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Old 05-13-2022, 01:29 PM   #1
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RPM gauge

Can someone tell me why would my rpm gaugestart jumping up and down unless I get over 3000 rpm. It doesn't work while you are at idle. About half trolle it starts jumping all over. Do I just need to replace the gsuge.
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Old 05-13-2022, 02:00 PM   #2
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Very likely a wiring problem. Check every connection and the condition of the grounds.
Many diesels use a tap from the alternator for the tach signal so check there.
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Old 05-13-2022, 02:30 PM   #3
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Yeah, put a ac voltmeter on the alternator W terminal. More indirectly, a dc voltmeter on the D+ terminal to help analyze.
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Old 05-13-2022, 04:51 PM   #4
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Rpm gauge

Dave. Thanks for the info. Without knowing much about alternator but I did check to see if I had any loose connections. Found none. How do I ck with my volt meter. I mean with positive and negative leads. Where to put them. Thanks in advance.
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Old 05-13-2022, 05:07 PM   #5
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RPM gauge

Diesel engine?

If so, is there a tach drive generator?

If diesel, and no generator/sensor, then very likely a wire on the alternator.

How many alternator wires or studs are present? A pix of the back of the alternator would be good to view. Any marks?
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Old 05-13-2022, 08:20 PM   #6
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Rpm gauge

I'm trying to download the picture I took. Obviously don't know how. Will keep trying.
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Old 05-14-2022, 05:37 AM   #7
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What engine?
Not all engines use the alt as a source for the Tach
On those thatvdo I believe it's one of the smaller wires likely with a plug terminal. Check for any corrosion, clean, lube & replace.

My Yanmar has a sensor that picks up a signal from the flywheel and has 2 leads from it. With this arrangement there are a couple causes if intermittent signal and tach jumping or not responding at all.
Wire terminals that slip onto the sensor- corrosion, clean then grease
Sensor adjustment / cleanliness - position / proximity to flywheel is important as well as being clean. Careful if removing to know where it is screw it is gently to get contact and count the revs. Then back it all the way out and clean. Replace to get contact and back it out the same # turns.
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Old 05-14-2022, 06:54 AM   #8
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You appear to have a Delco “one wire” alternator. Does not have a tach signal.
There must be a different signal source.
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Old 05-14-2022, 05:35 PM   #9
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It's a 1978 perkins 1-408 diesel. Found a site that says prior to 1980 they didn't use alternator but suppose to be a round plate 3" after of alternator bracket on starboard side of engine. This supposed to have a gear in it. Mine does not have any plate on starboard side.
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Old 05-14-2022, 06:59 PM   #10
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Perk 1978 4108 shop manual lists #41 as Tach Drive
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Old 05-14-2022, 08:23 PM   #11
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Thanks Bacchus
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Old 05-15-2022, 05:29 AM   #12
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Thanks Bacchus
No problem. I know next to nothing about the engine but a quick search turned up the manual. I scanned the pages referenced in the index for tach but couldn't find a sensor / sending unit. Maybe a more thorough dig would find something.
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Old 05-15-2022, 01:16 PM   #13
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Everywhere the books states the location of my tach sending unit is supposed to be, it is not. I did read that the perkins engine was built all over the world so they would not all be the same. I found this sending unit on the port side. In the manual it says provision is made, on the right side of engine, for a drive at half engine speed to be taken from the oil pump spiral gear to a tach.
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Old 05-15-2022, 01:25 PM   #14
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Quote:
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Everywhere the books states the location of my tach sending unit is supposed to be, it is not. I did read that the perkins engine was built all over the world so they would not all be the same. I found this sending unit on the port side. In the manual it says provision is made, on the right side of engine, for a drive at half engine speed to be taken from the oil pump spiral gear to a tach.
That looks like a pressure or temperature sending unit.
It may be time to trace the wires back from the tach to the engine.
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Old 05-15-2022, 01:28 PM   #15
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Those are oil lines next to it.
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Old 05-15-2022, 03:02 PM   #16
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It's not a tach drive.
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Old 05-16-2022, 12:58 PM   #17
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Knot yet, that's not a good option. The wires go down from the flybridge through the wall in the head and then to the engine room. Was hoping someone on here new where to look. Thanks a bunch
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Old 05-16-2022, 01:38 PM   #18
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You don't have to physically touch or see the complete wire length. After confirming the wire color at the helm tach headed to the engine follow as far as possible to make sure there isn't a connector at / near the helm.
If a bundle is head towards the ER start looking for wire bundles in ER headed to the engine. You may need to separate a bundle to search for that wire color. Colors should not change in the middle of a run especially if hidden. More often, if wire color changes it will be at a connector which should be more visible. Actually checking at connectors is a perfect place. If you find the colored wire on the helm side of the connector check yhe position to note the color that that position is mated to. If same fine, but if not start following / looking for the new wire color along the new cable run... another connector may enter the picture.
I've been successful with the above procedure when tracking small alternator wires to the helm... and I did find some color changes at connectors.
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Old 05-16-2022, 04:28 PM   #19
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Don, I hate acting so stupid but which wire do I need to trace
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Old 05-16-2022, 04:34 PM   #20
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Don, I hate acting so stupid but which wire do I need to trace
1 or 2 of those wires are probably for lighting and should marry with others nearby.
Of the remaining wires, 1 or 2 may also connect to busses for the ground or positive.
That leaves the signal that should remain separate and head toward the engine room.

After reading Diver Dave's next post, I like the gray wire now.
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