Tachometer Resistor Value

[SteveJ]

Here is the situation. I met a young man who bought an 83 N/A that has a turbo motor swapped in. There are two blue wires coming off of the negative on the coil. One went to the power transistor, and the other was cut. The car runs in this configuration, but the tachometer does not work. When the cut wire is reconnected, the car dies. The owner said that the tachometer did not register when the wire was connected.

I put a voltmeter on the negative of the coil and checked the voltage when connecting the cut wire. The voltage went to zero. I measured the resistance of the blue wire to ground (assuming it goes through the resistor, to the tachometer and to ground). It was about 850 Ohms.

What is the value of the resistor supposed to be? It just seems like we're grounding out the coil when the wire is connected to the tach.

[Zed Head]

I measured 2.3 kOhms (2,300 ohms, to be clear) on my spare from a 78 280Z. That's on the negative side, between the negative post and the tachometer. In series. I have also used the resistor without the tachometer connected between the negative post and ground, to run an engine on a stand. I think that it might also "condition" the Pin 1 signal to the ECU, since they're all branched off of the same line on a 280Z. Maybe absorbs voltage pulses or something. The ECU seems to need it to work right.

Edited November 11, 2013 by Zed Head

[SteveJ]

One thing I didn't try was to put my clamp ammeter on it to see if there was much current flow. I was also disappointed that I didn't think to bring a rheostat that I have to see about adding resistance.

In the 280ZX, I didn't see the tachometer signal branching to the ECU. Has anybody seen it differently from me?

[Zed Head]

The attached 1983 Electrical diagram suggests that there should only be one blue wire off the coil. Tried to figure out where the tachometer gets its signal but the later FSM's are much worse than the early Z FSM's in describing and illustrating how things work.

The turbo motors use a Crank Angle Sensor (CAS) which I would guess also feeds the tachometer. Probably through the ECU.

Edited November 11, 2013 by Zed Head

[SteveJ]

Actually, both the turbo and N.A. wiring have a wire going to the resistor. Look at the full wiring diagram. In the turbo, there are two wires. In N.A. the wire is WB coming off the coil and is spliced with a blue wire elsewhere (unless the wiring diagram has a mistake since in the EL section, the wire coming off the coil is blue).

Digging deeper, I looked at the wiring diagram for the 81. It shows the blue wire splitting off with one branch signalling the ECU on pin 18 and the other branch going to the resistor for the tachometer.

This is turning into a good mystery...