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Starter solenoid issue?

Posted: Tue Jul 02, 2019 6:58 pm
by GeoffTE
Curious to know what/why this has happened in the first place and how it came to be fixed. Tried to start SAR (my 5 for those that have led a very sheltered life :lol: ) and most unusually she didn't fire up as quickly as normal, in fact she actually didn't fire at all though she tried. I found then that when I turned off the ignition she continued to turn over albeit very slowly. I have the original solenoid on the inner wing as well as the one on the starter motor itself. I also have an engine kill switch before the solenoid so that I could stop the engine turning over. However when I turned this on again the engine continued in its efforts to start. Tapping the solenoid, the one on the wing, with a hammer ( lightly :roll: ) returned everything to normal, but attempting to start it again with the key and the whole shenanigans started again. I reasoned that the solenoid was sticking closed and that tapping it freed it up, so thought I might try and take it apart. However I first checked the wiring and found that it's earth wire was not as tight as perhaps it should be. Tighten it up and everything fine again :? Why would that be the solution to the problem? After all there was enough of a connection to allow the solenoid to operate to begin with. I dunno, my shrinking brain cells are struggling to reason it out, but I'm sure some of you lads can help me out :mrgreen:
The battery did seem a bit low, but I'm not connecting that with this issue.

Starter solenoid issue?

Posted: Tue Jul 02, 2019 7:42 pm
by AJL Electronics
The issue is that with low battery voltage, the contacts in the solenoid are not held firmly together while the motor turns. That causes a resistance across the contacts. As we know from Ohm's law, that causes heat which welds the contacts together. End result is what you found. A sharp crack breaks the whiskers of weld so it returns. With a decent battery the contacts close firmly again once the whiskers burn away and everything appears back to normal.

Starter solenoid issue?

Posted: Tue Jul 02, 2019 8:33 pm
by GeoffTE
Thanks Andy, that sounds very logical. The battery WAS low as I said, but I had a long run to the noggin on Thursday albeit with lights on on the way back so should have been ok.