Accessories work but not when motor is running

[Jenfeist]

I have an LT 40 HD with 4500 hours on mill, the last 12 hours running on a new motor. Here's the problem: when the motor is running or ignition switch is in run position the for/rev and up/down motors do not work. When the ignition is in accessory position they both work fine. (I haven't used the blade guide motor for years). I tested the hot leads coming out of the circuit breaker block going to each drum switch. They have 12 volts in both the accessory and run positions of the key switch. So is this a bad ignition switch or a circuit board problem? According to the 1992 series wiring diagram the up/down doesn't go through the circuit board. 

[charles mann]

What about running a jumper from the acc to the ign terminal and see if the motors work with eng running. 

Im running into a similar issue with getting my eng to even crank. Iv got 12v from the master switch to the push button and 12v on the closed terminal, down to the relay. Same for the high idle relay, power to the input side, 12b to the "S" on the relays when the master comes on and the starter button is closed. But eng no crank. I can jump the relay and it works. Chances of 2 new relays being bad i find slim, but not impossible. 

[Jenfeist]

I am considering trying that but am only "dangerously knowledgeable" about electrical testing. I have created a dead short in the past, several years ago...I did set up a jumper wire and insulated pliers to hold it with but haven't applied it yet. Wondering if I should check grounding contact and bar that runs under mill frame. Maybe this evening...we have drawknifing to finish before below zero weather moves in for a few days.

[charles mann]

I am considering trying that but am only "dangerously knowledgeable" about electrical testing. 

you and me both. 
i burned out a rectifier the other day by jumping a terminal. luckily, i don't think i need it. poor grounds can make a man pull his hair out.
i know you said you had voltage from the cb block. do you have voltage to the drum input connection? if so, do have voltage on the output side of the drum? if so, then keep checking the wires all the way to the motors. if no voltage out of the drum, it may be the drum switch. 
worrying about shorts, put a fuse inline on the jumper lead. to much draw, it'll pop the fuse. 

[Magicman]

You need to take a test lamp (or voltohmmeter) and determine where you have voltage when the key switch in the various positions.  Jumpering 12v. to the switch's "on" position would be a normal troubleshooting test.

[Southside]

A power probe is worth it's weight in gold when trying to diagnose issues like this. You can apply ground or power to a circuit and eliminate what is working and what is bad, IE switch, wire, motor, relay. About $60, combine that with a multi meter and the Gremlins become a whole lot less aggravating. 

[charles mann]

Any progress to report? 

[Florida boy]

I would look for grounds . Most items in 12v ground through frame . I've been having up down problems on my manual mill .this is how I test for grounds. Take a meter or test light and hook red lead to 12v from battery and use black lead to test for grounding....

I was on a service call once where a  fan was not working. Looked at taps on fan. Looked good checked voltage had proper voltage. Thought fan was bad so I started to take it down. Once I pulled th taps loose I could see corrosion in between the connection points. Had voltage on the part I touched with meter . Reconnected and ran fine... Dissimilar metals like to corrode in places that aren't apparent. Best of luck