Hello,

I recently installed a factory tach in my b110. It's working, except for it reads about half the speed of what the engine is actually rotating. If it's idling at 1000rpm, it shows 500. It moves up the range just fine, just reading half speed the whole way. Any idea what would cause this?

I wired it into the stock harness as shown in the wiki.

Ignition system is pertronix, aftermarket coil, and there is a ballast resistor still.

Check the earth. (Although a poor earth usually results in a high reading) or alternatively the trim pot has been adjusted on the rear of the tach. E.g it’s out of calibration. If you have access to a automotive multimeter. (Displays rpm). Rev engine to say 3000 rpm as read on multi meter and adjust tach trim pot until the needle reads the same. You should be good to groom there.

Did you get it used? A previous user may have converted it for V8 use (changed a resistor)

You can also, test it on any car just connect the wiring loop to the coil. Between coil - or coil + and the existing wire. and connect 12V and ground

Yes got it used. thanks both of you for the info. I'm happy to dig into it. Two great ideas.

Any idea what the factory resistor rating is?

If the stock Datsun tachometer has been modified to work on a V8, this causes the problem of reading half of actual engine speed

8-cylinder tach reads half of actual engine speed when used on a 4-cylinder engine

I would test it on another car, to rule out an unusual problem with your car. No need to fiddle with the 2nd car's dash or modify the wiring, just open the hood, unscrew the -coil wire and hook up the tacho temporarily. See if it reads the same "half speed" on another 4-cylinder car (or reads correctly on a Ford V8 truck)

I've never modified a Datsun tachometer but they say just add a resistor and make some simple change to the internal wiring. I suppose you could remove the tachometer housing and see if there is some obvious mod going on

1200 tachometer


* Yellow ARROW: adjustment slot

To remove tachometer guts from the case:
0. Remove the tachometer from dashboard
1. Remove nuts + washers (yellow circles)
2. Remove the wire loop
3. Remove the plastic wire block
4. Remove the case screws (red circle)
Pull the tachometer guts from the metal case

look for a resistor that is added

The Datsun tachometer V8 resistor mod is from the book "Jags That Run" to convert tachometer. I couldn't find a pic but here is the same mod done to a VDO tachometer 4-to-8 cylinder with 750ohm resistor added (violet-green-brown stripes, the gold stripe means 5% tolerance)

If you see an extra resistor, please take photos. I don't have any photos of the 1200 tacho guts

Attach file:

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  VDO tacho with extra resistor.jpg (104.84 KB)


  ResistorChart.jpg (53.51 KB)

Thanks, I’ll start diagnosing this pretty soon. I had an aftermarket tach in the car before, it was fine. But it wasn’t an inductive tach like this one. Also I swapped all the wiring harnesses out for OEM harnesses of the same year since, and installed this tach. I’ll hook the old tach back up as a reference.

Also I’ll start by wiring this into another car. It’ll be pretty easy, I’ve got a 70 510 with the same style tach. I’ll go from there and report back.

I wouldnt trust a factory tacho as far as I could throw it.

I once built an engine that should have been okay to 8000.
It refused to go past about 7600 no matter how much I ignored it's screams for mercy.
The next engine I fitted an aftermarket #OOPS# light. When I was showing 7500, the stock tacho was only ready 6200.
it was then I learned factory datsun Gauges were to be taken with a grain of salt..........

yes, all factory gauges are approximate. Ofter about +- 5%. The good news is that the Datsun factory tach has a user-adjustable trim pot on the back so you can easily dial it in

yellow arrow points to the adjustment. Use a small screwdriver