltage where necessary. All the obvious fuses are ok, and the cig lighter works.

I started checking voltages and grounds at the PCM, and i found several things.

1) C3-#9 doesn't show voltage. This is a white wire coming from the ICM. I suspect that this won't show anything until the engine is cranking however. any one know for sure?

2) C3-17, and 18 show good ground( continuity and low resistance) when the key is off. When the Key is on It doesn't. It shows continuity but with a Really High Resistance. When i do this test with C1 and C2 unplugged i loose the 18 ground all together, and the 17 ground is good.

Am i barking up the wrong tree here thinking it could be a grounding issue? i'm kinda at a loss as for what's going on. The fact i can't link up says that the pcm may be bad. I do appear to be getting power to all the correct pin outs as well.

any one have any thoguhts, or where to look next? any help would be MOST appreciated. thanks

Welcome to AF. Your suspicion about C3 #9 is correct. It is the timing signal to the VCM from the ICM.

Here's some things to check @ the VCM.

1. C3 pin #21. 12v "hot at all times" from ECM B fuse (underhood fuse block)
2. C4 pin #18. 12v with key in run & start from ECM 1 fuse. (" " ")
3. C4 pin #13. 12v with key in run from BRAKE fuse. (I/P fuse block).
4. The ECM 1 fuse also powers the ICM, the coil & the CKP all on the pink wire.

This out of a '98 C/K manual. Your '97 may be different, but I doubt it.

Post back what you find and we'll try to help more.

[rk396]

Thanks for the quic replies guys.

c3-#21 -> ~12v Key off
c4-#18 -> ~12v Key in run
c4-#13 -> ~12v Key in run

The power from the ECM-1 fuse seems to go to ground when the key is off, but shows voltage when the key is on. That seemed kinda strange too me as well.

All the guages, seem to work as well. The gas guage picks up when the key is on. So it seems that there is some kind of ground thing going on. I;m just unsure how to help narrow down the search. I'd hate to have to rip the entire harness out of there.

[777stickman]

Originally Posted by rk396

Thanks for the quic replies guys.

c3-#21 -> ~12v Key off
c4-#18 -> ~12v Key in run
c4-#13 -> ~12v Key in run

The power from the ECM-1 fuse seems to go to ground when the key is off, but shows voltage when the key is on. That seemed kinda strange too me as well.

All the guages, seem to work as well. The gas guage picks up when the key is on. So it seems that there is some kind of ground thing going on. I;m just unsure how to help narrow down the search. I'd hate to have to rip the entire harness out of there.

C4-#18. Did you check for 12v with the key in start also? This is important as it checks the integrity of 1 of the 5 contacts in the ign switch. Do not think that just because the engine cranks that all is good. The "crank" signal comes from a different contact than the one that powers the ECM-1 fuse in the start position.

Grounds can sometimes be a great mystery especially with all the electronic modules involved in todays cars. Don't start "ripping and tearing" just yet.

[j cAT]

Originally Posted by rk396

I have a 97 k1500, 4wd, 350 (5.7). For the past year or so i've been having problems with the fuel pump. Mainly that the connection inside the tank has been corroding and causing the pump to "fail". I seem to have

any one have any thoguhts, or where to look next? any help would be MOST appreciated. thanks

because you mention the corroding connection in the gas tank, are you sure your fuel pump ground is good. remove frame rail lug and reinstall. this problem is saying that the fuel is a path of current flow.

[rk396]

well i checked the following pins when the key is in start
c1-#4 -> 12V
c3-#21 -> 12v
c4-#5 -> 12v,
c4-#13 turned off when in key-start, but the schematic only shows it hot when in the run position.. which it is. so i'm guessing that is ok,

i didnt' have time to check the remaining pins, will hopefully get to that tomorrow.


i did run a jumper directly from the battery to the fuel pump ground, with no luck. I can get the pump to run by jumping the relay under the hood. so it seems that the connections from there back to the pump or ok.

i'll post the results of the remaining pins tomorrow.