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

[bedouin]

The boat has finally been re-launched today - yippeeee.

But I had trouble starting the engine - at first it seemed dead and the starter wouldn't turn at all. Then when I connected a multimeter (on Volts) between the solenoid supply and earth (to check the supply voltage) that connection alone was enough to allow the engine to start even though I presume the MM on V must have an input resistance in the hundreds of thousands of ohms.

Then I discovered that the single wire from the engine panel to the temperature sender had come adrift and reconnecting that fixed everything.

But what on earth was going on - why is that wire needed for the solenoid/starter to work and why did connecting the meter also "fix" it. I thought I understood the rudiments of the diesel engine but I don't understand this. Engine is Bukh DV24

 

[Boater Sam]

This could be one of life's great mysteries!
Bet the answer is not 42 though.

 

[Catalina36]

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

 

[bedouin]

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

No - I assure you it wasn't. I tested very carefully and it was the process of touching the negative probe to engine earth that allowed it to start turning over and removing it immediately stopped it.

I did then find the wire that had pulled away from the engine and re-fixing that solved the issue.

This is not a sophisticated engine and has no electronic anything. I can't understand why the multimeter had any impact at all

 

[James_Calvert]

Beds I've just acquired a Bukh 24 to replace our old 20. It starts OK, but needed a new battery to turn it over.

Used the old wiring harness. It is very easy to understand low tech.

I don't understand your problem at all!

So please share what you discover.

 

[VicS]

The boat has finally been re-launched today - yippeeee.
But I had trouble starting the engine -

Then I discovered that the single wire from the engine panel to the temperature sender had come adrift and reconnecting that fixed everything.

" the single wire " ... This is for the warning light then ... not one of the wires for a combined warning light and temperature gauge sensor?

Do you have a control panel with a combined start/stop key switch or a panel with separate off/run switch, start button and stop button?

Which terminal of the solenoid were you checking the volts on? The big one with the main positive supply from the battery isolating switch on it or the smaller connection to the solenoid coil

 

[Zing]

Some engines shut down if they overheat. If the sensor is open circuit on over temperature it will do the same thing if you pull the wire off. i.e. kill the engine or stop it starting. I presume your stop solenoid is on that circuit also?

 

[VicS]

Some engines shut down if they overheat. If the sensor is open circuit on over temperature it will do the same thing if you pull the wire off. i.e. kill the engine or stop it starting. I presume your stop solenoid is on that circuit also?

Can you show me this on the wiring diagram in the owners manual or the workshop manual. I dont think what you are saying applies

Workshop manual: http://bukh.dk/upload_dir/docs/FAQ/... - Workshop manual/Work Shop Manual DV 24.pdf

 

[Zing]

Can you show me this on the wiring diagram in the owners manual or the workshop manual. I dont think what you are saying applies

Workshop manual: http://bukh.dk/upload_dir/docs/FAQ/Bukh RME - Data/Bukh RME - Workshop manual/Work Shop Manual DV 24.pdf

I had a quick look. I don’t understand what is going on in the terminal blocks on the schematic. There doesn’t seem to be a detailed wiring diagram.

 

[sailor211]

Can you show me this on the wiring diagram in the owners manual or the workshop manual. I dont think what you are saying applies

Workshop manual: http://bukh.dk/upload_dir/docs/FAQ/Bukh RME - Data/Bukh RME - Workshop manual/Work Shop Manual DV 24.pdf

If my eyesite is OK on drawing P3 the water pressure sensor is in line and goes to the start stop switch via couple of junction boxes and what i asume are push in serminals in the harness.

 

[bedouin]

I had a quick look. I don’t understand what is going on in the terminal blocks on the schematic. There doesn’t seem to be a detailed wiring diagram.

The Bukh can come with two different control panels and both are shown on the diagram. Mine has a replaced form of the one without the gauges.

The stop solenoid is not involved in this - the wire connected to the sensor is not thick enough to carry the current for the solenoid - and you certainly wouldn't get that current through a digital multimeter on a V setting!

This engine is pretty much as simple/robust as it gets. No fancy electronics of any sort and can be started by hand without any battery at all

 

[VicS]

I had a quick look. I don’t understand what is going on in the terminal blocks on the schematic. There doesn’t seem to be a detailed wiring diagram.

I think you are referring to the connectors which may have an extension cable between them. Pins labelled 1 to 8. If so 1 connects to 1 , 2 to 2 etc They are drawn as mirror images of each other so are easy to follow,
.

 

[bedouin]

If my eyesite is OK on drawing P3 the water pressure sensor is in line and goes to the start stop switch via couple of junction boxes and what i asume are push in serminals in the harness.

Those are two alternative panels - the engine won't have both. Mine is like the top one on that page. The wire to the sensor comes from the switch, through the warning light and then to the temp sensor - with the negative being the engine itself.

The only thing that isn't apparent from the diagram is that there is a warning buzzer that should be commoned across all three warning lights - mine isn't working at the moment.

 

[VicS]

If my eyesite is OK on drawing P3 the water pressure sensor is in line and goes to the start stop switch via couple of junction boxes and what i asume are push in serminals in the harness.

No your eyesight is not OK. There is no such thing as a water pressure sensor

There is an oil pressure sensor which connects to the oil pressure warning light and a combined water temperature transmitter for the water temperature warning light and the temperature gauge ... one connection to each

 

[VicS]

The Bukh can come with two different control panels and both are shown on the diagram. Mine has a replaced form of the one without the gauges.
The stop solenoid is not involved in this - the wire connected to the sensor is not thick enough to carry the current for the solenoid - and you certainly wouldn't get that current through a digital multimeter on a V setting!
This engine is pretty much as simple/robust as it gets. No fancy electronics of any sort and can be started by hand without any battery at all

Who mentioned stop solenoid ?

(it is powered from the stop button or start/ stop ( key) switch depending on the panel)

 

[bedouin]

Who mentioned stop solenoid ?

(it is powered from the stop button or start/ stop ( key) switch depending on the panel)

#7

Mine has a stop button that was powered from a special "stop" position on the switch one round from off - but someone rewired that when they were working on the boat a couple of years ago - presumably because they didn't realise and so couldn't stop the engine.

 

[VicS]

#7

Missed that

Red herring anyway.

 

[sailor211]

No your eyesight is not OK. There is no such thing as a water pressure sensor

There is an oil pressure sensor which connects to the oil pressure warning light and a combined water temperature transmitter for the water temperature warning light and the temperature gauge ... one connection to each

Water temperature sensor

 

[Zing]

Having had my morning coffee, I took another look. Indeed, the stop solenoid is not wired as I suggested. It is an on to stop type anyway. Also I can’t see any reason why it would not start as you describe. Maybe the wiring was not done this way on your boat? Nice to see simplicity. No relays, nothing much to break.

 

[bedouin]

Having had my morning coffee, I took another look. Indeed, the stop solenoid is not wired as I suggested. It is an on to stop type anyway. Also I can’t see any reason why it would not start as you describe. Maybe the wiring was not done this way on your boat? Nice to see simplicity. No relays, nothing much to break.

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.