Erratic Tach

[C Masone]

Over the winter I rewired the panel to my M25XP according to instructions found here and now my tachometer is erratic. The needle is jumping around and as I throttle up the needle starts going down. I'm thinking that it has nothing to do with the wiring changes themselves but possibly a loose connection however I am usually good at the tidiness and tightness of my connections. Also think that the pulley could be slipping although it was not touched during the winter.

I cant get back to the boat till Friday, but will listen to all troubleshooting suggestions before I head back

 

[u079721]

In our case of erratic tack behavior the solution was to replace every spade and ring fitting in the instrument cluster and tighten everything down - which made it go away. My next step was to go find the sensor, but I never had to.

On our cluster at least there was a real cock up of connections, with like six terminals on one screw for some reason. As part of the retrofit I installed several tiny "bus bars" so that every screw only had to have one or at most two terminals on it.

 

[Lawrence B. Lee]

Neurotic Tach

Just a thought, but if you did the wiring update found in "these pages" that tells me you now have a 10ga. wire running from your starter button to the solenoid. If you were in that part of the engine did you change anything on your alternator? The tach terminal has to be isolated from the body of the tach and if anything were lose it could cause a problem. Frankly, if you have a full short there it could blow your regualtor up. Since you are not getting a full collapse with the needle still jumping around I wonder if you changed regulators. Some require different wiring routes.

Just some thoughts from someone who has run into all of the above along the way.

Larry Lee
Annabele Lee 32-200
Savannah, GA

 

[Brian K]

Funny you should ask this question. I also reworked the engine instrument panel on my 1988 E32-200 (with MX-25XP Universal). One of the goals was to get the tach positioned so I could read it without bending over to see the top half of the tach that is inconveniently located under the starboard locker lid (I digress, but who was the less than brilliant person that would design such a useless instrument positioning?)

Anyway, I have noticed the same problem with the tach over the past couple of years, and again this year. I'm suspecting the pick-up point wherever that may be. Again, feedback on what to try and where to find the pick-up location is truly appreciated!!

<O</OAnd, yes, adding a small buss bar or 2 greatly improves the wiring behind the panel.<O</O

 

[Lawrence B. Lee]

Brian,
I digress too but the brain surgeon who placed the tach guage in our instument panels was the same guy who designed the fail safe valves for the BP rig in the Gulf. Wow, what a pain. Anyway, the tach lead is the gray wire off of the alternator as I recall.

Larry Lee
Annabele Lee
Savannah, GA

 

[Brian K]

Larry, thanks, and yes my schematic also indicates a grey wire as the source for the tach. I'll have to look at the connection at the tach and make sure that is clean and tight.

FYI, I inverted my engine instrument panel and the tach is now about an inch lower than the stock arrangement. This did not cure the visibility problem but it helped. To get even greater visibility I rotated the tach (like you see in race cars) so the useable range is at the bottom therefore more visible from above.

Thanks again Larry,,, I' check my connection at the alternator this weekend.

 

[Chris A.]

Mine was reading high, rather than erratic, and I corrected this by adjusting the tiny setting screw on the back of the tach display.

Probably this is not relevant for your problem but it's an easy thing to try (use a jeweler's type of flathead screwdriver).

 

[Brian K]

Chris, how did you determine what the "real" RPM was? Does it have to do with what the tach reads when the engine is off? Mine does not read "0" when the switch is off, it actually reads about 1k.

 

[u079721]

Chris, how did you determine what the "real" RPM was? Does it have to do with what the tach reads when the engine is off? Mine does not read "0" when the switch is off, it actually reads about 1k.

The only way to determine what the "real" rpm is, AFAIK is by using an optical tack sensor to calibrate the tack meter. You put a bit of reflective tape on the engine shaft, and the meter shines a light and then measures the frequency of the reflected light from that piece of tape on the shaft as it goes around.

If you lived nearby I would loan you mine, but I bet you can rent them at a tool shop.

 

[Rocinante33]

I suggest you check the ground circuit for the erratic tach problem.

For an optical tach, check on e bay. They used to have those for about $25.

 

[Chris A.]

Yes I used an optical tachometer I purchased on ebay...

 

[exoduse35]

check that there is a good ground to he case of he alternator. many depend on the mounting bolts to the engine and will provide a poor ground under vibration. then double check he voltage and grounds at the gauger. You should have a full 12v on he pos spade going into he gauge, and 0 Resistance to ground on the negative terminal. check a the gauge side of the connection, (ie, if it is a screw lug check at the screw or if it has blades for a female spade check a the blade) Also make sure of the polarity (+is + , -is -... it happens) From there check the continuity, resistance and isolation to + &- of the signal wire. After that it is in the guage or alternator. Hope this helps, Edd