Hi,

I guess this topic has been already covered here, but since the search is not working, I couldn't find the right topics.

The RPM gauge has always been working on my 79 (well, since I got it last Autumn) although soon after I got the car it sometimes did a weird thing - when I switched engine off, the RPM gauge would slowly move to the very end of the scale and when I turned the ignition ON again, it didn't turn back to zero but it was clear that the mechanism wanted to push it towards that stop-end even stronger. Soon I figured out how to recover from that situation - I revved the engine to perhaps 4000 RPM momentarily which caused the needle to jump back to the proper position and the gauge began working again.

Two days ago I drove the car to work and a mate wanted me to give him a ride. We started up the car, RPM worked fine and suddenly while warming up, it just fell to zero and didn't work anymore. Revving the engine didn't fix it either, but it began working all by itself after having driven for some time. It also worked when I drove it home some hours later.

Since then, I've noticed two mornings that when I start up the car, nothing happens on the RPM indicator and it stays at zero. Yesterday I made it work again by revving the engine a bit. Maybe it will work today also but right now it's inoperative when I took the car out of the garage.

Could it be a loose connection somewhere? Where should I look? I don't know where the RPM sensor is on the engine.

P.S.

Today when the RPM gauge was not working, I stopped the car and noticed that the indicator moves a tiny bit near zero position when I turn the ignition on or off, so it's definately receiving power.

Thanks!!!

 

Tach is connected to HEI dist. There are 2 connections together- Tach & IG.

 

Thanks, I'm going to check that connection first. I also read from Corvette Action Center (where the search works ) that it could be a bad connection on the gauge panel. I installed a brand new printed circuit there myself so that can't be broken, but perhaps the plug is a bit misaligned. I'll try to reach it somehow under the dash.

 

I cleaned the plugs on the distributor and everything seems fine there... The distributor itself seems also pretty new. I don't know if it's stock, but it says "Delco-Remy" and underneath "Made in USA"

I took it for a test drive and after 10 minutes the speedo began working again. Before that, I revved the engine on stoplights and at one point the needle went to 7000 where it stayed and when I revved again, it came back to zero. It began working just randomly when I was cruising at 30 mph. Worked for the next 20 minutes until I arrived at home.

Doesn't seem like a bad connection near the gauge itself, does it? Perhaps it's the sensor on the distributor which needs to be rotated around for some time before it becomes loose. Any way to clean it or something?

 

I have no idea what I am talking about but until someone chimes in with some real knowledge here is a suggestion.

I have a '79 also. Suppose you disconnected the tach wire at the distributor and connected it to another tach? If the other tach works o.k. it would tell you it isn't the guage but the signal from the distributor somehow. Since I have a dwell/tach meter from the old days I use that tach when my head is stuck under the hood doing a tune.

The distributor near the coil is labeled "tach" at the wire that drives the tachometer.

 

I'd put my money on your tach circut board.

 

Hmm... Maybe it really is the circuit board although it's brand new. I remember that in a few places the connections on the board got slightly cut (like half or less) because those pins on the gauge block were so sharp that they simply cut into the board. However I believe I checked which ones were damaged and they were all cluster lights. Nothing got fully cut and all the lights work. Perhaps the RPM still got hit... I don't know why it would work only when the car has driven for some time, though.

I drove the car twice after I wrote the previous post tonight and the tach worked from start-up to shutdown each time. Although I normally leave the battery disconnected overnight (for some reason - I must quit doing this because I hate reprogramming the radio), I left it connected today and will see tomorrow morning if that might have had any effect on the tach. I read in some other posts that battery and the gauges are closely related and battery can cause problems to the gauges.

So, if everybody puts their money on the circuit board, I suppose I should pull out the gauge cluster and check the board / solder the wires if anything is broken?

 

Speedbird, I just posted this on another thread. . . I'm sorry it's to long to re-type but I hope it helps:

On your tach circuit, you will need to verify the following:

Make sure you have 12 volts to the tach

Make sure you verify the Tach ground

Make sure you verify continuity from the tach wire on the connector to the distributor.

Inline on our car there is a tach filter, this filter is held in place with one bolt to the intake manifold. If you do not have continuity from the connector to the distributor, un-plug the tach filter from the system. See if you have continuity from the dash connector to the plug where the filter plugged in. If you do have continuity to this connector then plug this connector to the “tach” connector on the distributor bypassing the tach filter. Now plug in the connector and see if it works. If this works inspect the filter to see if the wire is broke at the capacitor, if it is you can try to place a drop of solder in the capacitor to get the filter working again. This can be hit or miss. If the filter is bad we do carry a replacement style on the web site.

Testing without the filter will not hurt the tach in any way shape or form. If the tach works when you do this you need to replace or repair the filter.

If you have power at the tach and a verified ground with good continuity from the distributor to the tach and it is not working you have a board problem more than likely. If you want to test the tach by doing a hard wire test the help page listed below will tell you exactly how to do this. It is the same test we suggest you do before installing a tach with a new board in the car just to verify the tach is working properly.

I have a page on my web site that will help you in this repair: (and has the hard wire test). http://willcoxcorvette.com/repairand...lp.php?hID=252

I also have a page on the site on how to remove the tach from the car at this link:
http://willcoxcorvette.com/repairand...lp.php?hID=146

Should you need the tach board, I have them listed in the Forum Special section under this link and these boards are pre-tested and calibrated before we send them out. http://willcoxcorvette.com/advanced_...&keywords=tach If you enter the current sale discount code on check out, you will receive an additional discount on this item too. This sale code is DRWH3A and must be entered when checking out on the site.

On your printed circuit, the top wire on the tach side is your ground, the second wire down is the signal wire from the distributor and the third wire from the top is the power wire. Not this is if you were looking at the back of the tach with it sitting on our lap. So the wires in the car if you were facing the connector when un-plugged go to the passenger side top wire. This is the tach ground. The passenger side wire second from the top is the square signal wire from the distributor, and the third one down on the passenger side of the plug is the 12 volts to the tach.

I hope this make sense. . .

Now pull the tach, test your wires and if this is not enough info either post again or email me direct at Willcoxcustomerservice@willcoxcorvette.c om.

Willcox Inc.

Here is the printed circuit board for the tach and speedo showing the wires above.

 

Thanks so much, Willcox! I'll get to it soon and I'll keep you all updated.

 

Alright, yesterday I removed the gauges block from the dash and I had a feeling that the black plug to the gauges was a bit loose. I don't know this for sure, though. I checked the wires for continuity and they passed the test. I also started the engine and once running, all the 3 RPM contacts on the plug were loaded at roughly 14 Volts.

There is one thing on the circuit which I don't consider an issue myself, but I'm no expert. I drew an addition to the circuit image:



As you know, there are 3 metal "clamps" going into the gauge block and they connect to the printed circuit with two hands on both sides of the clamp. When I replaced the printed circuit, the clamps were so tight on the circuit that they cut into the circuit in the areas which I marked with orange color. They didn't cut the wire completely through, but almost.

Could this have anything to do with it? The reason I don't consider it the problem is that even if the circuit wire is cut from there, the metal clamp is still making it a closed circuit.

The RPM gauge worked right after I assembled the dash back together, but this morning it was again inoperative.

When I drove it around yesterday, each time I shut down the engine the RPM needle began moving towards the end of the gauge while the engine was "spooling down". Now it seems to be the standard behaviour that once the gauge is working and I shut down the engine, it gets a signal to move to the very end between the period when I have turned ignition off and the engine is still rotating. When it's idling normally at 700 RPM, the gauge doesn't have much time to move too far, but when it's at cold idle above 1000 RPM, the engine rotates so long after turning ignition off that the gauge has plenty of time to get itself stuck at the stop-end.

 

Anybody?

 

I have the same problem...Almost.
Just recently purchased a 1979 Corvette ,and noticed during the test drive , as soon as the ignition is on,the Tach jumps to 7000 at stays there..pegged.After starting the engine no change.Bought it anyway.
At home I disconnected the wire from the Tach connection at the HEI.Now when I turn the ignition on the tach will read whatever it feels like.With the engine running no change.Turn the ignition off and it drops back to zero.
Would really like to fix this cause although I could mount a aftermarket gauge somewhere....if there was room

 

"ll the 3 RPM contacts on the plug were loaded at roughly 14 Volts."

I'm confused by the statement that all three were roughly 14 volts. Only the power clip should have power on it.




The tach circuit clips should not cut the circuit board. They have nubs on them to put pressure on it and hold in place.



You said the pinch connector for the center was loose? That could be your issue and this is why it's best to leave the housing in the dash when testing the three clips. If you have an issue it will show up on the clips.

You stated you have continuity to the distributor on the signal wire.. test it with the housing in the car.

Make positive sure you do not have voltage on the ground or the signal wire clips....

Willcox

 

As usual, great info Willcox.
One of these days I have to buy a tach-board from you

 

On my '76 when I started it up and was returning from the Richmond Corvette Club show yesterday morning which was nice to see I noticed while driving on I95 south that the tach was reading zero. When I shut it off at home it went up to 500 rpms and stayed then off when I turned the engine on. I read off one of the search threads that I must first check the connector on the distributor and will do that today now that the engine is nice and cool. If it turns out to be the tach circuit board I may just go ahead and put a console mounted more modern tach in on the center console as my radio isn't there anymore, just blocked off. I don't push my car hard enough to red line it anyway and could drive it without the tach at all but like seeing it working one way or another. I'm tall enough that seeing the tach and speedo on mine when driving is not a direct line of sight anyway.

Just when you think your car is operating perfectly, zing...another thing fails. I guess at 35 years old it's bound to happen and is likely to keep happening.

lance

 

just because it is new does not mean it's good.

 

I've never liked the Vette tach on mine anyway. The thing tended to jump under certain conditions so when it stopped I thought about it, don't fully care about original restorations and have a new pedastal mount 3 3/4" tach due here tomorrow that is going to be installed Friday. I'll keep the original one in, unhook the signal wire to it, but I want one that works better than what was originally in the car and since my radio is mounted in the back shelf area on a mount I have that center lower instrument area blocked off and will mount it down on the console, tilted and angled up to me. I don't push the car hard so this will suit me just fine and this one has settable shift lights in it. maybe on the wheel column but I think the room is too small there with the tilt mechanism. We will see.

Lance