Tachometer Problem

[jcdozier]

The tachometer on my 240-Z maxes out (i.e., 8000 RPM) when the ignition is turned on. Reads zero when ignition switch is in the "off" or the "acc" position. I suspect a grounding problem, but haven't yet begun the search (it's mid-forties and windy here today). Whilst I wait for a warmer day, has anyone had (and fixed) this problem? BTW, the car has a Mallory electronic ignition - has been working well for months.

JIM DOZIER

[EScanlon]

Jim:

I hate to give you the bad news, but it sounds as though you fried one of the resistors inside the tach. I know this from personal experience. It is replaceable, although that is NOT what I did.

If you choose to fix, remove the tach from the dash. Open up the case by first removing the shroud and lens. Next, CAREFULLY remove the indicator needle by a firm and steady pull on the center directly in line with the shaft. (that is straight UP off the face) Next, remove the two screws on either side of the shaft, and you should be able to remove the face plate.

Next, go to the back of the Tach. Carefully remove the wiring harness and take note of which wire goes where. Presuming that you have the Loop wire 4 wire connector and not the 3 wire, take a close look at the loop connection on the back of the tach. This has a couple plastic pieces holding a LOOP of wire within. The wire is NOT bared, and the plastic pieces may be and probably are BRITTLE. Disconnect all wiring and then remove the "guts" of the tach.

You will notice that the tach is comprised of two circuit boards and the needle mechanism. There should be a resistor between the two circuit boards, and more than likely it is fried. Before you mess with it, very carefully jot down it's color stripe sequence, (i.e. gold, black, red, red, green, yellow etc.). This is how resistors are "sized" and you will need to know this when you go to Radio Shack to buy your replacement.

Resolder the new resistor and you should be functioning properly.

Reassembly is the reverse of these steps.

[jcdozier]

In addition to EScanlon's response, I have had a couple of direct responses as well. One suggests a problem between the positive connector on the coil and the tachometer, the other suggests the diode in the tach is shot.

This is a warmer-weather problem, but I will post the fix when I get it done.

Thanks for the input.

[EScanlon]

The "diode" referred to is more than likely the resistor I told you of.

The problem between the coil and tach is possible but impossible to diagnose at this point.

The reason for not being able to diagnose is simple, the tach works in an inverse method to what is commonly thought. It doesn't use the current from the coil to actuate the needle, doing so would actually be a problem as it would reduce current going to the plugs. What it does is sense the current on one side of a graduated relay, and based on the number of "ticks" from the coil, it graduates the current that DOES go to the needle assembly to indicate the number of revolutions. Since your tach goes to red line, the "choke" or limiting factor on the circuit is blown, that's why it is going to red line. In essence the needle assembly is receiving a full 12v full amperage current and not the graduated current that the internals of the tach are supposed to meter.

If you had stated that your tach didn't indicate anything when the engine was running, THEN you have an indication of wiring problems to the coil. The Tachometer in the 240's is an integral part of the firing circuit. If you disconnect it, the engine won't run without your inverting the connection to the coil FROM the tach. This is the most common area of trouble with upgraded distributors and the original 240 Tach.

But, just my 2¢.

Let us know what you find.