Possible ECM,Or just Lightning striking 3 times

I would never blame an ECM this early. 99.9% of the time, it is a wiring problem.

 

Ditto. ECM's do fail, but I have fixed 100+ wiring issues for every ECM I've changed.

 

For the cruise, you need a Cat program to monitor the switch inputs to see what is missing that allows cruise to be enabled. Most engines require park brake to be released, clutch to be engaged, cruise switch on and able to see a set switch input. Any of those could be missing and therefore not working causing your problem. You could have a bad clutch switch or bad cruise on switch for example. A simple test is to see if you can increase the engine rpm while parked for PTO mode if your engine programmed for that.

VSS could just be wrong ratio or tire size programmed into the ECM. If the speedo works, but is just off, the sensor and wiring are fine.

Now, are the engine brakes coming on when the fan is turned on, or is the fan coming on when the engine brakes are working? It is actually a programmable parameter option to make the fan come on too when engine brakes activate because the fan also absorbs energy to help slow down the truck. If the engine brakes are coming on with the fan when not commanded, then you have a short somewhere between the fan solenoid and brake circuits. This will be in the wiring harness close the ECM as the brake wiring and fan solenoid wiring will branch off to different directions shortly after the ECM. You would need a wiring schematic for your truck and be able to do pin-out wiring testing for that.

 

Yes, I understand. Since ECMs are not cheap, I try to find the less expense option to repair it. Not only that, but once you unbox and install an ECM, you bought it! You can't take it off and try to return it once you figure out it was actually wiring problem. Always make absolute certain it isn't wiring before going the ECM route. The next thing I would recommend is to find a shop that has a test ECM you can try. That is the ultimate test. If all your problems go away with the test ECM, then you know it was the ECM!

 

What kind of truck is this? I can see if I can find you a schematic. For odd issues, I usually ohm out the data link. Data link issues can cause goofy things to happen. I had a truck one time that would turn on the fan when the right turn signal was activated, as well as set some faults. It took a while to find, but the truck came with an Eaton Vorad system under the passengers side seat. The owner removed it, but left the harness and when he put the seat back in, he pinched the harness under the seat bracket. The Vorad was connected to the data link as well as the right turn to activate the right side radar unit. It took a few years to from the wires to chaff through, but the right turn was crossing power to the data link and that made a fault that turned on the fan.

You really need Cat ET to monitor some things and check parameters. That might give you a direction to go. You could double check all power and grounds at the ECM. Low voltage can also do some screwy things. You can get fault codes from your cruise switches (but with a cruise issue you might no be able to). Shut the on/off switch to "off" then hold the accel or resume switch for a few seconds until the check engine light flashes. The problem with Cat's is that it can only show active faults, not stored faults. Since you shutdown light is intermittent, I doubt it stays active to where you can get a flash code. If it's working, you should get the check engine light to flash, even if there are no faults is will flash 55 meaning no active faults. You need Cat ET to see stored faults.

If you really suspect an ECM, you can pay a dealer about two labors to put a test ECM on it and test drive it. It'll cost you a few hundred to check, but it's better than $2000 for an ECM if that's not the issue.

 

As mentioned, I've only ever seen 2 legitimate computer failures. Everything else has been wiring, switches or sensors.