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captbuddy 05-13-2022 01:29 PM

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.

KnotYet 05-13-2022 02:00 PM

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.

diver dave 05-13-2022 02:30 PM

Yeah, put a ac voltmeter on the alternator W terminal. More indirectly, a dc voltmeter on the D+ terminal to help analyze.

captbuddy 05-13-2022 04:51 PM

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.

diver dave 05-13-2022 05:07 PM

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?

captbuddy 05-13-2022 08:20 PM

Rpm gauge
 
1 Attachment(s)
I'm trying to download the picture I took. Obviously don't know how. Will keep trying.

Bacchus 05-14-2022 05:37 AM

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.

diver dave 05-14-2022 06:54 AM

You appear to have a Delco “one wire” alternator. Does not have a tach signal.
There must be a different signal source.

captbuddy 05-14-2022 05:35 PM

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.

Bacchus 05-14-2022 06:59 PM

1 Attachment(s)
Perk 1978 4108 shop manual lists #41 as Tach Drive

captbuddy 05-14-2022 08:23 PM

Thanks Bacchus

Bacchus 05-15-2022 05:29 AM

Quote:

Originally Posted by captbuddy (Post 1098708)
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.
Attachment 128649

captbuddy 05-15-2022 01:16 PM

1 Attachment(s)
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.

KnotYet 05-15-2022 01:25 PM

Quote:

Originally Posted by captbuddy (Post 1098811)
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.

captbuddy 05-15-2022 01:28 PM

Those are oil lines next to it.

rgano 05-15-2022 03:02 PM

It's not a tach drive.

captbuddy 05-16-2022 12:58 PM

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

Bacchus 05-16-2022 01:38 PM

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.

captbuddy 05-16-2022 04:28 PM

2 Attachment(s)
Don, I hate acting so stupid but which wire do I need to trace

KnotYet 05-16-2022 04:34 PM

Quote:

Originally Posted by captbuddy (Post 1099131)
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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