ECM communication issues/Engine brake seems stuck on

*I have a 2000 Mack Vision with the E7-427AC engine. It has had a slight miss for a while now, all else seemed fine. I did a valve adjustment on it a few weeks back, didn't help with the miss, also changed a leaky fuel line from the unit pump to the injector with no improvement.

Yesterday I pulled out of a pickup location, all seemed well then it was like someone reached over and shut off my key, dead in the middle of the road. I noticed the odometer display flashed "no data" for a few seconds before going blank. I immediately though I had a bus communication error, bad connection, corroded wire, etc. I had just run through a lot of road salt the night before which you know helps bring out these electrical gremlins. Checked around, found the power feed to the fuse block mounted on the driver side firewall was badly corroded, fixed that, cleaned up the ECM connections and it restarted but was missing worse. Limped home, about 170 miles, on the way home it was low on power and misfiring. After unloading at my destination I pulled out and made it about a mile up the road before it stalled out again, this time flooding a 4 lane intersection with heavy, grey smoke and raw fuel smell. It restarted and cleared up quickly without me doing anything so I drove it the last 60 miles home.

After getting it in my garage last night we pulled apart the wiring harnesses looking for shorts, haven't found any. It is acting like the engine brake is on low all the time, found we have 12v+ at both engine brake units constantly if the key is on. If we disconnect the wires to the engine brake units it seems to smooth out and run well, still a slight misfire but not as bad as it was yesterday. I am looking for ideas of what to check next, tried unplugging the engine brake dash switch to see if it had shorted out, even have 12v+ when the clutch is pushed in. I am thinking the ECM has shorted out and is sending an engine brake signal when it shouldn't but want to exhaust all other possibilities before I spend money on an ECM and trip to the dealer to have it programmed. Also, I am not sure if the engine brake units on this motor are supposed to have constant 12v+ and are connected elsewhere to a control circuit or switched ground? The units each only have one wire going into them that I can see external to the motor so I am assuming that wire is the trigger wire and should only have voltage when the engine brake unit is supposed to be activated. Maybe someone has a wiring schematic to confirm this?

 

I should also add, for the last 7 months or so that I have owned this truck, every time I shut off the key then restart it the odometer rolls back exactly 9/10 of a mile. I though I was crazy at first so I started writing my mileage down to the tenth, and it does this every time. It also flashed a vastly different mileage when I first turn the key on, something like 252,000 miles when the real mileage is 770,000. The 252k is not a trip reading or kms, I tried resetting those and it still does this. This is leading me to a ECM failure, assuming the ECM on Mack trucks control the odometer display also and it isn't a different module.

 

Thank you for your quick reply. I ordered the ECM and scheduled it into the dealer Monday as a waiter so they can install and flash it.

Another quick question, we also found today if we put 12v+ to each of the engine brake units the front one comes on/off as it should, the rear one trips the breaker in my Power Probe. Do you think the engine brake unit is shorted and that caused the ECM failure or are they two separate issues we just happened to find together? Either way I am going to replace the bad engine brake unit prior to the RCM, don't want to risk it.

 

BTW, Quality Truck, you say you used to work on a fleet of ABF units, funny I spent years towing them all over the southwest for the Albuquerque terminal. It was normal to tow two or three a day, and long distance swaps as well. From Abq we covered all the way to Liberal, KS, Salt Lake City, UT, Amarillo, TX, and Flagstaff, AZ, and of course all of NM fown to Laredo, Tx as well.