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Jeremy, Certified Truck/Bus Technician
Category: Medium and Heavy Trucks
Satisfied Customers: 3359
Experience:  ASE Med/HD Truck Technician, NATEF Diesel Engines and Electrical/Electronics Technician, EPA 608/609 HVAC Certified, Steering/Suspension and ABS/Brakes Technician
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I have a 04 Chevy c5500 with a U0100 code coming on when it

Customer Question

I have a 04 Chevy c5500 with a U0100 code coming on when it is driven down the road a few miles. When the code comes on it is when the truck is braking and it downshifts hard like its in "limp mode" and the service transmission light comes on. Any thoughts?
04 Chevy c5500
Allison Transmission 1000
6.6 turbo diesel
Submitted: 2 years ago.
Category: Medium and Heavy Trucks
Expert:  Jeremy replied 2 years ago.

My name is ***** ***** I will assist you.

It looks like this was in the light duty category and they just moved it over this one, medium/heavy duty trucks a few minutes ago.

U0100 - Lost Communication With Engine Control Module

- Faulty Engine Control Module (ECM)
- ECM harness is open or shorted
- ECM circuit poor electrical connection

This code will cause the transmission to act up because there is no communication going on.

Start checking the harness and the pins for any oil, corrosion, etc at the ecm to make sure there are no issues there. Check the pins for good power going to the ECM. Check the Orange, red, and blue wires. Those are the power in wires. See that they all have a good 12 volts on all of them.

If so, then send the ECM/PCM out for testing.

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Expert:  Jeremy replied 2 years ago.

I see you looked at my response. Just following up to see if you needed any more assistance. Thanks.