I didn't know how to succinctly describe my problem in the thread title but my work has a flatbed ford that we use as our "jump truck" to start up equipment that lays dormant around the terminal. I don't know who built this setup but it's actually really well done. Except for one part that I'll get to.
There are (4) six volt batteries in a box under the bed of the truck connected to a reel of jumper cables on the back. It works great. Every cable is labeled and routed neatly. Even has a nice wiring diagram!
Whoever wired it up has a switch to go from 12v to 24v for that extra kick on some of these old diesels. Well, at some point the guys complained about having to reel in 100ft of jumper cable after starting something. So they added a motor to help reel the cables back in. No one around here knows when or who built this setup but it works. Until yesterday. So instead of actually diagnosing the problem, the mechanic working on it said it was the motor. Yanked it off to discover it's an old ford starter motor. No big deal. Ran down to Napa and grabbed a new one, slapped it in and it didn't work. Looking at the wiring more we discovered this;
It's like a home made resistor! Now, the starter is only getting 6v already because it's wired off one of the batteries but with this resistor hooked to the new starter it doesn't work and just gets CRAZY hot, obviously. We decided to direct wire it to the 6v solenoid and it functions but is dangerously fast. It'll yank the cables right out of your hand if you're not paying attention. I presume the resistor was dropping the voltage even farther? I was tasked with replacing this resistor with something off the shelf. But I don't know what kind of resistor to use? My gut says not to use a resistor on a high draw motor like this, though. Would a voltage regulator be better? Or something else?
That may not be homemade. You can buy some monster wire wound resistors. https://megaresistors.com/products/motor-control-resistor/
A cheap route might be to wire one of these in series. They are basically a huge resistor and you can pick one up for about $20 at the hammer store.
Another dirty option is to pick up 25' of #14-#12 wire and hook it up in series. If that's too slow, shorten it until you get the speed you need. Then wind it to make your own resistor. Then it really will be homemade.
I am not an electrical engineer, but a starter is not a permanent magnet motor and I think there is a limit on how much you can use voltage to change the RPM before it simply stops working.
Some really old cars/jeeps/tractors were originally 6V but later converted to 12V, so is there any chance the old starter motor was 6V but the new replacement is 12V? A 6V starter running on less than 6V thru the crusty resistor might still work, but a 12V starter on less than 6V is no-go?
IMO rather than trying to find a different resistor, try to reduce the speed thru gearing (??) or find a motor/speed control that will work from 12V.
I would ditch the starter motor/resistor and use a proper gear reduction 12v motor. you can use a PWM controller for the speed if needed.
https://a.co/d/cGyLx6h
In reply to TJL (Forum Supporter) :
I agree that would be the better idea for control but with how poorly the guys here treat things, it needs to be ridiculously overbuilt. I'm not sure a small motor like that would have enough life in it? And if it did, it would be a hard sell trying to get the mechanic to redesign/rebuild the current setup.
In reply to Oapfu :
Definitely a chance. I pulled the number off the body of the starter and it was a regular ford fe starter. But, back in the day there was an electrical rebuilder just down the road that the port guys dealt with almost daily so it could have been custom.
In reply to Toyman! :
I think 12-14 would burn itself up though? That resistor looks like 1/8" rod that someone wound around a pipe. I'll look into the megaresistors.
