Military Vehicle Repair: starter, starter solenoid, fuse block

Question
Van,I have had to have my starter fixed again for the fourth time today. Its still 24 volt. Is there any parts that are inside that may not be heavy enough in it? The new selonoid is 24 volts. It seems to last a month or so then it quits working. The guy that rebuilds it asked me about a parallel switch. What is it and where is it? Is it the piece under the dash near the ash tray? Does it go bad and does it have anything to do with the starter going bad?Thank you in advance  

Answer
Hi Nate,
Under the dash is a relay.
It is operated by 12 volts, but switches 24 volts to the starter solenoid.
I have replaced several of them, but some of my replacements could have been just attempts to find a problem, and may not have actually been the problem.
I am lucky that I have the luxery of an ample parts supply, due to a large military post nearby, and me working for a govt. agency.

A weak area?  Loose or corroded wire connections may be one. Some of the trucks I have were in Saudi, and did seem to be a little heavy on corrosion, like in the fuse block, or the plug thru the firewall. Some of the fuse contacts would be green, and wiggling them would help.

I can see why he would ask about a parallel switch. If there is low voltage in the 12 volt pull-in circuit, you may have the relay making and breaking contact to the starter solenoid....they don't like that.
Also, if the relay 24 volt contacts are not making great contact, your 24 volts to the solenoid can be low....and they don't like that either.

If the starter engages and disengages several times like that, it can burr the flywheel ring gear, and will eventually require replacement of it.

What seems to be needing repaired on your starters? And if it is the bendix drive, have you rotated the engine by hand, checking all the teeth on the flywheel?
I have doctored burred teeth to help the starter engage, with pretty fair results.
And make sure the little bracket is not missing on the front end of the starter, that bolts to the block. Usually, if it is missing, the starter mount bolts will break off.

So as for your question about the switch, or relay, it is under the ash tray, up under the edge of the dash. There is a bracket with a couple things mounted to it.
The starter relay is about 3/4 inch thick, by about an inch and a half wide, and about the same high. Has a little mounting ear, and a black moulded plastic recepticle for the wire connector.
Seems like there is a purple wire, a red wire, and a couple more. You will find the hot wire there , probably the red one, will show 24 volts with the connector unplugged. The black wire is ground for the pull-in circuit. The other wire that is not purple, will be 12 volts when the ignition key is turned to start, and the purple wire is what carries the 24 volts to the starter solenoid.
Don't hold me to tightly to the wire colors....I am at home, and doing this from memory.
But that 12 volt wire there seems like is associated with a fuse that I have found corroded in the fuse block, ....crank maybe? It also goes thru the neutral safety switch on the steering column, which is another resistance possibility, although I haven't needed to replace one. The ignition switch itself is also a connection possibility.

If I were having to shell out for starter repairs every four months, I might be tempted to install a push button, connected from 24 volt supply, directly to the starter solenoid, and eliminate all that relay, safety switch, ignition switch, wire connection corrosion possibilities. Just keep in mind that the engine can be cranked in gear if you do that.

Van