got it.

the truck is at the construction site, so will work in these checks tomorrow between hammering nails to boards

thanks George

 

good evening.

please be patient with me. I'm retired and currently an owner-builder. I see the truck every day, but home building is taking top priority and we are pushing hard to be completely closed in before winter arrives. I just got home, another 12 hour day. Having the time of my life...

I will be sure to take this weekend "off" (from a construction aspect) as I have this issue and some other personal obligations that need my attention . Not having a truck is killing productivity, and I need it available. I will update everyone after i have completed the checks George requested.

Brad

 

No problem, we will be here when you need us. Were on your schedule.

George

 

hello all.

Took the day off today - with respect to house construction - to dive into my truck's no start issue and get to the root cause.

Disconnected the Coil and ICM connectors and verified continuity for the white/black wire between the ICM and Ignition Coil. That was expected - that wire produces the same result for the coil and ICM when connected to their respective components - test light illuminated w/o pulsing.

Back probed the white wire at the ICM and confirmed voltage at the ICM end of the white wire. The fact I have steady power to the ICM from the PCM, but no pulsing has me focused on the CKP and PCM. CKP is new.

I performed the CKP hand cranking test for the yellow wire three times (for repeatability purposes). Worked on my technique to smoothly crank the engine while observing the meter. I didn't go to the trouble of marking the harmonic damper, but it seemed the VOM would drop off from a solid 9.3 volts to zero approximately every 90 degrees of crankshaft rotation, which makes perfect sense for a V-8 and a 4 stroke otto cycle engine.

Checked the CKP pink wire - solid 12 volts.
Checked the CKP ground wire, using the VOM set to DC volts. Red lead connected to 12V source and the black lead to the purple wire on the CKP connector - 12volts on the meter, indicating a good ground from the PCM to the CKP.

I'm inclined to purchase a replacement PCM.

what else can I check ?

 

Everything that you did is reasonable with no problems found. That said:

Where did you probe the crank sensor yellow wire, at the crank sensor end or the VCM end? What did you use as your ground reference?

I would repeat your hand crank yellow wire voltage test at the VCM end of the yellow wire and then I would disconnect the wire harness at the VCM and crank sensor end and measure the resistance of the yellow and purple wire. I sure hate to indict a VCM without really testing the arriving crank sensor pulses during cranking at the VCM but to do it properly you need a scope. You can also read AC voltage at both ends or duty cycle (if your meter allows) at both ends with your meter. Duty cycle is actually the GM service manual technique. I would also verify all VCM power and ground connections before pulling that VCM trigger. I think in 15 years of hanging out here I have only seen an engine computer swap fix a problem once or twice but many more have been changed.


George

 

I probed the CKP yellow wire at the CKP connector. Tried both battery/chassis ground and the purple wire as ground sources.

I will recheck everything as you suggest. Then might throw another CKP at it before the PCM.

 

Your power and grounds are as follows on the VCM:

Power:

C4 (BLK): pin 13/brown, pin 18/pink
C3 (CLR): pin 21/orange

Grounds:

C3 (clr): pin 17/blk-wht, pin 18/blk-wht


George

 

good evening.

George - you refer to the PCM as the VCM. Every other information source I have found identifies the engine electronic unit as: PCM just curious why you refer to it as: VCM. and... I'm not clear with the "CLR" designation for PCM harness connections in your post above.

So... thinking about this no start issue today and that I get power where we would expect to find it - but the only problem - it's not pulsing on and off.

That pulsing begins with the CKP sending a low voltage signal to the PCM. I don't have access to an oscilloscope, and used the hillbilly method of handcranking the engine while the CKP is connected to a VOM. The CKP was a new aftermarket replacement and seemed to deliver expected results, but... not via an optimal test method.

I'm becoming less patient, badly needing the truck to purchase construction materials. So... I loaded the parts cannon this morning and purchased a Delco-Remy CKP. I will say right now - I had VERY-VERY low optimism the truck would start - and I was not disappointed. No start.

I have to believe the CKP is doing what it's supposed to do.

I think that leaves me checking continuity between the white wire on the CKP and the same wire going into the PCM. Which PCM connector and wire number am I looking for ?

If I get continuity in the wire between the CKP & PCM (and I expect I will - it's clean one owner truck, w/no buffoonery) - I think I'm replacing the PCM...

thoughts ?

Brad

I am left to replace the PCM.

 

GM labels their engine computers as VCM, PCM or ECM. A 95 actually can have either a PCM or VCM depending on the engine code. Yours is labeled as a VCM in the GM wiring diagrams but these three terms are used interchangeably. Since you dont have a 95 I think that there is only one engine computer for your truck but the programming is specific for your configuration.

Diagnosing problems like this is hard over the internet one question per day. If I was there with all my gear we could have solved this is one day or less but ping ponging it on a forum can be frustrating for a complicated issue so I understand.

The wiring diagrams supply the connector color to help identify the right one. I think they mean clear but I am not sure. I figured they meant natural. Mine are red and blue so I cant confirm. I alsways confirm by looking at some wire colors and positions.

The remaining unanswered questions are:

1) Does the hand crank signal make it to the other end of the two crank sensor wires at the engine computer. You can measure there and also disconnect and measure the resistance of those two wires

2) Does that signal maintain at cranking speed? This requires either an o scope or measuring duty cycle or ac voltage.

3) does this signal exit at the engine computer connector and does it make it to the ICM?

4) If no to the above, are the powers and grounds OK at the computer.

5) If you could prove a clean signal into the computer at cranking speed, verify no signal out and verified power and ground then replacing the computer is a pretty good bet. I just hate to see that component loaded into the parts cannon when its wiring at least half the time but if your crunched for time more than care about the expense then it might be the right think to do in your case.

I am going to give you your wiring diagrams in a couple of hours when I get home. I'll do my best to help remote control but I understand the frustration


George

 

Here is your circuit diagram


George