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Cat Man
Cat Man, Cat Engine Expert
Category: Medium and Heavy Trucks
Satisfied Customers: 8863
Experience:  I am ASE Master Truck Certified.
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1999 peterbilt 330 has 3126b cranks wont fire up. bypassed

Customer Question

1999 peterbilt 330 has 3126b cranks wont fire up. bypassed harness fires right up
Submitted: 1 year ago.
Category: Medium and Heavy Trucks
Expert:  Cat Man replied 1 year ago.
Thanks for asking your question. I will try and help you out.
So in removed the truck harness from the ECM and connected a bypass harness and the engine runs?
Have you verified the power and ground on the truck harness?
Customer: replied 1 year ago.
power and grounds are good
Expert:  Cat Man replied 1 year ago.
The only thing the ECM needs to run is power on pins 52 53 and 70. Grounds are 65 and 67.
I would suspect if you have power on these pins there must be a poor connection or bad wire that will not carry load.
Customer: replied 1 year ago.
52 53 and 70 are all powered 65 67 are ground
Customer: replied 1 year ago.
these are totally in working order . we had a truck repair company out and they cant get it going but , they did a harness bypass and the truck fired off
Expert:  Cat Man replied 1 year ago.
Thanks for the information.
You most likely have a problem on these power wires while cranking.
The voltage is most likely dropping too low while cranking. If the DC voltage drops below 7.5 while cranking, the ECM can not function. The voltage must stay above 7.5. Low voltage can be caused by a poor connection, bad wire, corroded battery cables, or a starter pulling too many Amps.
Customer: replied 1 year ago.
If it was the starter would it cause thew same issues when by passed?
Expert:  Cat Man replied 1 year ago.
it would. I must suspect power wires in the truck harness are not able to maintain enough voltage while cranking. I've seen this many times.

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