[sleepr]

The tach in my T/A doesn't work. The kid who had it before me said he replaced the distrinutor. I had the gauge bezel (if that's what it is called) out today and replaed some bulbs along with cleaning some of the contacts (got the water temp gauge working). BUt it didn't help the tach.

What should I start looking at to troubleshoot this?

 

[Keith Boisvert]

If he replaced the distributor and it's an HEI unit, the tach should be plugged into the side of it(it's marked "tach").
If it is, start checking your power and grounds. Usually it's just something simple, especially if it worked before he worked on it. You can also do a search on tach wiring in the electrical section of this site and you'll find plenty of diagrams and troubleshooting tips.

 

[docvette]

Tach

Doc hereimp:

Watch your Tach when you first turn the key on, Does it "Jump up " Slightly? This is a good indicator that power and ground are working properly. If not, look for a fuse...

If it does, check the wire going from the Tach input to the HEI terminal marked "TACH" for continuity.

If you have a "TACH" filter on the Manifold (Corvettes do) Then check that, It looks like an old Cap on a points distributor.

If you have one, and it's dead, don't bypass it!! you will damage the computer as well as the tach..It's function is a wave shaper on the high tension pulse that interfaces with the two. You may not have one at all...I don't know if T/A 's used them.

If all is good, and it still won't work, and you have access to a known good tach, Try that clipped into the existing wires, If it works, you will need a Tach Calibration board (usually dealer item) for your stock tach...Unless your handy with Electronics and a solder gun, then you can replace the LM 2917 VCO Chip on the board. Actually, the most common suspect is the 16 to 24 volt zenier Diode on the board (reverse polarity protection) it usually goes first from mis wires. So if your up to it, get both and have Fun.

Doc imp:

PHOTO:
Typical Tach Schematic

 

[sleepr]

Hey Doc,

I checked the today and the tach wire is hot from the distributor. It's a 78 T/A, and there is no cap/in line fuse. Although there is a wire spliced in about 8-12" in length. What should be my next trouble shooting step be?

 

[docvette]

tach

Doc hereimp:

This is a stock Cluster tach , right?

If so, pull the instrument cluster and look at where the terminals bolt into the PC board, try and locate the power, ground terminal and the high tension lead.

Using a DMM set for 12 volt or higher scale, check the power terminal, with power carefully on, for 12 volts. If it's missing, check your connections, or an instrument regulator, (if so equipped)

Next, with power off , set your DMM for Rx1 scale and check your ground for a hard ground. It should read 0.00 on the scale.If not or higher, you have a bad ground. Repair the ground circuit from the card.

Last, run a long jumper from the distributor, to your meter at the cluster and with your DMM set for Rx1 measure it...it should read 0.00, If not , check those splices, they are probably suspect, cut at the highest one, and check again, if you get 0.00 then replace the wire from that point out using properly installed splices , or a new wire..

If it's all good, and the Tach still won't run, then it's shot. If you can find a Tach repair/Calibration board, then order one. Or you may want to go to the Bone yard and get one out of another car..It'll save you a lot of money.

Doc imp:

 

[sleepr]

Thanks, Doc. I'll let you know what I find.