Engine Spontaneously Starting

[micwhite]

The saga continues without a solution yet. The electrician ive now got has found some dodgy work done by a previous so called electrician including that the isolation switches dont totally isolate the system and he seem to have put a switch on the negative cable somewhere which apparently could be the problem. I dont understand this bit so im going back for more explanation. The thing that puzzles me is why it is acting up now. I got rid of the dodgy electrician a couple of years ago and this self starting is a new phenomenon. It’s also puzzling that when it self starts it stops when the engine starts running. It doesnt continue to drive the starter motor (fortunately!). Anyway I’ll keep updating this thread till it’s solved.

[gonesail]

ghosts in the machine?

[donradcliffe]

If you open the switch on the negative cable, it SHOULD keep the engine from starting.

In any case, if the starter is energized by the normal methods, it is stopped by taking your finger off a button or letting go of a key. It is truly amazing that whatever circuit is energizing the starter also de-energizes it when the engine starts.

[Wotname]

Hi micwhite, chilly morning today .

Regarding your engine starting, obviously the issue is an intermittent electrical problem i.e. the hardest problem to solve. While I have a couple ideas, first let me ask -

1. are you using the Volvo supplied engine panel or a custom instrument panel?
2. What is the switching / charging arrangement between the start and house batteries?

[Mike Burch]

Assuming you have separate starter and house batteries, there will be some sort of device splitting the charging current from the alternator to to each battery bank. This can be a mechanical switch, a splitter diode or a voltage-controlled relay. That gadget keeps the two batteries separate, stopping one from discharging the other, but allows them to be connected together to be charged from one alternator. If it fails, it can mean that the house and starter batteries are effectively coupled together all the time, not just when the engine is running, and the battery isolation switch is effectively bypassed. I would look very carefully at any voltage-controlled relay and its associated wiring. I had one once—never again.
My own preference is to have two completely separate circuits—the factory-fitted alternator charging only the starter battery, and a second alternator charging only the house batteries.

[Stu Jackson]

Your wiring harness has dodgy connections.