I had this problem with my Vanagan. I tried replacing the battery, the starter even the alternator then I stopped listing to my mechanic. I put a trouble light on the input on the solenoid and wired it up to the cab so then when it did not start I would know if there was power going to the solenoid. There was! Just not enough. The run from the ignition switch to the engine in back is too long and it drops too much voltage by the Time it gets back to them selenoid that it can’t trip it.
What I finally did was, I went to the auto parts store and got a generic GM stand alone firewall mounted solenoid and mounted it on the firewall in back by the engine. I took the input wire from the old selinoid and extended it to the input terminal on the new solenoid. I ran a heavy red wire from the positive terminal of the battey to the positive input on the the new solenoid. I ran another heavy red wire from the output of the new solenoid to the the input terminal on the old selinoid. It sounds complicated but it only took about twenty minutes and the car never failed to start again and I live in Minnesota.
I hope the woman who called in about her Vanagan reads this before she buys a new starter.
You mean VanagOn? LOL…of course you did. Sounds to me like you have a MAJOR GROUND WIRE PROBLEM…did you look into that? You can run power to any old thang and without a good ground none of it will work.
We used this technique many,many times with 6V VW’s but I never had to use it with any 12V vehicle.
I have to take that back as I have used a slight variant of this technique with small block Chevrolets. The variation part was completly doing away with the GM technique of using the large terminal on the starter solenoid as a bus point but it also included using a FORD starter relay (not a GM relay like the OP reports).
“The run from the ignition switch to the engine in back is too long and it drops too much voltage; by the time it gets back to the solenoid it can’t trip it.”
The real trouble you had wasn’t due to the wire being too long and having excessive resistance. The voltage drop was happening because there is a faulty wire connection somewhere in the line, possibly the safety switch, if you have one. The amount of current it takes to turn on the starter solenoid is fairly small so a large gauge wire isn’t needed.
Yes the problem could have been a faulty ground or a bad connection somewhere else but my highly paid mechanic who replaced expesive parts were unable to find these faults. A bad ground connection or loose or dirty connection would have been corrected when the starter was replaced but this did not fix the problem. If it was a problem with a safety switch the trouble light connected to the selenoid input would not have lit up. If it take so little current to trip the selenoid then how come the trouble light connected to the selenoid lit up but the selinoid could not trip? The second solenoid solution was fast and cheap and worked.
So you just recreated the circuit but in miniature… OK…works for me