Tacho not reading

[Challenger_7]

Hello everyone, hope you all are well and safe
I'm new on this forum and this is my first post.
I just concluded a full ground up restoration on my 74 C3 (Manual 454) Stingray which took a good many years to finish.
But having seen the final outcome and felt the power and smooth ride, feel it was worth the trouble and money

Now that I have the car back, I am looking at the few hitches that still need sorting out.
The first is the Tachometer. Mine had a mechanical one and I changed over to electronic, including the distributor and all needed items.
When I first started the car at the mechanic's shop a few weeks back, the Tachometer was reading but at least some 1500 (estimated) rpm less.
The car still had to undergo some repairs in the meantime, but now that I have taken delivery, ithe tacho t is not working anymore.
With the ignition off, the needle is at Zero. When I switch on, it slides backwards and stops at the 7000 rpm, and when I start the engine, nothing happens.
The needle stays at the 7000 rpm and when I switch off, it slides forward again to sit on the Zero.
The only thing I notice is that as soon as the engine picks up when starting, the needle shoots forward to approx 1000 rpm for a split of a second before resting at the 7000.
All the wires, as is the case with every item on the car, are brand new from Zip Corvette.
I checked around for any possible loose connections but found none.
I will also add to this, but not sure if related, that the oil pressure and water temp gauges either were not working or stopped working on the instrument panel.
The fuel gauge and clock are working perfectly.

Any advice as to where to look at?
Thank you for any suggestions.

Cheers

 

[gdh]

If the mechanic installed it, take the car back and have him fix it properly.

 

[nascar03]

I agree with @gdh above..... have the mechanic fix it.
The experts in this field seem to be WILLCOX CORVETTES?

http://repairs.willcoxcorvette.com/wp-content/uploads/2015/03/corvette-tachometer-testing-removal-board-installation-video-link-1978-1982.pdf

Have him/you look at their site for assistance.
good luck

graham

 

[Rruuff Day]

I agree with @gdh above..... have the mechanic fix it.
The experts in this field seem to be WILLCOX CORVETTES?

http://repairs.willcoxcorvette.com/wp-content/uploads/2015/03/corvette-tachometer-testing-removal-board-installation-video-link-1978-1982.pdf

Have him/you look at their site for assistance.
good luck

graham

Link didn't work for me Graham.

 

[Challenger_7]

Thank you both for pitching in.
The mechanic is not an electrician, and he installed all the new harnesses in accordance with the wiring diagrams and a bit of guidance from my side too. One can't go much wrong as everything is colour coded with clips that fit in with its counterpart with very little chance of errors.
I am seeking the help of an old timer electrician who might be able to help and I'm sure we will get to the bottom of it, but needed some guidance in case anyone experienced this before.
Graham, thanks for the link to Wilcox Corvettes, I will check it out but on a quick glance the gauges there are of a newer model.
Nevertheless, will go through and see if there are any similarities or clues that might help.

Much obliged
Farouk

 

[smokindave]

Hello everyone, hope you all are well and safe
I'm new on this forum and this is my first post.
I just concluded a full ground up restoration on my 74 C3 (Manual 454) Stingray which took a good many years to finish.
But having seen the final outcome and felt the power and smooth ride, feel it was worth the trouble and money

Now that I have the car back, I am looking at the few hitches that still need sorting out.
The first is the Tachometer. Mine had a mechanical one and I changed over to electronic, including the distributor and all needed items.
When I first started the car at the mechanic's shop a few weeks back, the Tachometer was reading but at least some 1500 (estimated) rpm less.
The car still had to undergo some repairs in the meantime, but now that I have taken delivery, ithe tacho t is not working anymore.
With the ignition off, the needle is at Zero. When I switch on, it slides backwards and stops at the 7000 rpm, and when I start the engine, nothing happens.
The needle stays at the 7000 rpm and when I switch off, it slides forward again to sit on the Zero.
The only thing I notice is that as soon as the engine picks up when starting, the needle shoots forward to approx 1000 rpm for a split of a second before resting at the 7000.
All the wires, as is the case with every item on the car, are brand new from Zip Corvette.
I checked around for any possible loose connections but found none.
I will also add to this, but not sure if related, that the oil pressure and water temp gauges either were not working or stopped working on the instrument panel.
The fuel gauge and clock are working perfectly.

Any advice as to where to look at?
Thank you for any suggestions.

Cheers

I would start by checking your ground wires.
Too me it sounds like a ground short.
A good way to check is start with a new ground wire to a known ground and start by grounding engine and then check your other ground wires to that known ground.
A dirty ground will give you symptoms you are having.

 

[Hanko_]

I'll agree with smokindave; I'd be inclined to connect a long (8ft) wire to battery negative, to give you a definite reference. (A thin wire is ok). Bring the other end into the cabin and connect it to the (-) lead of a digital voltmeter, set to read DC volts. Take the (+) voltmeter lead under the dash and read different points. The black ground wires should be daisy chain connected to all gauges and read 0.0 volts. Any voltage reading means you have a poor connection somewhere. Now also read the (+) at the battery, will be ~13.8 volts. Go under the dash again and check the non-ground side of the gauges. Most gauges will read a variable resistance through the sender to ground. e.g., if the gas tank is ½ full, the sender will read ~7 volts. Your oil press and water temp will operate in the same fashion. Gauges are actually voltmeters with a scale printed to a specific use. I expect you'll find that the 3 faults are related to each other.
FWIW, I am a proverbial old timer electrician, (licensed in '72)

 

[dodosmike]

Willcoz sells new circuit boards for tachs. Ernie is the guys name, posts a fair bit on the "other corvettefourm".
His repair and troubleshooting guides are available on the website, although can be problematic to find sometimes.

1974-1977 Corvette Tach Board Connections | Willcox Corvette, Inc.

Tach Board Connections 74-77

repairs.willcoxcorvette.com

 

[Challenger_7]

Thank you all for the advice which sounds very helpful.
Hanko Many thanks for the step by step way of doing things. That is what I was hoping for and will give it a shot myself for now as the expert old timer I referred to is currently unavailable for a few days, and you seem to fit the qualification well better
At this point I do not believe I need a new board as the Tacho is brand new (photo attached). In fact everything in the car is.
Anyway will do the step by step way suggested by Hanko and will take it from there.