Erratic Tachometer Readings
Thread Starter
Melting Slicks
Joined: Jan 2014
Posts: 2,478
Likes: 254
From: Downers Grove Illinois
Erratic Tachometer Readings
I have a 1977 Corvette that has the typical erratic Tachometer readings. I replaced the filter and just recently replaced the circuit board on the tachometer. Before putting the tach back in the car I connected 12V and ground and the tach went right to 0, as expected. When I put it back in the car I am again getting a erratic reading and when I turn the ignition on without starting it the tach does not go to 0. I suspect either the 12V or ground wire is bad. I have a stronger suspicion it is ground wire, but without ripping the dash completely apart does anyone know how that wire is routed? I know the radio wiring looks a little kludgey, but other than that don't see anything out of the ordinary. Thanks,
Jim
Jim
Thread Starter
Melting Slicks
Joined: Jan 2014
Posts: 2,478
Likes: 254
From: Downers Grove Illinois
I disconnected the tachometer wire under the hood and then turned the ignition on. I noticed the tach moved slightly telling me that the 12V and ground wire must be connected. It sits about 1000 RPM below 0. When I start the car now with the tach wire connected it sits at about 1000 RPM below 0 and if I slowly increase the engine speed it eventually jumps up around 3000 RPM. I think it may have a bad ground wire, but will have to pull things back apart to check, which I am not looking forward to. Last time I put it together I broke 3 wires going to bulbs and had to solder them back to the little brass center conductors.
Pro
Joined: Aug 2013
Posts: 591
Likes: 3
From: Monroe Washington
Perhaps take another look that the solder you added didn't jump over close to other lines... My tach didn't work when I bought the car... But later found out the shop didn't extend the line to reach the new distributor location during motor swap... I extended it, plugged it in, and all is well in my case... Good luck! I've found tracking down wiring issues can be a lot of. 'FUN' with these cars. Lol
Pro
Joined: Aug 2013
Posts: 591
Likes: 3
From: Monroe Washington
When my tach was disconnected... It would jump when I turned the car over... But as soon as I got the motor running it would move to 300-400 Rpms... Occasionally going to higher numbers when I shut off the car or while it was cranking before it started...
Thread Starter
Melting Slicks
Joined: Jan 2014
Posts: 2,478
Likes: 254
From: Downers Grove Illinois
This is a good one. I took everything back apart after measuring 12 volts from the pink and black wires going to the tach. The problem was one of the nuts that hold the tach board down wasn't tight. I tightened all 3 to be safe hooked it to a 12 volt source and it read 0 RPM again as it should. Once I put the gauges back in the left dash panel I hooked it up to the tach connector. With the ignition on before I started the car it still read 0, good so far. I started the car and for the first time since I bought the car the tach reads right. I still need to put it all back together. I learned to be sure everything is tightened properly when putting things back together. I think the socket wasn't quite deep enough for the job. I hope this helps someone else from making the same mistake. The odd thing was I tested it the first time after putting the board on the tach and applying 12 volts had it read 0. I guess it made an OK connection at that moment.
Last edited by JimLentz; Jul 18, 2014 at 08:28 AM.
Thread Starter
Melting Slicks
Joined: Jan 2014
Posts: 2,478
Likes: 254
From: Downers Grove Illinois
Thanks. The other problem I had was when I put the dash back together the first time I broke 4 wires to the bulbs for the gauges. Now that I bought the assembly manual I see the wiring harness is supposed to sit above the gauges and I used tie wraps to hold it up. This time when I re-assembled that side I didn't break any wires as they weren't running into the wiring harness.
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