M1078A1 “warmup” switch popping C/B

[Wingnut13]

Hey all, anyone have their warmup switch short out and trip the CB77? Just happened to me yesterday and haven’t had a chance to look into it yet. Didn’t know if there were any usual locations for this problem to check. Search function didn’t return anything useful (could be my fault)
Thanks, Wingnut

 

[GeneralDisorder]

You have the Retard/Off/Warmup switch for the PAC brake I take it?

Just a guess - PAC brake solenoid shorted?

 

[Wingnut13]

Yes, warmup/retard switch. Didn’t have an issue with the retard. Will check again.

 

[GeneralDisorder]

Retard only engages the PAC brake when you are driving and you lift off the throttle...... the Warmup mode engages it at idle. You are supposed to use it in cold weather in conjunction with the fast idle and it is stated explicitly to NOT DRIVE while it's in use:

http://pacbrakeoem.com/wp-content/uploads/L5914.pdf

 

[Ronmar]

The pac-brake solenoid gets its power from different sources for the two different modes. In warmup it originates at CB77 via the trans neutral relay K26 and the brake/warmup switch Then on to the solenoid and ground.

In brake/retard mode the solenoid power originates at the ECU And is enabled by a relay controlled by the trans control unit.

Since the CB77 power is available all the way to the warmup/brake switch whenever the trans is in neutral, I agree the solenoid or it’s wiring to the switch may have a short. MCB77 is supposed to be a 15A, is it working right? You can plug in a standard automotive 15A fuse or swap it with a different 15A breaker…

The ECU power output is probably current limited, so it is not allowed enough current to pop any other circuit breaker, but if the solenoid is pulling so much current the solenoid may not be performing any work when the brake is commanded on either… a high current draw thru that circuit may also set a code in the ECU…

 

[Wingnut13]

Finally got around to checking on my Pacbrake solenoid. Thankfully their website has good information on the solenoids they use. Assuming what is on our trucks… is what the website states, the 24v solenoid should ohm out at 60 Ohms. Mine checked out at 38, which is low, but not the dead short I’m looking for.

 

[Wingnut13]

Ronmar thanks for the reply to this, got busy with work and kinda just forgot about it. Yes I’ve been buying new manual resetting C/B’s and replacing as I go. I’ve replaced a few so far.

 

[Suprman]

If its cold out theres a solenoid relay that controls the intake grid heater its a common failure

 

[Wingnut13]

Copy that, will look into it. This is just as I flip the warmup switch…dead short. I think it may be time to put the dash panel up and have a look.