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Old 22-03-2020, 21:37   #1
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What is this? off Volvo Penta MD22L

I have been troubleshooting a gremlin in my engines electrical system and I came across this part and I have no idea what it is so was hoping someone might be able to shed some light for me. (it needs the corrosion cleaned off)

https://photos.app.goo.gl/vXH5uxufUPoVbinu9

https://photos.app.goo.gl/NX4qB7W1ScvSZC729
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Old 22-03-2020, 22:01   #2
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Re: What is this? off Volvo Penta MD22L

Pretty sure that’s a fuse. IIRC they call that one a “ground fuse”, one cable goes to a relay, the other to a ground connection.

See page 124 in this manual, pretty much the same for the whole family.
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Old 22-03-2020, 22:04   #3
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Re: What is this? off Volvo Penta MD22L

Thats correct, its definitely connected to the engine ground. I tested it for continuity however there appears to be none?
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Old 22-03-2020, 22:21   #4
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Re: What is this? off Volvo Penta MD22L

Does everything function? I think that provides the ground connection for a fair part of the controls.
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Old 22-03-2020, 22:30   #5
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Re: What is this? off Volvo Penta MD22L

well there is a bit of a gremlin in the system.. I am having issues with warning lamps and the tacho which have all kind of occurred around the same time. The engine starts and runs otherwise fine.
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Old 22-03-2020, 22:55   #6
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Re: What is this? off Volvo Penta MD22L

Can you tell if the glow plugs operate? The fuse is 55A (I think) and protects the negative for instruments and glow plugs. The MD22 is usually isolated ground. The starter and alternator should have connections to battery negative, but not the block. When the relay is energized by turning on the ignition switch the rest of the system (besides starter/alternator) gets negative connection through that fuse. If the glow plugs work then the fuse should be good. In which case I’d start looking at the relays and instrument wiring.
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Old 22-03-2020, 23:39   #7
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Re: What is this? off Volvo Penta MD22L

Quote:
Originally Posted by Dsanduril View Post
Can you tell if the glow plugs operate? The fuse is 55A (I think) and protects the negative for instruments and glow plugs. The MD22 is usually isolated ground. The starter and alternator should have connections to battery negative, but not the block. When the relay is energized by turning on the ignition switch the rest of the system (besides starter/alternator) gets negative connection through that fuse. If the glow plugs work then the fuse should be good. In which case I’d start looking at the relays and instrument wiring.
If the glow plugs operate and the ground to the instruments and glow plugs is distributed through that fuse, then the fuse is good and is unlikely to be the problem, as described above.

But...the corrosion on the terminals probably is, even if it doesn't interfere with the operation of the glowplugs.

It might also indicate the general condition of the rest of the electrical system, which is notorious for electrical 'gremlins', many of which are brought on by (in my eyes) undersized wiring and those infamous circular, 22 pin, twist-to-connect Volvo connecters (if your engine has them).

May seem daunting, but I'd start with an as-complete-as-possible disassembling, cleaning and reassembling (with some sort of antioxidant 'grease') of every electrical connection in the system causing problems, giving especial attention to grounds and positive supplies, and when you're satisfied everything's kosher there, going on to check all the connectors for corrosion and/or loose wires or pins. Again, they are notorious for having both power and ground faults, and are sometimes hidden from view.

If you have a connector 'flopping around' near the instrument panel, that is a prime suspect...
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Old 24-03-2020, 07:09   #8
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Re: What is this? off Volvo Penta MD22L

As mentioned above, the item you are holding is a fuse, about 50 amp, that connects the main wiring harness ground to the engine block.

The 22 series engines were built with an isolated ground system, where the block is used as part of the ground system in only three instances: starter motor use, glow plug use, and stop solenoid use. At those times current will flow thru that fuse. There is also a grounding relay which is energized to make the connection from the harness to the block, and this relay is controlled by the key switch in the three instances mentioned. Diodes in the harness near the ignition switch prevent feedback between the three positions on the key switch.

I would suggest working thru the harnesses as mentioned by Jim, looking at the 16 pin connectors for corrosion or other faults.

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Old 24-03-2020, 07:15   #9
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Re: What is this? off Volvo Penta MD22L

Thanks for all of the awesome information provided!
I will be visiting the vessel tomorrow and will be well equipped for cleaning, greasing and testing. This should be done regardless!

Not super confident I will find the fault as I have performed continuity tests between the inputs and outputs of the alternator and as close as I can on the instrument. This appears to be OK..

Would there be any advice in testing tacho output of alternator with standard multi meter, or how should I test?
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Old 24-03-2020, 07:29   #10
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Re: What is this? off Volvo Penta MD22L

Depends on how fancy the “standard” DVM. Needs to be able to measure frequency/Hz. The alternator generates AC electricity (hence its name), the frequency of that AC is proportional to speed, and that is what is used to drive the tachometer.

If the alternator is putting out power then there is AC, no way around it, which leaves you with a bad connection somewhere (could be inside the alternator).

If the batteries are full and the regulator turns the alternator down to almost no power output this can lead to a wild tach reading. Easiest way to test this, turn on some load that takes 5-10A, force the alternator to increase power output a little.
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Old 24-03-2020, 07:48   #11
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Re: What is this? off Volvo Penta MD22L

The alternator drives the tach thru the grey wire connected to the “w” terminal on the alternator. Put your VOM on “ac volts” and place one lead on the “w” , terminal and the other lead on the black ground wire. You should see some output, and you can trace that grey lead up to the instrument panel and tach.
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Old 24-03-2020, 21:33   #12
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Re: What is this? off Volvo Penta MD22L

@Doug, I note that the solid grey wire you mentioned on the alternator only has continuity to the grey/black wire to the tacho.. not to the solid grey wire at the tacho.

@all A little more background here. When the issue first occurred, I was getting no charge from the alternator. I attempted to manually excite it by putting a bulb across the D+ but was still unable to get it to generate power. I since purchased a replacement (https://www.marineenergy.com.au/prod...98aab85b&_ss=r), I note its self exciting. I also note that I have performed the tests below with D+ and W connected and disconnected, no notable difference.

1) I am reading hertz across the white and negative terminal (you can see the terminal labels on the link above). I am getting inconsistent readings between 40-70hz at idle. The meter seems to lock on a reading, freeze for a few seconds and then reset back to zero. To test the meter i tested the AC power coming from my AC inverter and the readings appeared to be consistant. The Alternator is charging the batteries.

2) It was stated that this engine is supposed to be ground isolated? I note this replacement alternator appears to have a grounded body. Is this a problem?

3) another issue is the alternator warning light. I appear to be getting a 7V difference (reading +7V) across the black excite connector and ground (-). Is this expected behaviour, I understand this to be driving the warning lamp.
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Old 25-03-2020, 07:59   #13
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Re: What is this? off Volvo Penta MD22L

The tach is equipped with two leads, grey and grey/white because some of the engines which also used that panel had inductive tach drives which require twin leads. This shouldn't be a problem.

- grounded tach body- You need to attach the black ground lead from the old tach to the case ground of the new tach. If you don't do this the charging will be erratic to non-existent.

- with the key on, but the engine not running, the brown exciting lead should show about 12v when disconnected from the alternator, but might show +7 volts or lower if connected to the alternator......depends on the internal resistance of the alternator. When the engine is running, the exciting terminal should show 12 volts.
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