Tachometer Problems

[Hack71]

Bought a tachometer with vacuum gauge conversion kit from Classic Industries. I have stock electronic ignition from a newer vehicle, Classic Industries stated it would not work with electronic ignition. I though that didn't sound write as I remember running many tachs on breaker point ignition then switching to electronic with no problem. I checked around and everyone I talked to said it should work, some people said there was a filter to take out some of the electronic "noise" and make the tach more accurate. Installed the tach and vac gauge and they looked and worked great, for 500 miles about 3 weeks, I smelled something hot amp gauge spiked up and tach quite working. I looked over all electrical tach and truck and found no problems other than the tach that quit working. I figured it was a bad tach or it simply burned out. I installed another new tach drove for about 2 weeks and 800 miles and same thing happened. I double checked all electrical and grounds and found nothing wrong other than the tach not working.
I don't want to buy another tach and have the same result, I don't want to go back to breaker point ignition either.
Any Ideas I want to retain the factory look of the dash, I use the tach simply for looks I do not use it for anything else, but I would like it to work.

[AussieinNC]

This sounds like the tach is receiving too high a signal voltage....where are you feeding the input signal from?

What exactly is the year of the hei distributor?

[AussieinNC]

You should be connecting to the Tach terminal on the hei cap....not the main feed power circuit.

[Hack71]

Connected to the tachometer connection on the distributor, distributer is probably from late seventies era.

[Hack71]

Tach worked good when installed, but didn't last long.

[AussieinNC]

CX 1533 unit? Did you buy the engine harness as well ?

[AussieinNC]

Or CW 1008, that explicitly says not suitable for HEI dist ?

[68c10airstream]

Same thing exactly happened to me even though mine is ls powered. When my alternator went crazy and voltage spiked i smelled the burnt electrical fumes in the cab, looked down and the tach was dead and my voltage gauge was pegged to the max (i swapped a 73 and up voltage gauge with the 68 face overlay). Had alternator rebuilt, new tach and same thing in a matter of weeks. This time i saved the tach by shutting the key off early. The tach will need a 10 ohm 1/2 watt resistor installed just behind the face.

I now installed a bigger alternator (new ac delco) with a longer belt and all is good so far. Original alt was 105 amp, bigger alt is 145 amp

[Already Gone]

Same problem for me but different. I installed an original tach with vacuum gauge dash in my truck. The truck has a Petronix Distributor and coil.. My tach works but reads about 800/1000 RPM high at 60/65 mph. I called Petronix and they told me install a resistor, I have the number, which i did and it didn't change anything. I've put 1500 miles on the truck and the tach is still working so no frying anything on mine. Someone I think on here mentiond that the origianl tachs don't work well with HEI's but your problem is a new tach isn't working with a HEI. I'm on board here to see if anyone has a solution.

[paul blair]

Sub

[Hack71]

Thanks for the replies, I really appreciate having this site to air problems out and get everyone's experiences. The part numbers were CX1543 and CX1138, as said Classic Industries stated it wouldn't work and they were right twice, the kit came with harness for tachometer and I had to provide my own tubing hook up for the vacuum gauge. I am confident in the wiring. It could have been a spike in voltage from the regulator that took them out, if that happened that probably wouldn't have been the fault of the HEI. More than anything I want a solution or maybe a new idea to mount a factory looking tachometer in the dash. Does anyone know if the bad tachometers are repairable? Voltage seems to be consistent the truck had a new alternator 2,000 miles ago the regulator looks vintage but seems to be working correctly.

[68c10airstream]

Hack71, if you are using the old style regulator i would suggest upgrading to a gm internal regulator. Get with VETTEVET in the electrical section to see what is needed, it's not tough and you will end up with a much better system. I checked my failed tach and it had an obvious burnt 10 ohm 1/2 watt resistor sitting on a little board right on the backside of the dial face.

[72blu]

Since you guys are on the topic of tachometers... can you add an inline fuse to the tach harness in order to prevent a meltdown should it short for whatever reason? If you can what size fuse would be appropriate? 5A?

[Jcfcamaro]

Good question 72blu

[Hack71]

Found the problem. Voltage swings from the external regulator was burning out the tachs. The regulator wasn't responding fast enough to the voltage spikes, the voltage spikes burnt a resistor on the circuit board. I replaced the regulator but still didn't change it, I upgraded the alternator to one with an internal regulator reworked the wiring a little. I installed the third tach and everything is operating perfect. I replaced the resistor on the circuit board of the bad tachs and they work good now.

[Shinysideup]

I guess LMC assumes it is the upgraded HEI that fries the tachs, but it is the lack of upgrade to the charging system doing it all along. Thanks for the update to your ongoing issue.

[ls1nova71]

Originally Posted by 68c10airstream

I checked my failed tach and it had an obvious burnt 10 ohm 1/2 watt resistor sitting on a little board right on the backside of the dial face.

Originally Posted by Hack71

I replaced the resistor on the circuit board of the bad tachs and they work good now.

I also had one burn out on me, also with an LS, so don't think the PCM is putting out too strong of a signal, but after changing the tach, I never had another problem. The bad tach has an obvious burned resistor on the back, so is this the resistor you replaced? I cant see the colors on the bands to tell what size it is! Guessing its the 10 ohm 1/2 watt one?