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Hi guys …new to the forum not sure if this is the correct forum to post this…
long story short bought a nice 81’ cream puff but of course it had a wonky Tach. Tach had power but it was all over the place when driving. At that time I didn’t know what I was looking at but I’ve since gotten an education on the 81’ Speedo cluster.
my question to the gurus is this; …if I have another 81 Tach outside of the vehicle I SHOULD be able to wire the tach board to hot, ground and run a signal wire from the HEI to the signal post on the tach board …and that outside tach should operate normally correct?
I have a different tach jumped to a 12v tractor battery and a jumper to the HEI Tach port. The Tach goes to zero when powered to the battery but when I start the car I’m not getting any needle movement…
You issue with the tach in the car is most likely a bad tach board. Common failure and replacement tach boards are readily available from Corvette vendors. There are numerous threads on here about this issue. A recent one, maybe just 6 months ago highlighted where to get a good one. Perhaps do a google search on the topic. Google search seems to work much better than the search feature on this forum. Yet google will bring you back to the forum in most cases.
As per your test. Perhaps use power and ground from the car, not a remote battery. I should think the tach is not likely to work without a common ground for both power and tach signal. And perhaps your test tach is also no good.
You issue with the tach in the car is most likely a bad tach board. Common failure and replacement tach boards are readily available from Corvette vendors. There are numerous threads on here about this issue. A recent one, maybe just 6 months ago highlighted where to get a good one. Perhaps do a google search on the topic. Google search seems to work much better than the search feature on this forum. Yet google will bring you back to the forum in most cases.
As per your test. Perhaps use power and ground from the car, not a remote battery. I should think the tach is not likely to work without a common ground for both power and tach signal. And perhaps your test tach is also no good.
Thanks for the quick reply! …yes I’ve got a new tach board on the outside tach but I’m not certain that tach works (eBay purchase). It does swing to zero when powered.
You did answer a question I had tho was if the Tach needed to be powered and ground to the vehicle not just a test battery. I’m a nuts and bolts guy with a very basic knowledge of electronics but it did occur to me that it might be an issue using a test battery. The only reason I did was to have shorter jumpers. I’ll try it with the cars power.
Chuck Fever
Just wire your external tach up to the car to see if it works..... that's what I did with mine when I converted to an electrical tach. Mine needed a board but I bought one and tested externally before tearing the dash apart to install it.
BUT
Check the tach filter before tearing the dash apart.
You can make one cheap enough, for a couple of bucks, if you how to use solder and heat shrink. Used to be able to buy the components to build one at radio shack but I reckon you'd have to buy them online nowadays .....
Chuck Fever
For a couple of bucks I make one (I did actually --- ended up removing it when I wired my tach to the MSD box--- didn't need the filter and it would make the tach bounce above ~3500 rpm).
Chuck Fever
For a couple of bucks I make one (I did actually --- ended up removing it when I wired my tach to the MSD box--- didn't need the filter and it would make the tach bounce above ~3500 rpm).
Thanks for the reply! …yea that was my next hurdle after bench testing a Tach externally was replacing the filter. I’ve unplugged it to bypass it. The new filters have a ground wire …I don’t see that the OEM filter has a ground wire. Wondering if it’s grounded just the way it’s mounted.
Hey guys just an update…I used the power and ground at the under hood light and the bench test was a success 👍
I don’t know why it didn’t dawn on me that t
he signal and the power had to have a common ground …electricity is a Voodoo science to me 🤣
I also connected the signal wire to the dash side of the OEM filter and it works good also.
so now I just have to get the beer ready to pull the Speedo cluster next weekend. The only thing I’m concerned about is getting my fat plumber hands up there to disconnect the VSS on the back of the Speedo without breaking anything…
Another 1981 Tachometer question
