skidsteer with a VERY STRANGE problem

TCBoomer · May 6, 2004

Hello All
So here's the deal...machine is a 68? Mighty Mac model 8A skidsteer/loader with the Orian NHC gas engine. The electric fuel pump has just about died. It's an AC pump, and does run but very weak and won't supply enough fuel to run the tractor, but still will pump some fuel. If you fill the carb bowl...it starts, runs and shuts off fine, or til it runs out of fuel. THe replacement pump is a Federal-Mogel/Carter brand. The problem with the new pump installed is the engine won't shut off with the key. Otherwise runs fine. If you cut the B+ to the pump or disconnect the voltage regulator feed to a terminal board (pump is wired from the same board) then the tractor will shut down (without using the key). Reinstalled the defective/weak pump again, fiil the carb bowl with fuel and it starts and runs until carb runs out of fuel. and again...it DOES shut off with the key, each and every time. Reinstall another new Fed Mog pump and same as before...can't shut it down with the key.

I realize a seperate relay would solve this problem, but the owner is the type that 'needs to know' and would like to know why the replacement pump is causing this problem. It has me baffled too! Doesn't make sense...

Any ideas or thoughts anyone?
Thanks
Bob

bgott · May 7, 2004

You probably need a diode in the B+ line to the pump. You want to install it so power goes to the pump but can't come out. The AC pump probably has one built in. This is the first time I've heard of someone having a problem with a fuel pump but it happens on custom electric cooling fan installations all the time. The little motor rotates after shutdown enough to generate current that backfeeds and runs the vehicle. This might not be the problem but unless someone comes up with something better I'm sticking with it. /forums/images/graemlins/smile.gif

TCBoomer · May 7, 2004

Thanks bgott
Sounds logical. I've heard of something similar regarding the cooling fan, but a small fuel pump...?? Hmm...wonder if I would be able to prove this by bench testing the pump and then checking for current leakage after shutdown. I'll have to try it. The diode would be easier to install than the relay, so that sounds like the probable fix.

Good answer and thanks again!!

TCBoomer · May 8, 2004

bgott

Bingo...you called it. The weak AC pump has a diode and the replacement doesn't.
Good call, I never even gave it a thought that the pump could generate enough of a signal to keep the engine from shutting down.

Thanks again...