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Old 19-09-2019, 05:26   #1
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Perkins Prima M50 engine ground

Hi, on our b oat we have an 1990 Perkins m50 diesel engine.

I am quite hapy with her but she has one quirk that moest other engines don't have.

The engine has no grounding and has a Ground relay for when the glowplugs need to be used.

As ground is not traveling through the engine, this engine was not fitted with anodes.

Here goes the problem:

I have disconected all wires, the drive shaft, the gas and gear levers and somehow i am still measuring 12 volt ground on the engine block.

Is it possible that it travels through the salt water from the raw water intake? or perhaps from the boiler? I just don't understand as all wires are disconnected.

The 12 v i am measuring is weak though.
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Old 19-09-2019, 06:54   #2
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Re: Perkins Prima M50 engine ground

Please explain where you are measuring 12 Volts: where are you attaching each lead to?
What do you mean by a weak 12 Volts?
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Old 19-09-2019, 09:01   #3
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Re: Perkins Prima M50 engine ground

You are describing a momentary ground engine. Where a relay momentarily grounds the engine block only during activation of the starter motor or the glow plugs. During normal running operation the block is not grounded. If you can measure voltage between the battery positive and the engine block, you may have fault. Use a 12 volt test light bulb to make the check. If the bulb lights when connected between the battery negative and the block you have a battery positive fault. If the bulb lights when connected between the battery positive and the block you have a battery negative fault. This fault can be through the wiring, a sender or maybe the alternator. Start disconnecting wires until the light goes out. Testing with a meter can give you confusing results.
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Old 19-09-2019, 09:31   #4
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Re: Perkins Prima M50 engine ground

I attach the + lead to the battery and the minus lead to the block. i measeure 11.5 volts. so there is some resistance as battery reads 12.7v.

I also measure with a light (led).
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Old 19-09-2019, 09:36   #5
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Re: Perkins Prima M50 engine ground

Indeed, the ground relay is for shutting down the engine and for the glowplugs. All sensors, starter and alternator are isolated from ground and have there own wiring.

I indeed measure a fault, as i read 11.5 volts between battery positive and engine block. Even when all cables + engine throttle and F/R disconnected.

I also mesure this between a bronze fitting for the raw water which is isolated from the block with rubber piping which indicates that is travels through salt water?

(i also use a led buld measuring device which indicates if it is + or -.)

I would think that i have a leak somewhere into the saltwater or so?
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Old 19-09-2019, 09:42   #6
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Re: Perkins Prima M50 engine ground

Quote:
Originally Posted by Tjurd View Post
Hi, on our b oat we have an 1990 Perkins m50 diesel engine.

I am quite hapy with her but she has one quirk that moest other engines don't have.

The engine has no grounding and has a Ground relay for when the glowplugs need to be used.

As ground is not traveling through the engine, this engine was not fitted with anodes.

Here goes the problem:

I have disconected all wires, the drive shaft, the gas and gear levers and somehow i am still measuring 12 volt ground on the engine block.

Is it possible that it travels through the salt water from the raw water intake? or perhaps from the boiler? I just don't understand as all wires are disconnected.

The 12 v i am measuring is weak though.
I heard this engine is a nightmare.
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Old 19-09-2019, 09:44   #7
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Re: Perkins Prima M50 engine ground

If you need to fix it it is....

But its actually very reliable.
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Old 19-09-2019, 14:59   #8
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Re: Perkins Prima M50 engine ground

Try measuring the resistance (ohms) between the engine block and the -ve terminal on the battery.

The resistance value will help in determine where to look next.
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Old 19-09-2019, 15:45   #9
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Re: Perkins Prima M50 engine ground

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Originally Posted by Tjurd View Post
If you need to fix it it is....

But its actually very reliable.


They have their quirks but do live quite a long life if carefully maintained. The raw water pump and its drive and the timing belt spring readily to mind and adjusting the valve clearances is a fairly complex operation compared to most other engines.
Do you actually need the engine to be above ground because the vessel has a metal hull?
It's a bit of a long shot but some engine mounts will allow the lower end of the adjusting bolt to contact the engine bed if the rubber to bolt bond fails. Wire braid and shielded fuel hoses can also ground out your engine block. The raw water earth idea is worth investigating and only involves removing the hose from the gearbox oil cooler to verify or dismiss.
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