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1999 Mack RD688S code 6-4 no start wiring issue


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Having a fairly large issue with one of our Mack trucks. My knowledge of the electrical system inside the cab has kind of hit its limit on this one. Dad started tinkering with it and said he had code 6-4 come up while pulling codes. Once I got to it, we really couldn't even get it to come up. Problem seemed to get worse. Truck cranks but won't start, then next time it will start but not stay running. Spits and sputters and dies. To me, it is an obvious issue with the EECU, like power or ground not holding. We pulled the kick panel on the passenger side to get to the VECU and ABS module. When we turn the key to the run position, the parking brake light and ABS light come on (normal). The main rats nest of a harness coming from the VECU and ABS module going up behind the dash, when wiggled or touched, will make those two lights go out. Essentially killing power. The blower motor for the HVAC also dies with it.

Now, it also seems to time out after about 10 seconds regardless what we touch. The whole process we're going through goes as follows: Turn key on, buzzer and lights come on. Either touch main harness or wait 10 seconds, lights and buzzer go out.

The breaker on the second from the bottom row and far right seems to be linked. When the key is on and we remove and reinstall it, everything starts the above mentioned process again. So I'm not sure if that is essentially key power there or if that is a circuit that has a short in it causing the issue. 

That is about all I can think of at the moment. I also want to apologize ahead of time for my awful description skills. These trucks are a bit out of my knowledge base. I'm used to John Deere Ag or Army stuff.

Thanks again, I will update the more we tinker with it.   

 

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this model has an acc. relay to the left in the pic. i think there is 4 or 5 cubes there... switch thee acc. relay with the headlight relay and see if issue stops.. also there is a ground cluster on a 6mm bolt to the right in the pic...  clean it... theres another behind the cluster to the left..  clean it.... clean the ground breaker on the firewall, and the 5/8" bolt on the left side of the engine block and behind the left steer tire where that grounds bolts on, this for a 9-2 code, however,,  a bad ground  can do this.. I also made a repair last month that I have made about a dozen times.... right below the Engine fuse box on the firewall by the steering shaft, the large harness may rub against the bracket on the firewall, and it cuts into the wires and causes shorts or a complete dissconnect.  before you spend thousands for an ECM... jojo

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