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I have one peterbilt 2004 model 379 does not start a

hi i have one peterbilt...
hi i have one peterbilt 2004 model 379 does not start a mechanic came to see him but says probably computer
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Answered in 7 minutes by:
3/19/2016
Technician: Cat Man, Cat Engine Expert replied 6 years ago
Cat Man
Cat Man, Cat Engine Expert
Category: Medium and Heavy Trucks
Satisfied Customers: 10,344
Experience: I am ASE Master Truck Certified.
Verified
Thanks for asking your question. I will try and help you out.
When you say it will not start, do you mean it cranks over but does not run?
Also, what engine do you have? Is it a Cat?
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Customer reply replied 6 years ago
is caterpilleris not run
Technician: Cat Man, Cat Engine Expert replied 6 years ago
Does the yellow check engine light come on when the key is turned to the on position?
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Customer reply replied 6 years ago
if the yellow light is turned onno is not turned on
Technician: Cat Man, Cat Engine Expert replied 6 years 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.
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Technician: Cat Man, Cat Engine Expert replied 6 years 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.
Cat Man
Cat Man, Cat Engine Expert
Category: Medium and Heavy Trucks
Satisfied Customers: 10,344
Experience: I am ASE Master Truck Certified.
Verified
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