Tach Trouble

Hello all. I have a basic understanding of wiring but have a problem I was hoping someone more experienced than I could help with. I'm having trouble with one of tachometers on our 1979 Tollycraft Sedan. I was hoping someone might have some tests I might try to determine what sort of repair might be needed. Our boat has twin Mercruiser 255s and two station controls. Three of the tachometers work flawlessly, but the starboard lower helm tach can fluctuate dramatically. Sometimes it reads true, sometimes it reads 500 - 1500 RPM higher. Sometimes I can smack it and it will work correctly again. (My first method of repair on almost anything) Sometimes it will not. When I turn the instrument lights on the tach always creeps up to higher RPMs. The gauges are all original on our boat and was really hoping to repair rather than replace as I don't think I would be able to find an original any way. Does anyone have any thoughts on what I might try to correct this. Thank you for any comments or thoughts. Roger

 

I am under the impression that you may have a "dry" connection. Either one of the soldering joints is oxidiced or a bad internal connection. If you do have soldering points, re-solder them. If you have a mechanical one, the "clutch" may slip or the spring may get tangled up. A watchmaker in that case may be of assistance.
Bert

 

Thanks for your help Bert. I'm thinking I will pull the tach out and have a little look inside and see what I can see. I may have more questions for you at that time. Thanks again for your time. Roger

 

If turning something on affects it then you have an earthing fault ..try a tempary cable from the tacho to the engine earth ..I assume its driven by the alternator ...

 

Thanks for your suggestion Pistnbroke. I was originally thinking it was a connection somewhere but couldn't grasp which one would cause such a thing. A temporary ground is a quick and easy test to try. I'll give it a go. Thanks again, Roger

 

FIRST LAW OF AUTO ELECTRICS

If operating one circuit causes another to be affected its an earth fault .

 

Would you put money on that one, I mean I might be wrong and all that as Im not really sure and im not an auto electrician but lets say a little bet like a million dollars just to make it interesting . As I say im not sure 110% but im willing to have a go. Gosh I hope im right I dont want to loose a million dollars, by the way how would you want the money?

So if I could explain a circuitry that had a problem of two circuits affected by each other and was not a ground or earth prob I would win right?

 

Frosty can I share that million with you? Poor Pistnbroke can maybe afford a million , but I don't think two million.
Bert

 

The guy says if he turn on the lights the tacho goes funny....I cannot guarantee the accuracy of his information or his competance in testing the earth.......
SECOND RULE OF AUTO ELECTRICS
Test dont guess
THIRD RULE OF AUTO ELECTRICS
If you dont know what you are doing leave it alone or get someone who does.
FORTH RULE OF AUTO ELECTRICS
Frosty is not always right.

 

There is a FIFTH RULE OF AUTO ELECTRICS
One should use the words : Most likely, or a great chance, it will save 2 million

In the olden days rule 1 to 3, this could be very much true, but in today's world with micro controllers and computers in an auto, aute electrics has now to cope with electronics and that is sometimes a different kettle of fish.
Bert

 

Quite right bert ..I usually add to the earth rule ..unless the system has an ECU ...but I wanted to keep it simple for Frosty.

 

damn bert I though I had im there.

Thats what you get from school teachers, slippery as a snake.

Ask him what an ECU is ille bet a million he knows now he's been googling it .

 

Damn Frosty, I have spent the 500.000 also already.
Bert

 

My Tach works again!

I went down to the boat today optimistic about trying pistnbroke's ground fault test. The test connection made no difference, and I do possess the skills to make a proper test. While considering BertKu's soldering repair suggestion I thought I would check the other connections. When I wiggled the signal connection, which was loose, the tach snapped back to being accurate again!

I am new to this forum so I am a little unclear as to how the million dollars factors in but I am sure that as the originator of this thread I must be entitled to a portion of it as well. I'm thinking ten percent or so would do just fine. Thank you all for your help with my issue.

 

Glad you got it sorted yourself --feels good don't it?