jyounane

Hi,

I have a 75/76 tach in my 69 with HEI. It has a home made tach filter installed. This has just been installed to replace the mechanical tach.

Problem is... it fluctuates wildly from zero to full scale while cranking but settles instantly and reads correctly once engine starts.

Where should I start looking? This doesn't seem normal. A search on the forum hasn't got me anywhere.

TIA and
Joe

Big2Bird

From that symptom, my best guess is the 12volt feeding the gauge is dropping during cranking. Put a VOM on that lead while cranking.

jyounane

Thanks. I'll check that out. I'm pretty sure it's connected into the "pink" ignition circuit.

Joe

0Willcox Corvette

Your car will drop while cranking..

Where is your 12 volt power going to the tach coming from?

The Home Made filter is interesting... The filter is designed to knock the edge off the square signal coming in from the distributor. Oddly enough though, the chip on the factory board and on our reproduction board can read either the square or rounded signal... so I doubt this is your issue.

Willcox

jyounane

Thanks Willcox.

I'll check the voltage source while cranking.

The "home made" tach filter design is fairly standard...





Joe

Kully

0Willcox Corvette

Well that makes no sense..

The purpose of the tach filter is to knock the edge off the square signal from the distributor before it hits the tach.... and prevent voltage spikes from hitting the board.

The tach board (even the original GM board) is designed to read both the square and rounded signal so removing the filter for testing shouldn't hurt the tach board.

However if there is another issue such as a defective coil in the distributor, or a defective analog movement then you might have an issue... Our boards have a fuse built into them so if the fuse is blown on the board then there is something else wrong with the car. The best thing you can do at this point is send me the tach and the board and I can test them. If the fuse is blown it's 14.00 to fix... if the movement is bad I can tell you in about 30 minutes. So if you are interested in this email me at service@willcoxcorvette.com.

Willcox

Bills75

Willcox Corvette, I just purchased a 75 coupe and the Tach does not work. So if I am reading this correctly, if I replace the old Tach board with a new Tach board from Willcox, I dont need to hook up the old filter?

Just want to make sure that I don't need to purchase a filter also.

Thanks,

Bill

0Willcox Corvette

I would still run the filter, but you bypass it for testing.

Willcox

Bills75

So is there a way to pin point if it is a filter issue or board issue?

Thxs

0Willcox Corvette

Well I'd start with bypassing the filter... Unplug the filter and plug up the tach direct. Both our boards and the GM boards are capable of reading both a square and rounded signal so if doing this for testing you will not hurt the tach, just eliminate and narrow down the issue. I've said it 100 times... as these gauges and our cars age, analog movement failures are becoming more of an issue.

We had a fella send his tach in the other day, he purchased a China made board with washers, he had the original board and then he purchased our board... nothing made the tach work... turns out the analog movement was the issue and when this happens the only fix is ti replace the tach. Unfortunately the only reproduction tach is made over-sea's and this tach has multiple commons unlike the original... so we tweaked our boards (because eventually these will fail too) so that we could fix either the original tach's or the imported ones. Original boards used sine and co-sine with a similar common, repro's don't. Reproduction boards imported are not fused nor are the new replacement tachometers. All our boards are fused, even the low fuel warning modules as well as the delay timers. This prevents damage to the board, it prevents damage to the car and it makes for something that can be repaired cheaply if there is any issue.

If you have replaced your tach board and the tach isn't working my suggestion is to go to our tech page first and try to figure the issue out.. and if you can't send us the tach and board and we'll put it on our test machine to see where the issue is.

Willcox.

Bills75

Thanks, Ill go this route to try and isolate which part is bad.

Bill