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Old 07-29-2021, 07:23 AM   #1
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Cummings 555 tack

The VDO tacks (4- 2 up and 2 down) on our 1989 42' Oceania trawler with Cummings 555 don't typically read until about 1'000 RPM's, the port engine sometimes doesn't work until the engine is warmed up and sometimes doesn't work at all. Could this be sending units going bad, where would they be mounted?

Thanks for any help Jack
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Old 07-29-2021, 07:32 AM   #2
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Most marine diesel tach's of this vintage use the a signal from the alternator to determine engine speed. They need to be calibrated at time of installation and can be thrown off if you change the alternator pulley size. Intermitant operation is usually due to a lose wire/bad connection, bad ground wire to the dash or slipping belt. Black grime in the area of the belt will indicate a slipping belt.
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Old 07-29-2021, 07:49 AM   #3
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See if the tachs jump up if you put a small dc load on like a dc cabin light.
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Old 07-29-2021, 12:45 PM   #4
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The V555 tach drive is NOT driven off an alternator, at least on mine. Mine is
a 1976 model. It is possible they changed things but I doubt it as the sender drive was integral with the PT [injection] pump.

Find the PT pump in the engine VEE at the back of the Vee. THe tach drive is attached to the side of the PT pump. From the engine front that sender will be on the LH side of the PT. The unit is a variable voltage generator.

Check the wire to it. The wire terminal needs to be tight to the output terminal, the terminal crimp needs to be tight on the wire. If there is corrosion or looseness at the terminal or the actual crimp that can cause goofy readings.
I have often just recrimped the crimp which will break any oxidation up and has worked.

I have reterminated many of my connections over the years. Be prepared to do that.
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Old 07-29-2021, 10:58 PM   #5
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Welcome to TF, but as you have a very specific query, I have moved you to the Power Systems section from Welcome Mat for better exposure of your problem.
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Old 07-30-2021, 08:31 AM   #6
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Thank you all for this information, I will ck into all your suggestions.
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