replaced tach, still doing same thing
- Thread starter azmusclecar
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Borrowed from Nasty Z/28 forum:
The tach filter is exactly what its name indicates. It filters the high frequency interference generated by the solid state HEI so the tach sees only the firing pulses and not the interference. You can run without it, and if your HEI is elecrtically clean enough, you won't notice the difference. If you have electrically sloppy parts in the HEI, the interference they generate might cause the tach reading to fluctuate, but it still should be usable.
You should have continuity through the two wires of the filter, but not from either wire to ground.
This is a schematic of a GM tach filter used in pickup trucks. It is similar to the Camaro filter. GM was generous enough to put the schematic in the truck service manuals, but I haven't yet found it in the Camaro manuals to compare it.
![[IMG]](https://cdn-0.gbodyforum.com/proxy.php?image=http%3A%2F%2Fdsl00271.btigate.com%2F%7Egary%2Ftachfilter.jpg&hash=e970bab6703e22409fdb8c3ed15bc32f "[IMG]")
As you can see from the schematic, you should be reading some continuity through the input and output wires in the range of 15k ohms. It should show pretty much infinity reading to ground from either wire if it is disconnected from the system. The Camaro filter resistance readings might vary.
The tach filter is exactly what its name indicates. It filters the high frequency interference generated by the solid state HEI so the tach sees only the firing pulses and not the interference. You can run without it, and if your HEI is elecrtically clean enough, you won't notice the difference. If you have electrically sloppy parts in the HEI, the interference they generate might cause the tach reading to fluctuate, but it still should be usable.
You should have continuity through the two wires of the filter, but not from either wire to ground.
This is a schematic of a GM tach filter used in pickup trucks. It is similar to the Camaro filter. GM was generous enough to put the schematic in the truck service manuals, but I haven't yet found it in the Camaro manuals to compare it.
As you can see from the schematic, you should be reading some continuity through the input and output wires in the range of 15k ohms. It should show pretty much infinity reading to ground from either wire if it is disconnected from the system. The Camaro filter resistance readings might vary.
Copied this from the C3 Vette Registry Forum:
Tach filter is mounted on the intake manifold very close to the distributor, visible from the driver's side. 1975 is the first year with the electric tach due to the switch to HEI that year and thus the tach filter. You can check to see if the tach filter is the culprit by bypassing it. Unplug the tach filter from the distributor, unplug the brown wire coming through the firewall from the tach to the tach filter, plug the brown wire from the tach directly into the distributor. If the tach works properly, you have a bad filter. You won't hurt the tach by bypassing the filter. It is only there to filter out electical noise from the circuit which can give wonky tach readings.
Tach filter is mounted on the intake manifold very close to the distributor, visible from the driver's side. 1975 is the first year with the electric tach due to the switch to HEI that year and thus the tach filter. You can check to see if the tach filter is the culprit by bypassing it. Unplug the tach filter from the distributor, unplug the brown wire coming through the firewall from the tach to the tach filter, plug the brown wire from the tach directly into the distributor. If the tach works properly, you have a bad filter. You won't hurt the tach by bypassing the filter. It is only there to filter out electical noise from the circuit which can give wonky tach readings.
Well.......i plugged the tach wire directly from the distributor outlet to the tach wire going through the firewall bypassing the
filter and the tach STILL reads about 500 high at idle and gradually increases the error as the speed increases.
I pulled the tach filter off the firewall cleaned up all contacts and nothing changes. I'm at a loss..........
Thanks to all that posted ideas and directions.
filter and the tach STILL reads about 500 high at idle and gradually increases the error as the speed increases.
I pulled the tach filter off the firewall cleaned up all contacts and nothing changes. I'm at a loss..........
Thanks to all that posted ideas and directions.
I'll toss this one in, do you have the correct coil for the engine? Your coil should have a white wire for the tach lead, if it's yellow you got the wrong coil, Pull the coil cover off & check. Might be the cause, might not but can't hurt to rule it out. Here's a old thread that might help with the coil.
gbodyforum.com
Ignition coil differences
I'm wondering what the difference is between HEI ignition coils. My Pontiac had a stock coil which I think had red and white wires in it. I've seen the coils sold in stores and there were always two choices of red/white or yellow/white wires depending on what your car came with. What is the...
gbodyforum.com
I'll toss this one in, do you have the correct coil for the engine? Your coil should have a white wire for the tach lead, if it's yellow you got the wrong coil, Pull the coil cover off & check. Might be the cause, might not but can't hurt to rule it out. Here's a old thread that might help with the coil.
Ignition coil differences
I'm wondering what the difference is between HEI ignition coils. My Pontiac had a stock coil which I think had red and white wires in it. I've seen the coils sold in stores and there were always two choices of red/white or yellow/white wires depending on what your car came with. What is the...
I'll pop the top off today and see what lies underneath. Can't hurt......
Odd how I find joy and a sense of accomplishment when I am able to fix thinsg like this, but OH THE RABBIT HOLES you go down
trying to find the answer.
I hope a DIY celebration lies somewhere ahead here so I can move on to other challenges.
Copied from the link to the Gbodies ignition coil posted in #17 by Pagrunt
COIL IDENTIFICATION
"There are two types of coils that are integral with the distributors. They can be identified by the different color tach leads and the part number stamped on the coil.
The coil with the yellow tach lead (#1875894) is used on all Cadillac (except Seville), Chevrolet and Oldsmobile Toronado engines.
The coil with the white tach lead (#1876209) is used on all Buick, Oldsmobile (except Toronado), Pontiac and Cadillac Seville V-8 engines and all V-6 engines.
These two coils are very similar in appearance, perform identically, can be installed in any integral coil distributor, but are of opposite magnetic polarity. This was done to match the high tension coil to the magnetic pickup coil. The misapplication of these coils can cause hard or no starting and/or backfiring while cranking."
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Now look at the pic......do I have the wrong coil???????? Yellow wire? Not white??????
COIL IDENTIFICATION
"There are two types of coils that are integral with the distributors. They can be identified by the different color tach leads and the part number stamped on the coil.
The coil with the yellow tach lead (#1875894) is used on all Cadillac (except Seville), Chevrolet and Oldsmobile Toronado engines.
The coil with the white tach lead (#1876209) is used on all Buick, Oldsmobile (except Toronado), Pontiac and Cadillac Seville V-8 engines and all V-6 engines.
These two coils are very similar in appearance, perform identically, can be installed in any integral coil distributor, but are of opposite magnetic polarity. This was done to match the high tension coil to the magnetic pickup coil. The misapplication of these coils can cause hard or no starting and/or backfiring while cranking."
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Now look at the pic......do I have the wrong coil???????? Yellow wire? Not white??????
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