I'm figuring it's probably just the guage being a lil bastid on me but I want to see if anyone has any other ideas on it.

So my tach acts very odd. Some days it works perfectly fine and is very accurate. Other days the needle is all over the place or will be sitting down at like zero and if it blip the throttle it'll bounce a little and go back down. This morning for example, while it was warming up I gave the throttle a good shove and the tach went to 2000 and bounced down to zero and around there. Sometimes the tach just doesn't want to go over 2000. It'll go from 500 to 1800 just fine but over that and it acts all wierd on me.

Distributor is a Procomp 50k volt HEI,for any that are curious on it. Something similar happened to it once a few months ago but it went away and it only recently came back like last month on and off. It's still very intermittent because it'll do it for a little while then start working fine. Engine runs fine, no missing or anything so I'm thinking the guage is just being a dork.

 

Kalway, what year is the car?

Willcox

 

Assuming all connections are good/solid, then it sounds like it could be the tach board itself. If it is these can either be replaced w/an aftermarket replacement or many times the original board can be repaired for approx 1/2 the cost.

Please IM me if you need more info on repairing the tach board.

Good Luck!

 

his profile says "1981"

 

Yup it's an '81, with no mo 'puter. Basically the description in my profile for mods would tell ya what's on it.

I'm thinking it's probably the tach board, too. However I'm sure I have a few shorts throughout the rats nest of wires in the car, too. I fixed one the other day that resulted in my check engine light coming back on. However, the issue has been sort of intermittent. During lunch it was acting up, but on my way home it worked fine. Go figure.

 

Probably not a short but a grounding issue with the board.

 

There is a ton if info archived on this forum in regards to the electronic tach for the 1975-1982 cars. You might want to just run a search. Or send me a email direct and I'll send you this file. willcox@willcoxcorvette.com. I copied what I found a few years ago and have it in a series of email files. It’s a large file, but it’s pretty complete.

But, I think I would check the connection to the coil on the distributor first, then I would go to the tach filter. The tach filter looks like a capacitor and bolts to the intake manifold. With age, the wires that run in and out of the tach filter can become worn and loose inside. Intake gasket changes, Distributor changes and just moving the wires around can cause them to break loose. If they are still intact, you can put a tiny drop of solder inside where the wire hooks to the filter.

You can bypass the filter, (it’s purpose is to prevent voltage spikes and false signal). And plug the tach wire in direct to the distributor for a quick test. But in doing this you also run the risk of a voltage spike and taking out the tach completely. I doubt it will happen, but just so you know….

What I do know is that if you have a needle that moves at all the main suggestion seems to be a loose wire or connection point.

 

Excellent information there willcox, thanks a ton!

I'll check it out tomorrow morning and see what I find. I have a strong feeling that you're most likely right. I'll e-mail ya if I don't figure it out.

 

I can vouch for ACECO(George"s) tach board work.
he redid mine and it has worked flawlessly since.

 

I don’t know who the guy was that posted this info, but he defiantly did his home work. I combined what he posted with what I knew.. Printed it out and sent it to the shop for a manual on how to on these tach’s. It’s bailed us out more than once on trying to figure them out.

 

Looks like that filter is probably the culprit. How do I replace it and where do I get one?

 

Hummmmm http://willcoxcorvette.com/product_i...ducts_id=24001

But before you purchase this, are you sure that a drop of solder won't fix the problem... I'd try this first and then look for a replacement.

Willcox

 

Before I try soldering it, how do I get it off to solder it?

 

The filter should be grounded to the block via it's bolt on the the intake manifold. So, check that the ground is good there, then also check that the wires into and out of the filter are both good. If you have a broken wire going into the filter, it's not too late to actually re-solder the thing, as someone said above, provided there's enough bare wire sticking out of the filter. Each end of the filter is sealed up with a blob of epoxy, and the wire normally breaks off flush with this - so, if you unwind the tape at the end of the filter and "pop" the blob of epoxy off with a screwdriver, you should end up with enough wire sticking out to solder onto. Make sure the wire's very well cleaned, and use plenty of flux to make sure the solder actually takes. Then, once you've soldered a new wire on, you can wrap another bit of tape around the end and re-fill the thing with some fresh epoxy. It's a bit of a mission, but I did this at a time before they were making the repro ones, so it was worth while. But if you don't fancy the hassle, you can buy a repro one from somewhere like Ecklers. There's an internal circuit diagram somewhere on the forum for these things, and I'd suggest you first check the continuity of the filter with a ohmmeter before spending any time fixing the wires. It should have a resistance from end to end of about 16Kohms.

Good luck!

 

Just to make sure that I'm fully sure it's the filter.

Lately the tach pretty much doesn't want to work at all. Last night it teased me in to thinking it might be working for a second but it would only go up to 1k RPM then go back down after that. This morning, it just sat at zero and didn't really move much. When I turn the car off it resets itself back to like 500, I believe.

Filter doing this, correct?

 

The filter is there only to suppress electrical spikes, etc. Usually they are either working OK or open. Now it could be an intermittant connection at the filter doing what you describe, but an intermittant could also be elsewhere too. I'd suggest as Wilcox indicated above to bypass the filter for a quick test. If the tach starts working then it's likely the filter. If nothing changes, then the issue is probably somewhere else.

However, don't just leave the filter bypassed if it does work. Others will tell you it isn't needed but GM put it there for a reason. Over time the spiking can damage the tach board itself if the filter is eliminated. FWIW.

Good Luck!

 

I was going to try that but where do I unplug it? All I see is a plug in the distributor for the tach that goes in to the filter and out and then goes to maybe another plug and back in to the firewall. How would I bypass it without snipping wires?

 

Just unplug the plug on the firewall side of the filter and plug it directly into the tach terminal on the distributor cap. The two plug styles should be identical, if it's stock wiring...

 

So I plugged the tach straight in to the distributor. Needle just sits at zero still. Figures, tach board must be dead.

 

Probably so. Could be the wire between the filter and the tach but this is unlikely. (but possible) Too bad but it sounds like it's time for either a possible board repair or repro replacement.

The only other thing I might suggest is to try hooking up an aftermarket tach right at the point where the wire connects to the factory tach. If the aftermarket works then it's probably the board. If not then perhaps wiring is the issue.

Good Luck w/your continued troubleshooting.
PM me if I can provide any additional help w/your board.