Electrical Issue with 6.5 (no start)

[SIG220]

In my last thread I was trying to diagnose what I thought was a no start issue due to a fuel system problem. I couldnt get the truck to run and decided to bring the truck to my friend who is a mechanic, and has worked on 6.5's before (and happens to own one). He looked over it and said there doesnt seem to be any problem with the fuel system itself. When he plugged the truck into his scan tool, it was showing that the IAT was 300*f, the ECT was over 300*f, and rpms were about 800 with the key in the on position. He did find a couple of wires that needed repair but that hasnt solved the issue. He tried swapping out the ecm with one from another truck to see if it would still show those temps and the rpms and it did. He then put the original computer back in, unplugged the entire wire harness inside the engine bay and it was still showing those numbers. The only code it was throwing was for an ECT failure or something (I forgot to ask him what the number was). Are the IAT, ECT, and RPMS, in any way related?

Any ideas on what could be goin on? Thanks!

[DmaxMaverick]

Start with verifying battery health and grounds. Either can cause all kinds of problems, including what you are seeing. One battery with a dead cell or one weak ground is all it takes.

[SIG220]

The truck has two brand new batteries. I will mention double checking grounds. Would a bad ground cause issues like this?

[JohnC]

The PCM grounds to the engine and all the sensors at the back of the intake manifold on the passenger's side head. Check that one twice.

[SIG220]

The PCM grounds to the engine and all the sensors at the back of the intake manifold on the passenger's side head. Check that one twice.

Will do thanks!

Another question: Will the PCM from a 1995 Tahoe work in a 1994 Tahoe? I have access to a running '94 and was going to try my PCM in that truck to rule out a bad PCM.

[JohnC]

Will the PCM from a 1995 Tahoe work in a 1994 Tahoe?

Pro'lly not. Too many differences between the bodies. PCM failures are rare.

[SIG220]

UPDATE: Okay, its been a while and the truck still isnt running. I dont want to go throwing parts at it until I have a good lead as to what the problem is. The truck has been sitting for a while. I gave up on it over the winter. I hooked it back up to my friends scanner at his shop and here is what it tells me...

Engine RPM:806
TPS- 19%
Speed: 1km/hr
TDC Offset: 157.61 degrees
DES Injection Timing: 11.5 degrees
Glow Plugs: 11.7 volts
Fuel temp: 39.2 degrees F
Wastegate: 82%
Boost: 0
EGR Pressure: 53kpa @ 2.12 volts
Pedal Position: #1=0v, #2=.62v, #3=.56v
Barometer: 10kpa @ 0 volts
Coolant temp: 304 degrees F
Desired Idle: 19
IAT: 296 degrees F

The truck is also showing three codes in the history. They are:

#81- shift solenoid B Circuit
#82- shift solenoid A Circuit
#83- Torque converter quad driver module

We have tried swapping two other known working ECM's and all of them show the same codes, temps, info. etc. Grounds have been checked and double checked. Is there anything above that stands out to anyone?

[Robyn]

Fuel filter, lift pump operation are always good places to start.

The PMD is always a likely suspect in a no start situation.

Battery + and - connections can cause many issues on these rigs.

Make sure of all the connections and verify good voltage to the chassis supply point.

The starter is on most models run from a different feed and can work fine but the rig not start.

Make sure that both batteries are topped off and that the intertie between the two is good.

94 trucks feed the chassis from the RH battery to the junction on the RH firewall and 95 trucks feed from the LH battery to the junction box behind the ABS unit.

Seeing all those wierd readings makes me question voltage and grounds.

If the RH battery is grounded and the LH is not all sorts of wierd crap happens.

The stock battery cables can see the ground eyes at the block fail as well as the side terminal ends fail. They may look fine outside but inside where you can't see they corode and loose connection.

If at all doubtful replace the cables.

With OEM cables and lots of years on them I can almost guarantee that they are not in the best of condition.

Again, you can't really tell by looking at them. The rubber covers hide the bad news.

The connectors are crimped onto the cable and some earlier versions were soldered but not to the core of the cable.

Once you are sure of supply voltage and good grounds then proceed to the fuel filter, lift pump and then the PMD


Keep us posted


Missy

[JohnC]

What kind of scanner is it? Have you tried the scanner on a different vehicle? If it's not the scanner it must be the grounds. (My WAG...)

[Plowboy81]

From my understanding it will crank but not fire, is that right?

[SIG220]

Wow, its been a little while. LOL

Thats correct it turns but no start.

BTW since my last update the truck got a new injection pump still nothing. I havent touched it in a long time but plan on getting started on it again soon.

[GMTech97]

Well if ect is reading 304f its not gonna cold start. Will be pulling back fuel and not turning glow plugs on cause ecm thinks engine is hot

[GMTech97]

I would look for a short to ground on the ect sensor signal circuit

[JohnC]

I would look for a short to ground on the ect sensor signal circuit

You have so many out of spec readings I think you need to take a bigger picture approach...

[GMTech97]

I agree but if you take one component and one circuit you know is incorrect and start your diagnosis there it is very likely it may lead to the bigger problem then it doesnt seem so overwhelming trying to look at everything at once. Many of the engine sensors all share the same ecm supplied ground.

[scot5146]

Hey before you buy a bunch of parts like I did, double check that the electrical conections to the starter are good and working... Mine was conected but very poorly which caused a ton of electrical codes and issuse with starting...

Basically the power supply cable which conects to the starter was grounding out...

I would double check that, I was in almost the exact same issues and all it took for me was a new end on the cable and a socket to tighten it down... (DONT RULE OUT SIMPLE STUFF UNTILL IT IS CHECKED)

good luck
Mike

[SIG220]

Thanks again folks! I will definitely check those things.

As a side note. My friend who has a '94 Tahoe 6.5 hooked his scanner up to his truck and it didnt read any codes. Then he completely unplugged the engine harness from the computer and it nearly identical codes/readings as my truck is giving (plugged in or not).

EDIT: This was using the same snap on scanner.

[SIG220]

Well if ect is reading 304f its not gonna cold start. Will be pulling back fuel and not turning glow plugs on cause ecm thinks engine is hot

I will have to double check with a friend of mine (its been a while) but Im pretty sure that the glow plugs were still heating up. we pulled one and turned the ignition to "on" while it was still hooked up and it got red hot.

[racer55]

Thanks again folks! I will definitely check those things.

As a side note. My friend who has a '94 Tahoe 6.5 hooked his scanner up to his truck and it didnt read any codes. Then he completely unplugged the engine harness from the computer and it nearly identical codes/readings as my truck is giving (plugged in or not).

EDIT: This was using the same snap on scanner.

From this test alone I would say that you have an open circuit between the ecm and the firewall on a ground wire,best guess.

I would identify the grounds as they connect to the ecm and splice new wires out to the engine ground locations and then check the results,a wiring diagram for your vehicle would be invaluable for this.

[16gaSxS]

How about pulling the codes with the ole paper clip trick and checking the codes via the check engine light.