Testing a Tach.
Le Mans Master
Joined: Aug 2000
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From: Semper ubi, sub ubi
Cruise-In VI Veteran
Cruise-In VII Veteran
Re: Testing a Tach. (e3pres)
You should be able to test it by just wiring it directly to your HEI distributor's tach port. If it is a stock 'vette electronic tach, there are three terminals that go to the tach itself. One is a ground, one is 12v and the other is the signal line from the tach port on the distributor.
There is supposed to be a tach-filter that goes in the signal line, it is a simple band pass filter that is used to smooth out the signal from the distributor. Your tach will probably work without a tach filter, but it will probably jump around and not act exactly right, but you should be able to see if it can move or not.
If it is already in the dash and just quit working then there a couple of possibilities for failure. The first one is that the signal line is broken or not connected to the tach port on the distributor, also the tachfilter might be dead, this is usually caused by one of the two leads breaking off, due to age. The filter is mounted on the back of the manifold, so engine heat can cause the plastic coating on the wires to become brittle and break.
The second possibility is that the circuit board itself has failed. If everything leading to the tach is intact then the circuit board is probably fried. Zip sells a replacement board that has more modern components and fits right into the old tach case.
Go to http://www.corvettefaq.com and look for articles on repair the tach, and making your own tach filter. The tachfilter is easy to build from parts at Radio Shack.
Good Luck.
PS a signal generator can be used to test the tach also.
There is supposed to be a tach-filter that goes in the signal line, it is a simple band pass filter that is used to smooth out the signal from the distributor. Your tach will probably work without a tach filter, but it will probably jump around and not act exactly right, but you should be able to see if it can move or not.
If it is already in the dash and just quit working then there a couple of possibilities for failure. The first one is that the signal line is broken or not connected to the tach port on the distributor, also the tachfilter might be dead, this is usually caused by one of the two leads breaking off, due to age. The filter is mounted on the back of the manifold, so engine heat can cause the plastic coating on the wires to become brittle and break.
The second possibility is that the circuit board itself has failed. If everything leading to the tach is intact then the circuit board is probably fried. Zip sells a replacement board that has more modern components and fits right into the old tach case.
Go to http://www.corvettefaq.com and look for articles on repair the tach, and making your own tach filter. The tachfilter is easy to build from parts at Radio Shack.
Good Luck.
PS a signal generator can be used to test the tach also.
Le Mans Master
Joined: Oct 2002
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From: Las Vegas - Just stop perpetuating myths please.
Re: Testing a Tach. (Smokehouse69)
Hello Smokehouse,
While on the subject, my mechanical tach. (on my '74) has a lot of lag. When compared to an electric tach., it's off by a few hundred RPM (low) at the low end and over a thousand RPM (low) by 5,000 RPM. Is this the norm or is my car just goofy? Any cures? I had always heard mechanical tachs. were more reliable but now A'm disapointed and looking at electronic replacements. Any suggestions? :confused:
While on the subject, my mechanical tach. (on my '74) has a lot of lag. When compared to an electric tach., it's off by a few hundred RPM (low) at the low end and over a thousand RPM (low) by 5,000 RPM. Is this the norm or is my car just goofy? Any cures? I had always heard mechanical tachs. were more reliable but now A'm disapointed and looking at electronic replacements. Any suggestions? :confused:
