Hello -

My wife's '86 Z-28 (original LG4 305, now 350) tachometer is basically reading double (e.g. 1200 at idle).

A buddy at work says there should be a tach "filter" inline between the distributor and tach wire. I see no such device-the wire comes from the tach outlet on the distributor and disappears into the firewall harness.

Is this guy correct? Is this something I need?

Any suggestions or assistance is appreciated.

Rick

 

All my wiring diagrams do not show any type of filter of buffer between coil and tacho, (White wire). Do you have a stock HEI ignition system?...if so make sure you have a good earth between engine block and the coil frame.

 

I appreciate your response. The car is pretty much stock, including the stock HEI, only the long block was changed out from 305 to 350, and a new Magnaflow exhaust with high-flow cat.

The fellow at work has an '84 Corvette, and he mentioned the tach filter.

Anyone else have any thoughts on the matter?

Rick

 

My '74 had a large resistor in line on it's tach wire, I had no idea what it was for, but when it broke off I knew I needed to fix it. I don't think you need one on these cars though. Did you buy this car like this or did the tach just go crazy on you one day? If it was bought this way you might have a V6 tach in there or something like that.

 

The tach has always pretty much read double since I acquired the car. It was always a V-8 car, and it has about 80,000 miles on it now. All the other gauges read normally, only the tach is odd.

When I disconnect the tach lead and plug in my tach/dwell meter, it reads normally, so the signal from the tach output on the distributor is correct.

Rick

 

The capacitors on the back of the circuit cards on the back of the tach go bad over time. I would perhaps try replacing them, that's what I'm going to try.

 

Yes, I figured that it would be a capacitor. I looked at some other threads in this Forum and saw some traffic regarding that, the Pontiac Fiero being similar, and changing out the capacitor. I don't feel that I have the expertise or the equipment to tackle that one. I have a soldering iron and a digital volt/ohmeter, but this is one area I am not in my comfort zone.

I don't drive her car that often, so it's not really troubling me so much, but it would be nice to know what I'm really doing when I work through the gears.

How painful is it to pull the dash cluster? I guess I would be willing to have a look, anyways. I typically work on '60's and '70's cars, they are certainly less complicated-but I do love our 3rd gen's!

Rick

 

This resistor was a ballast resistor used to lower voltage after the engine is started to reduce wear on ignition components. It also makes the engine much easier to start by effectively doubling the voltage provided to the ignition coil when the engine is being cranked. Some manufacturers used them and some didn't. If it was used it was normally on a point type ignition.

 

Must have been, I thought it was on the tach wire, but I guess not... I didn't question it's function, just figured I should leave it in there. The car was converted to an HEI setup though by the previous owner.

 

Found a good article on fixing the thirdgen tach problem here:

https://www.thirdgen.org/forums/elec...meter-fix.html

The tach behavior should be controlled by an R/C filter in the cluster. Seems like the resistor network used there has a tendency to fail, and a new resistor or potentiometer should fix it.