Newcomer Needs Help w/Tach
Thread Starter
Drifting
Joined: Aug 2004
Posts: 1,763
Likes: 0
From: Fayetteville NC
Newcomer Needs Help w/Tach
Just bought my first Vette, a '75, a few months ago and I'm thrilled with it even though it's not show quality like so many I've seen on this forum. Regardless of that, I'm pretty hot stuff now where I live. I don't have lots of cash to spend on it but have wanted it for so long. I know what ya'll mean when you say, "Better to spend the money up front and ride, vs getting a cheap one and spending all your nickels and dimes on it". But you have to learn and that's what I'm doing. I'm getting it to the point where it just needs paint to be presentable. It's some shade or red but not one I'm familiar with. Maybe oxidized red. Here's my problem. The tach dosen't work and probably needs the replacement circuit board. All the dash instruments light up except the emergency brake light and right turn signal. These two lights happen to be in the tachometer facing. Need to know what relationship these have to each other and what might the problem be. Will a good circuit board make the two lights come back on? I doubt it. Thanks.
Melting Slicks
Joined: Dec 2000
Posts: 2,198
Likes: 2
From: Eastern Connecticut
First thing to do is check that the tach housing is grounded. Bad grounds are the source of 75% of all electrical gremlins. Unfortunately I don't know how the tach is grounded, probably a strap to the under dash metal. Joe
PS welcome and have fun!
PS welcome and have fun!
Drifting
Joined: Nov 2003
Posts: 1,634
Likes: 4
From: Northern NJ
St. Jude Donor '04 & '05
Welcome to the insanity! 
Along w/the ground, I would also check the tach filter before replacing the PCB. Should be a round metal "filter capacitor" looking device bolted to the rear of the intake manifold. It is wired inline between the distributor and tach and is there to filter out any electrical spikes, etc to keep them from damaging the tach board circuitry.
To test it, simply use a jumper wire across the two filter terminals at each end. If the tach starts working, then you have a bad filter. If not, then chances are it's the tach PCB, assuming all your wiring is in place, etc.
If it is the filter, I can provide you with a schematic of the circuit (only two resistors and two caps) which you can build with parts from any electronic parts house.
Good Luck w/your project.
Along w/the ground, I would also check the tach filter before replacing the PCB. Should be a round metal "filter capacitor" looking device bolted to the rear of the intake manifold. It is wired inline between the distributor and tach and is there to filter out any electrical spikes, etc to keep them from damaging the tach board circuitry.
To test it, simply use a jumper wire across the two filter terminals at each end. If the tach starts working, then you have a bad filter. If not, then chances are it's the tach PCB, assuming all your wiring is in place, etc.
If it is the filter, I can provide you with a schematic of the circuit (only two resistors and two caps) which you can build with parts from any electronic parts house.
Good Luck w/your project.
Racer
Joined: Jun 2004
Posts: 252
Likes: 0
From: Shawnee KS
Congratulations. By the way, this is what you will be looking at.
I'm currently chasing down electrical gremlins in my 75 as well so I can be of a little help.
The E-brake light is on the same fuse as the tach so there may be a connection but I am betting it's a bad bulb. A new board will not make these come on. Do your oil pressure and fuel gauges work? They are also on the same fuse. The turn signal indicator is on a different fuse. My guess on both - burned out bulbs. These can be pulled out from the back if you have small hands but if you are replacing the tach board, I would wait.
There is wire screwed down to the back and that goes in to the harness and is grounded near the top of the door pillar (in front of and a little below) the light switch. I don't see how you could check this without taking th LH dash out. I couldn't see it without doing this.
As far as removing the left dash. Here's a good thread about it.
http://forums.corvetteforum.com/show...t=dash+removal
Good luck. Let us know if you have any questions along the way.
I'm currently chasing down electrical gremlins in my 75 as well so I can be of a little help.
All the dash instruments light up except the emergency brake light and right turn signal. These two lights happen to be in the tachometer facing. Need to know what relationship these have to each other and what might the problem be.
Unfortunately I don't know how the tach is grounded, probably a strap to the under dash metal.
As far as removing the left dash. Here's a good thread about it.
http://forums.corvetteforum.com/show...t=dash+removal
Good luck. Let us know if you have any questions along the way.
Race Director
Joined: Apr 1999
Posts: 11,520
Likes: 13
From: CORVETTE 77 385 C.I. TEXAS
1 Have some faith in Chevy's electrical system. 10
(Do not have any faith in Jag's Lucas system.) 0 (1-10 scale)
2 Check for connections. Clean connections.
Tach - Check connection at dist. & wire.
Bulbs - Check for burned out.
(Do not have any faith in Jag's Lucas system.) 0 (1-10 scale)
2 Check for connections. Clean connections.
Tach - Check connection at dist. & wire.
Bulbs - Check for burned out.
