Tach working backwards

[Bedrest2]

Anyone ever see a tach work backwards? I have a Volvo Pent 2.1, Tach
never worked when I purchased the 1985 Bayliner. Got a tach at the Auto
Parts store hooked up, and needle went from just below zero backwards to
the 8, when running 7400@idle and 5000 when reved up. Checked wiring 3
times looks correct, I'm an Electrical Engineer, and worked in Maintenance
for 10 years, on production equipment, So I don't work on cars, but this
should not be this difficult. Any help? Greatly appreciated, thanks.

 

[littlebookworm]

Re: Tach working backwards

Hi, welcome to the site. I just checked the dash wiring diagrams that I have and there seem to be only a few possible reasons for the problem you're having, as I see it. Your automotive tach might not be designed to work on a marine engine, which is highly doubtful since they're all pretty much the same, or you have two wires switched - the signal wire (which goes to the coil) and the purple (I believe) wire. I suggest you double-check your connections compared to the original tach. I'd also suggest that you check that the tach sending wire (ususally gray) is connected to the positive terminal on the coil. Let us know what you find. Good luck and safe boating. Hy

 

[Bedrest2]

Re: Tach working backwards

First let me thank you for the reply Littlebookworm.
I'm confused, you say sidnal wire(grey) hook to plus side? All diagrams I have
been able to find show signal(grey) to minus side. I don't doubt you, but I do
not have a diagram for my motor wiring yet (book in route). What I do have
is Grey(minus on coil) to Purple(to points) together in harnis plug, to grey/brown, signal wire to tach, Ground(Black) direct to battery Minus, Red
to 12 volt plus. Motor is running fine, could some one have put the coil in backwards? Just bought the boat 2 weeks ago.
Anyone willing to send me a diagram? Thanks in advance. and happy boating

 

[Bedrest2]

Re: Tach working backwards

Update to my issue;
Never assume the new Tach just bought is good, I've purchased a 2nd Tach,
got a shop manual, and now have a working Tach. Was not hooked up wrong
it was a bad, brand new Tach. Thanks for reading and hope it helps some one.