View Full Version : Electrical Issue with 6.5 (no start)
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. :confused: 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
12-30-2009, 17:58
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.
The truck has two brand new batteries. I will mention double checking grounds. Would a bad ground cause issues like this?
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.
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.
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.
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?
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
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
11-10-2010, 22:16
From my understanding it will crank but not fire, is that right?
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
03-08-2011, 23:57
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
03-09-2011, 00:04
I would look for a short to ground on the ect sensor signal circuit
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
03-09-2011, 17:03
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
03-09-2011, 17:47
;)
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;)
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).:confused:
EDIT: This was using the same snap on scanner.
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.
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).:confused:
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.
How about pulling the codes with the ole paper clip trick and checking the codes via the check engine light.
jonflies
04-30-2011, 15:22
any updates to this? Whatever happened?
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.
Yeah I will need a wiring diagram either way. Im just waiting for the truck to be moved to a new location so it can be worked on. Thanks for the tip.
How about pulling the codes with the ole paper clip trick and checking the codes via the check engine light.
We did this as well and it was showing the same codes as the scanner.
Once I get the truck in a good place to work on it Im gonna completely replace all ground wires, clean up the connections, etc. Then I'll go from there. Ive also considered pulling the harness from the dash considering the number of mice that have been living in there. It seems as though there may be multiple problems with the truck that could have me running around in circles.
any updates to this? Whatever happened?
I just sent you a pm. Again sorry it was such a late response.
Wow... This thread is still open lol!
So... It turns out after all that compression was too low to fire half the cylinders:(. We discovered this in the middle of spring. We pulled the engine and tore it down. Im still debating what to do with the motor, but...
We found a 1994 pickup with a good running motor for sale locally. The owner claims that it was replaced by the local chevy dealership in the late '90's or early 00's but the dealership has no record of anything other than the injection pump being replaced. This doesnt bother me as everything with the motor seems to be ok. The truck is going to be picked up tomorrow and we'll pull the motor. Hopefully then we can get the casting number and date from the block to figure out what we're dealing with.
Im not 100% sure that the compression was the only reason the truck wouldnt start (especially considering the weird information the scanner was reading), but Im sure it wouldnt have started considering the numbers were well below gm specs for minimum compression.
Powered by vBulletin® Version 4.2.5 Copyright © 2024 vBulletin Solutions Inc. All rights reserved.