Engine wont stop

[John Silburn]

I've just fired up my A4 in my 1970 Hugnes 29, after it was sitting on the hard for four years. While it was reluctant to start, it eventually did, and ran for about 20 minutes, then quit. When I tried to restart, there was no power, and it appeared that the -ve battery clamp was loose. I tightened it, and restarted. After about 20 minutes, I tried to shut down. Everything kept going. I finally just choked the engine off, but noticed that all power was still on to all the instruments, despite the fact that the key was in my pocket. I assumed the ignition switch was faulty, took it out and checked it out with a multi-meter. It works fine. I reinstalled, and as soon as I turn on the batteries, all power is on to the engine as if the key were turned on. When running, the output of the alternator is 13.5 to 13.7 V at the output terminal. A friend who also has an A4 said it's probable that the diode in the alternator is shot, allowing current to backflow through to both sides of the ignition switch. Does this happen? Is it possible to just replace the diode?
Can I just place an in-line diode on the output side of the alternator? If so, should a 5 amp 12v diode be adequate? Thanks for any tips.

[sastanley]

I do not know much about electrical systems, so I can't help with the analysis...however, it is a fascinating concept.

I recently had the 'diode plate' on my alternator replaced and it was about a $12 part.

[John Silburn]

Further to my saga of the ignition staying on, I think I've found the problem. After taking off the alternator and the starter, I found that the ignition was still staying on, so I checked the 8 wire rubber connector, and found that there was a direct short on the female side of the connector closest to the batteries.between the +ve lead from the battery to the -ve side of the coil, so as soon as the batteries were turned on, the coil was on, and hence the whole ignition system. On a temporary basis, I'll just wire the coil separately. I'm going to take the opportunity to clean up the wiring on the instrument panel as it has always been a rat's nest since we bought the boat in 1985. A bench test of the old alternator showed it was running fine, but I had to go through the usual exclamations of " I never seen a alternator this old", from the guy running the test. No diode problems, so thats a relief.

[sastanley]

Wow..good troubleshooting!

I get that comment from the Mennonite shop regarding starter/alternator too, however, they recognize the telltale orange. "Hey, is this from an Atomic4? Wow, that's old..most people have upgraded, blah blah blah.." :rolleyes: