Odd tachometer problem - any ideas?

[joeydurango]

My 72 K10 has a factory tach (albeit one that was added later, as it isn't on the SPID). I'm running HEI (so was the PO, but I dropped in a new distributor about 9 months ago).

Most of the time the tach works perfectly, and is actually accurate (I've checked it against my timing light across a wide range). Sometimes, however, I'll start the truck up and the tach will do nothing at all - it will just be dead. More oddly, sometimes it will react to engine RPM, but only up to the highest recent RPM, stay pegged there, and slowly bleed back down over the next several minutes. (Or, it will stay at highway RPM when I turn it off, then drop back if I cycle the key.) Even more oddly, from either dysfunctional state, one of two things will happen: 1) I'll turn the truck off, and the next time I fire it up it will be back to normal, or 2) I'll be driving along and suddenly the tach will just spring into action. It is the strangest thing and is slowly driving me nuts.

I have checked the connections on the back of the tach and at the distributor, and they appear to be fine. I have even checked them while the truck is running with the tach not working (unplug and replug at the distributor, fiddle with the fitting on the gauge), to no avail. The tach just seems to have a mind of its own.

I suppose I don't have a thorough understanding of how it works, beyond receiving impulses from the distributor. For example, I know my speedo gets "bouncy" when it's cold out due to extra friction in the cable; but I'm not certain of potential causes for the tach issue. Anyone seen this before, or have any ideas? Thanks!

[davepl]

Just a stock HEI unit being the only change? No ignition box or tach adapters or geegaws inline?

Then I think you are looking at a defective tach. Could be as mundane as it binding on the face or shaft, but more likely some electronic part has failed in the "return to 0" part.

[HO455]

All the symptoms of a bad solder join inside or a bad connection/broken trace on the printed circuit component on the back of the guage panel.

[joeydurango]

I was afraid someone would say that. Was hoping for some "Oh, check this fitting, it may be loose", but I figured that would be too good to be true.

Dave, nope, nothing extra. Just HEI > brown wire > tach.

Drove it today and it was dead for the first 10 minutes. Then it sprang to life. Sleeping on the job!

[FlounderGDI]

It might be a grounding issue. There are 3 lugs on the back of the Tach... Coil, 12V+, and one not labeled... To get mine to work I had to run a ground to the third lug because mine wasn't grounding through the gauge cluster properly.

[joeydurango]

Bringing this one back, one year later. Still driving my truck. Tach still acts funny. But over a year's worth of driving, I've learned a few things about it. The following are true:

It appears to be temperature dependent (although I don't know where that temperature matters, more later):

-This spring, when it wasn't 70+ degrees in the garage all the time, the tach would be dead when I fired the truck up. After 5-10 minutes of driving, it comes partially to life (pegging at highest-reached RPM and not dropping), and then after another 5 minutes or so, it begins working normally and accurately.

-During the summer, when it was quite warm in the garage overnight, the tach began working normally as soon as the truck was started.

-Now that we're back to fall and cooler temps, it's doing the exact same thing as described from the spring, above.

-Finally, in any of the above scenarios, it always works if you shut the truck down and come back to start it while it's still warm. If you let it sit for say, an hour, it will work. Let it sit for several hours and if it's not really warm outside, the tach won't work until the truck warms up again.

I haven't opened the heater vents at all, so it seems to be tied to engine warmth (maybe even ambient under-hood warmth?), not in-cab warmth.

It's a puzzler. Is anyone aware of any reason any electrical connection or ground might depend on reaching a certain temp? I don't get it.

[paul blair]

Did You ground the TACH as FLOUNDERGDI mentioned!

[joeydurango]

Thanks for the input... but obviously it's grounded, or it wouldn't work at all! Perhaps it's intermittently grounded, but I'm failing to see how temperature could affect the ground. Someone must have had this issue before... it's a rare issue on this board that has been had only once!

[LockDoc]

-
I would try the ground wire also. Easy and cheap enough to do..... If that doesn't help then I would make up a new wire from the HEI to the power source.

LockDoc

[joeydurango]

Well, dang it, okay. I'm outnumbered. Easy to re-do the ground wire. I'm just one of those people that really wants to know the "why" of a problem. I would be kind of bummed if a new ground fixed it, and I didn't understand what was going on in the first place.

I'll keep you all posted, for the next guy that has this issue...

[HO455]

To troubleshoot this problem I would let the truck get cold and then put a portable heater in the cab and point the discharge under the dash and get things hot under there and see if the problem disappears. Then you know the basic location of the issue. If it is the cluster and a ground wire fixes the problem most likely the screws that hold the cluster together have gotten loose and when the parts get warm they move slightly and the ground is restored. This is a milliamp circuit so it doesn't take much resistance to alter readout. If the heater or ground wire trick doesn't help then I would disconnect the factory tach lead at the coil and the guage and run a separate test wire through a window and see if it makes the tach work correctly. Another easy check would be to swap the HEI cap and coil with another one. Somewhere you have a spot that is not conducting until it warm. The gap is closed by the themal expansion of the components.

[joeydurango]

That's great info and a good idea. When I get time perhaps I'll try this, before running a new ground. Thanks!

[Steeveedee]

The temperature as a factor means that there is somewhere in the circuit that is broken, and when it gets warm enough, the materials expand and make a good enough circuit for the tach to work properly. This also explains intermittent and/or flaky operation- a connection that isn't solid can give those results.

[custom10nut]

Originally Posted by HO455

All the symptoms of a bad solder join inside or a bad connection/broken trace on the printed circuit component on the back of the guage panel.

The tach operates separate from the printed circuit. I had similar issues with my original tach, and after tracing wires, I found that the printed circuit doesn't contact the electrics of the tach

[joeydurango]

Okay, update time. Yesterday I finally had time to dig into this a little more. I checked out the back of the tach more closely and it didn't have a ground wire run at all - I believe stock they relied on grounding through the bezel.

I ran a new ground wire, and no change.

I checked that I had power to the fuse block - yep. I checked that I had power to the tach - yep. So, now I know the tach is getting switched 12v, and that the ground is good. The back of the entire cluster looks nice and clean, in excellent condition, for what it's worth. There are three options I can see:

1) The distributor is faulty. This is a new HEI distributor I dropped in there about 18 months/3000 miles ago, so I doubt this as a cause. Also, this problem existed with the old distributor.

2) The tach is faulty internally somehow. As I said, in excellent shape, but it could happen, I guess. Don't know how they work inside.

3) The wire from the distributor to the tach is faulty.

To begin checking these possibilities, I guess I'll start the truck and see if there is power to the tach at the brown wire a) when the tach is cold and not reading, and b) when the tach is warm and reading properly.

Finally, my latest question: What should the brown wire read at idle? 12v? Or does the voltage go up with RPM? Maybe I need to rev the engine to get a reading on my multimeter? I am no electrical genius.

[57rookie]

There is highly likely a 90+ percent chance that your tach gauge is failing internally. I would buy a reproduction replacement.

[harpo231]

Hmmmm

http://67-72chevytrucks.com/vboard/a...1&d=1510957175

[joeydurango]

That's an evil lookin' gremlin. Also an evil lookin' homemade Chevy emblem. I like it.

So I guess that means no one knows what the brown wire should read at idle? I'll just see what I get, if anything. ZAP!

[harpo231]

[tommy3horse]

Howdy, i too have the same issue with an aftermarket tach. Seems to work sometimes only, but works when i rev the motor. I am subscribed!

Tom

[ray_mcavoy]

Originally Posted by joeydurango

So I guess that means no one knows what the brown wire should read at idle? I'll just see what I get, if anything. ZAP!

The brown wire connects to the negative side of the coil. When the points are closed (or the transistor in the HEI module is conducting), that side of the coil is connected to ground. When the points are open (or the transistor in the HEI module is not conducting), that side of the coil is no longer grounded so there will be voltage present. So with the engine running, there will be voltage pulses present on the brown wire. The tachometer responds to the frequency of these pulses (which is directly proportional to the engine speed).

You can connect a 12V test light to the brown wire and it should glow at nearly full intensity with the engine idling. And should get dimmer as the engine speed is increased. With a voltmeter connected to the brown wire (and set to DC Volts), I believe you should see a reading of 10V or higher if the connections are okay.

Originally Posted by HO455

All the symptoms of a bad solder join inside or a bad connection/broken trace on the printed circuit component on the back of the guage panel.

Originally Posted by custom10nut

The tach operates separate from the printed circuit. I had similar issues with my original tach, and after tracing wires, I found that the printed circuit doesn't contact the electrics of the tach

True, the tach does operate separate from the printed circuit on the back of the gauge panel. However, there is a small printed circuit board (with soldered-on components) that is part of the tachometer assembly (located right behind the tach face). And if all the external connections check out okay, the symptoms sure do sound like there is a bad solder joint on the tach board.

[joeydurango]

Ray, now THAT is what I'm talking about, thank you! I love this board. I knew someone out there knew how these worked and could provide a little more detailed info. It was just a matter of having the right person look at this thread.

I will check with the multimeter at cold idle and after the tach has warmed up and begun working, and report back.

[joeydurango]

Okay, final update for now. Looks like several of you were correct, and that the problem is in the gauge itself. I have power to the gauge. I have a solid ground. And as of today, I know that I have the same distributor signal to the gauge, whether it's working (warm) or not (cold). So, whatever broken connection is expanding and making contact with heat is indeed inside the tach, probably on the circuit board Ray mentioned above.

Since it works once warm, I will leave it alone for now. If it gets worse, I may try to disassemble the gauge and repair it, or just replace it. I dislike threads with no resolution, so I figured I'd post a final word!

Tom, since you subscribed for info, I'll say that my tach does what you've described for a short period of time between it not working at all (cold) and working perfectly (warm). When it's in between, it only registers the higher RPMs. Given what I've learned here it sounds like yours may have a connection that is tenuous, and that doesn't change with temperature, unlike mine. But to be sure you could do what I did and check power, ground, and signal to the back of the gauge with a multimeter. Good luck!

[Trackbar]

I have been doing some checking on this subject from other people having the same or simular issues. There is a Tachometer Filter in the "Classic Industries" catalog for the HEI swap. Part # MG019219 $34.99. Found on page 77 of the new catalog. Have not tried this myself, havent got that far in my build. Hope it helps.

[1SG.Rob]

No ideas here, but my tach goes out as soon as I turn the headlights on. I've got bigger things to do on the truck and this particular issue is just quirky enough that it makes me laugh about it. I will fix it eventually.