Why won't my engine start without the temp sensor connected

[James_Calvert]

Bedouin, my old Bukh 20 which I think was electrically identical to the 24 was quite happy to start and run with the water temperature alarm sensor disconnected. The wire fell off quite often...

I suspect you have something else going on. Maybe making or breaking a connection somewhere by your very presence. I had something like that once, turned out stepping on the engine steps somehow distorted the switch panel and opened up a dry joint.

 

[Bilgediver]

Look for an intermittent loose connection somewhere which was temporarily corrected by poking about with multimeter probes which were probably pure coincidence.

I suspect this too and in particular the small wire connection on the solonoid.

 

[VicS]

It must be the starter solenoid that isn't operating but the -ve is clearly okay - and the +ve seems to be fine, but it is when I connect the meter to measure the 12V at the solenoid that it starts. So connecting a digital MM with very high resistance across the solenoid coil makes it operate. But that connection has to be maintained, as soon as I disconnect the meter the solenoid stops operating.

The act of connecting the meter to the solenoid coil must be somehow or the other be bridging or closing a bad connection

Disconnect the coil clean and check the connections and check the coil for continuity.
They can take quite a large current ... Ive measured 40 amps on my car solenoid to initially pull it in

 

[Zing]

That's the whole point - the engine is designed to be robust and fool proof and can start and run without any electrics at all (not sure how you stop it though )

That is what has got me confused - I cannot begin to guess the mechanism that causes this behaviour.

It must be the starter solenoid that isn't operating but the -ve is clearly okay - and the +ve seems to be fine, but it is when I connect the meter to measure the 12V at the solenoid that it starts. So connecting a digital MM with very high resistance across the solenoid coil makes it operate. But that connection has to be maintained, as soon as I disconnect the meter the solenoid stops operating.

It is sounding like a dying solenoid. They can become temperamental. Maybe damaged contacts or a partial wire break. A cheap and easy fix hopefully.

I’ll bet there is a way to stop it without the solenoid. A stop button or lever on the fuel pump to depressurise the fuel pump is usual, or a decompression lever on the engine or a fuel shut off valve.

 

[bedouin]

Bedouin, my old Bukh 20 which I think was electrically identical to the 24 was quite happy to start and run with the water temperature alarm sensor disconnected. The wire fell off quite often...

I suspect you have something else going on. Maybe making or breaking a connection somewhere by your very presence. I had something like that once, turned out stepping on the engine steps somehow distorted the switch panel and opened up a dry joint.

I would normally agree about this being random but I did check very carefully and I am certain about the impact of connecting the meter. Remove the -ve meter probe from the earth point and the starter stops immediately. Repeated several times. (to be clear the +ve prob was clipped on with a croc clip so no chance disconnecting the -ve move the +ve in any way) Also reproducible by disconnecting the temp sensor wire.

I assume the engine is not supposed to work that way - so I suspect there may be some other missing connection somewhere - but can't think what it can be given the symptoms. A missing earth/-ve would be one candidate but everything else seems fine