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Cat Man
Cat Man, Cat Engine Expert
Category: Medium and Heavy Trucks
Satisfied Customers: 8724
Experience:  I am ASE Master Truck Certified.
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99 gmc c8500 with cat 3126....cranks but won't

Customer Question

99 gmc c8500 with cat 3126....cranks but won't start.....also scan tool can't communicate with pcm says COMM ERROR. ..suspecting electrical issues. ....all relays n fuses check out good.....even swapped pcm with sister truck....no change...any advice CATMAN?
Submitted: 12 months ago.
Category: Medium and Heavy Trucks
Expert:  Cat Man replied 12 months ago.
Thanks for asking your question. I will try and help you out.
Expert:  Cat Man replied 12 months ago.
Does the yellow check engine light come one when the key is turned to the on position?
Customer: replied 12 months ago.
No it does not that's why i suspect it's electrical n after checking all relays n fuses i swapped the pcm sisnce i have sister trucks available but still same symptoms continue. .....also my Hvac system is inop n not battery positive at the control head unit(hvac).....abs module communicates on scan tool but not the pcm.....
Expert:  Cat Man replied 12 months ago.
Thanks for the information.If the yellow check engine light does not come on when the key is turned to the on position and the engine will not start, there are only 2 things that can cause this.The first and most common is the engine ECM is not getting the correct power and ground to function. The ECM should have power on pins 52, 53, and 70 of the rear most ECM connector. This is the truck side harness connector. 70 is the key switch wire. The grounds are on pins 65 and 67. You can remove the connector and check for power on these pins. If you do not have power on these pins, you most likely have a blown fuse in the electrical system. I would check all fuses first.If you do have correct power and ground to the ECM, you most likely have a failed ECM. I have seen this many times over the years. To confirm the ECM is bad, you would need to install a known good ECM on the engine or you can take the ECM to the dealer to be tested. Unfortunately the ECM cannot be repaired. Replacement is the only option. I am sorry. I wish I had better news.Please reply back if you need more help or have more questions. I am happy to assist.
Customer: replied 12 months ago.
i already checked all that n all checked out good that is why i went ahead n swapped the ecm with a sister truck n my symptoms r still the same.
Expert:  Cat Man replied 12 months ago.
Thanks for the information.
If you have correct power to the ECM and a good ECM.
I must suspect the yellow engine light is broken. Also I would suspect the data connection to the service tool is also bad as you can not communicate.
There must be a problem with either the engine speed timing signal or the injection actuation pressure system. If there is power to a good ECM, that's all that's left other than a low fuel pressure problem. The engine needs at least 25 psi fuel pressure to start.
Please reply back if you need more help or have more questions. I am happy to assist.
Customer: replied 12 months ago.
I believe the issue is electrical because the ecm swap didn't change anything. ....the scan tool says COMM ERROR when i try to communicate with the ecm but its works fine with the bendix module n also i have no battery positive at my hvac control head unit but yet all the fuses n relays check out good.....it reads 14v on my DVOM but does light the test light(pinA on Hvac control head unit harness)
Expert:  Cat Man replied 12 months ago.
Thanks for the information.Al the engine ECM needs is good power and ground and it should start. If the ECM is good, I must suspect a problem with the engine. You may need to connect your service tool directly to the ECM to read it.
Customer: replied 12 months ago.
The scan tool works fine on 2 other sister trucks but when i connect it to the casualty truck it instantly kicks out and says COMM ERROR 4.......I'm triple checking the ecm power n ground again as we text.......but i still don't understand why my hvac has no power also.....
Expert:  Cat Man replied 12 months ago.
Ok. Let me know how you make out.
Expert:  Cat Man replied 12 months ago.
You may want to check the power on the wires to the ECM when it's plugged up. You may have voltage when it's unplugged but lose it when plugged to the ECM.

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