Tach Interference
Thread Starter
Intermediate
Joined: Jul 2018
Posts: 29
Likes: 8
Tach Interference
Hey Everyone,
First time poster here! So I have a 1982 CE that I am currently fixing up that has been in my family for the last 20 years and I'm sure I'll have a ton of questions so here goes my first issue.
When I got the car the tach wasn't working, so I purchased a new tach circuit board and got it going. The problem now is that I have interference in the tach and it's pretty bad. I always assumed it was the tach filter, so I put a new one on today, but it didn't make a difference. Here's what is going on:
The tach seems to be reading the RPM accurately and it fluctuates with the throttle like it should, however any time there is a change in the electrical system i.e. the brake pedal gets pressed, the turn signal is turned on, or the windows get rolled up or down, it causes a spike in the tach reading (it will bounce up and down while the turn signal is on).
I disconnected the tach wire from the distributor to see if other electrical pulses still affected the gauge and they do... So I know there is interference coming from somewhere other than the tach wire, I'm just not sure where to begin.
I think I have ruled out the tach itself, the circuit board attached to the gauge (it's new), and the filter (also new).
Any help would be greatly appreciated!
First time poster here! So I have a 1982 CE that I am currently fixing up that has been in my family for the last 20 years and I'm sure I'll have a ton of questions so here goes my first issue.
When I got the car the tach wasn't working, so I purchased a new tach circuit board and got it going. The problem now is that I have interference in the tach and it's pretty bad. I always assumed it was the tach filter, so I put a new one on today, but it didn't make a difference. Here's what is going on:
The tach seems to be reading the RPM accurately and it fluctuates with the throttle like it should, however any time there is a change in the electrical system i.e. the brake pedal gets pressed, the turn signal is turned on, or the windows get rolled up or down, it causes a spike in the tach reading (it will bounce up and down while the turn signal is on).
I disconnected the tach wire from the distributor to see if other electrical pulses still affected the gauge and they do... So I know there is interference coming from somewhere other than the tach wire, I'm just not sure where to begin.
I think I have ruled out the tach itself, the circuit board attached to the gauge (it's new), and the filter (also new).
Any help would be greatly appreciated!
Le Mans Master
Joined: Jan 2001
Posts: 6,656
Likes: 1,357
Can you borrow an after-market tach to see if it's the circuitry and not the tach?
When I was about to install my '76 electric tach in my '68 that's what I did to ensure that everything was working ok while waiting to receive my electric tach. It should let you know if there is something wrong with the vehicle as opposed to something being wrong with the Corvette tach itself.
If there is interference with the after-market tach, then it should be the car wiring... if not, then you probably got a bad circuit board. or the tach movement itself may be erratic. Should be an easy check with tearing apart the dash since you can set the external tach anywhere.
When I was about to install my '76 electric tach in my '68 that's what I did to ensure that everything was working ok while waiting to receive my electric tach. It should let you know if there is something wrong with the vehicle as opposed to something being wrong with the Corvette tach itself.
If there is interference with the after-market tach, then it should be the car wiring... if not, then you probably got a bad circuit board. or the tach movement itself may be erratic. Should be an easy check with tearing apart the dash since you can set the external tach anywhere.
Last edited by carriljc; Feb 17, 2019 at 11:13 AM.
Thread Starter
Intermediate
Joined: Jul 2018
Posts: 29
Likes: 8
The original was just hanging there so it wasn't grounded, but the new one is bolted to the intake. I'm still getting the interference in the tach needle when the tach wire is disconnected (the needle is dead with the car running like this, but turn signal and power windows etc still cause the needle to bounce a little.) I'm guessing there's something crossed in the electrical system somewhere.
Thread Starter
Intermediate
Joined: Jul 2018
Posts: 29
Likes: 8
Can you borrow an after-market tach to see if it's the circuitry and not the tach?
When I was about to install my '76 electric tach in my '68 that's what I did to ensure that everything was working ok while waiting to receive my electric tach. It should let you know if there is something wrong with the vehicle as opposed to something being wrong with the Corvette tach itself.
If there is interference with the after-market tach, then it should be the car wiring... if not, then you probably got a bad circuit board. or the tach movement itself may be erratic. Should be an easy check with tearing apart the dash since you can set the external tach anywhere.
When I was about to install my '76 electric tach in my '68 that's what I did to ensure that everything was working ok while waiting to receive my electric tach. It should let you know if there is something wrong with the vehicle as opposed to something being wrong with the Corvette tach itself.
If there is interference with the after-market tach, then it should be the car wiring... if not, then you probably got a bad circuit board. or the tach movement itself may be erratic. Should be an easy check with tearing apart the dash since you can set the external tach anywhere.
