Tachometer repair
Thread Starter
Racer
Joined: Apr 2007
Posts: 443
Likes: 0
From: Rexton New Brunswick
Tachometer repair
Has anypone out there repaired their tach? I think the problem with mine is the circuit board. I am conflicted about whether to try it myself or have it done. How much of a pain is it to get the tach out?
Melting Slicks
Joined: May 2006
Posts: 2,590
Likes: 403
From: Nashville TN
St. Jude Donor '09
That's going to be my next project too. I've been avoiding it, but time to jump in. I was looking for a <$100 project and I really need to get it working to help with some other projects.
Several on CF have used this guy . . . interesting reading if nothing else tach board
You might also do an advanced search. . . there is a lot of good info about replacing/troubleshooting electronic tachs here. Good luck!
Several on CF have used this guy . . . interesting reading if nothing else tach board
You might also do an advanced search. . . there is a lot of good info about replacing/troubleshooting electronic tachs here. Good luck!
Race Director
Joined: May 2006
Posts: 16,528
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From: Dayton, Ohio
Melting Slicks
Joined: May 2006
Posts: 2,590
Likes: 403
From: Nashville TN
St. Jude Donor '09
I beleive they are two different filters. The one you are referring to filters EMI/RFI (?) from getting into the radio signal/power supply. The one he is talking about protects the tach from voltage spikes and is mounted on the intake manifold and connects between the distributor and the tach. The tach filter is not required for the tach to work, but damage to the tach may occur without. This filter is known to go bad and block the signal to the tach and he is suggesting a way to by-pass the filter for testing purposes to verify the tach itself is working.
Intermediate
Joined: Mar 2006
Posts: 28
Likes: 0
From: Cary NC
Can someone tell me where the tach filter is located? what does it look like, maybe a photo?
My tach (1976) is now pointing to 3000, it does not appear to move while driving but, at other times it reads 4000 or 3500. I'm wondering if it is the filter.
Thanks,
Mike
My tach (1976) is now pointing to 3000, it does not appear to move while driving but, at other times it reads 4000 or 3500. I'm wondering if it is the filter.
Thanks,
Mike
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Burning Brakes
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From: THE OLDER I GET THE BETTER I WAS! NORTHERN ONTARIO
George ROCKS!!!!!!!!!!!
Race Director
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From: Dayton, Ohio
Racer
Joined: Jul 2005
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From: Winnipeg Manitoba
I beleive they are two different filters. The one you are referring to filters EMI/RFI (?) from getting into the radio signal/power supply. The one he is talking about protects the tach from voltage spikes and is mounted on the intake manifold and connects between the distributor and the tach.
It is not needed to protect the Tach circuit from voltage spikes.
The Tach circuit board already has plenty of protection.
When its mounted on the manifold, its supposed to reduce noise going into the cab on the Tach signal line.
Race Director
Joined: May 2006
Posts: 16,528
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From: Dayton, Ohio
In the above wiring diagram there is a caution statement next to the tach wire -circuit 121. It says that grounding 121 will damage the HEI.
Racer
Joined: Jul 2005
Posts: 499
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From: Winnipeg Manitoba
One cap is on the red wire and the other is on the black.
Note that the red and black wire are actually the same wire,
Therefore the capacitors are actually in parallel, on the same wire,
but in two different locations. What's the point of that.....
The capacitors provide a local source of charge for the switching of the ignition coil.
When the electronic switch opens and closes, current will come from the caps
instead of coming from the pink wire which runs all the way back to the ignition switch.
RF noise is caused by switching currents in wires.
Since the pink wire runs through a good portion of the harness
and is right next to all the other wires, you don't want it carrying a switching current.
If you short the white wire to ground, you will have placed 12V directly across the coil.
This will probably burn it out. When it burns out, it could short.
Now the HEI is trying to switch the battery to ground.
I would be more concerned with HEI damage due to the white wire being shorted to 12V.
If that happens, the electronic switch is trying to switch the battery to ground.
Last edited by Kilroy1024; Dec 8, 2007 at 11:45 PM.
Team Owner
Joined: Jul 2006
Posts: 22,190
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From: Kannapolis NC
The capacitor labeled C100 in the diagram on the WHITE wire comes from the distributor and bolts to the intake near the driver side valve cover. That's the tach filter. If you want to bypass it, you have to unscrew the bolt holding it to the intake to remove the ground and then short around the filter. My 78 actually has two wires coming out of the distributor, one through the filter and one bypassing it. You can choose which wire to connect. If you don't unscrew and disconnect the mount, and if the filter is shorted, bypassing will have no effect. If the filter is open, the tach won't work, but bypassing it will make it work. Keep in mind that the tach board could be bad, or the connectors behind the tach board, or the tach movement itself could be sticking or slipping.
Last edited by 65GGvert; Dec 8, 2007 at 10:19 PM.
Racer
Joined: Jul 2005
Posts: 499
Likes: 1
From: Winnipeg Manitoba
The capacitor labeled C100 in the diagram on the WHITE wire comes from the distributor and bolts to the intake near the driver side valve cover. That's the tach filter. If you want to bypass it, you have to unscrew the bolt holding it to the intake to remove the ground and then short around the filter.
First off, C100 in the above diagram is a connector pin.
Probably at the firewall connector.
I would recommend that you DO NOT short either of the capacitors in the schematic above.
Please review the schematic! one side is labeled BAT, the other is ground!
Very bad things will happen if you bypass these capacitors!
The tach filter that you can short out is a different kind of filter used on earlier models.
It is a series filter that would appear on the WHITE wire in this drawing.
Team Owner
Joined: Jul 2006
Posts: 22,190
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From: Kannapolis NC
As I said before, disconnect the ground screw and then short around filter. I didn't say short 12v to ground. If you're still having trouble pm me and I'll send you a troubleshooting sheet that shows you exactly how to find out if it's the tach or filter or board.
