BLDun

I have a '76 coupe and replaced the gauges about 6-8 months ago with auto meter's. The tach, along with all the others, have worked great until yesterday. Took it for a ride over to Daytona and it worked great but on the way back I noticed that the readings on the tach continued to drop. Engine ran fine without any problems but the tach kept dropping until eventually it was reading about 500 rpm's while I was running at about 60-65 mph. Any ideas what might cause this?

Thanks
Barry

mikem350

If it was slowly dropping I would suspect an electrical problem. Either the filter capacitor on the tach lead or the tach itself.

The stock tachs have a circuit board inside that can fail. Call Autometer!

BLDun

How would I go about checking the filter? I suspected an electrical problem as well but didn't know where to begin checking it. I will give Autometer a call and see what they say.

Thanks

BLDun

I cannot locate a tach filter anywhere on my car. But here's one for you. I started it up after sitting all night in the garage and the tach works perfect. Doesn't make sense unless the wire to the HEI vibrated loose somehow or something. I'll keep an eye on it and see what gives.

Thanks

7T1vette

My guess would be that the tach filter was going bad and that it was particularly sensitive to heat. The hotter it got...the lower the reading. That filter should be mounted to the intake manifold near the tach; and a lead from the tach should be connected to it. It looks kinda like one of those radio noise suppression capacitors. Several of the Corvette parts vendors sell replacements.

BLDun

I have looked and looked and cannot find a tach filter anyplace near the tach or on the intake. The HEI is an aftermarket one so I'm wondering if the po that installed it didn't put a filter back on??

0Willcox Corvette

Tach filter location on your car is pictured below.

BLDun

Sorry, No such animal!!

BLDun

That wire - A- that Wilcox shows in the drawing plugs into the tach slot on my distributor and then goes directly into the firewall.

0Willcox Corvette

Follow the white wire from the distributor cap...

BLDun

No white wire. there is a yellow, red, and black wire that come out of the bottom side of the distributor and then plug in to the underside of the protruding part of the distributor cap. Then there is a large red wire coming from the firewall that plugs in and a black wire that comes from that same hole in the firewall and plugs in beside the large red one. No white one I can find at all.

BLDun

Please don't leave me hangin!! You asked me to follow the white wire but there is no white wire I can find and none visible in the photos I posted, so where do I go from here? Is it possible one was never put on when the engine was rebuilt? Should I put one on and if so, where does it go?

I am tending to agree with 7T1vette in that I think the problem is heat induced. If I go to the garage right now and start it the tach will work perfectly. It wasn't until about 1/4 of the way home from Daytona that it began to act up. Can you think of anything other than a filter that might react to the heat and cause this?

Thanks
Barry

0Willcox Corvette

The signal wire is the forward connection on the distributor...