coil too much for tach?

[zed]

I’ve now ‘blown’ two rev-counters/tach’s on my Hardbody truck. They ran off the neg pole on the coil (blue wire). They each lasted about six months, then the needle starts bouncing, and then they die altogether. I’m wondering if the coil is too much for them – it’s a 40kv, 1.8 ohm. I tried putting a diode on the tach to coil wire – didn’t work, still blew. Now I’ve ordered another rev counter – but before I put it in, I would appreciate any suggestions on keeping it alive? What about a bigger resistor?

[datzenmike]

Odd. The Hardbody coils have a resistance of between 0.84 and 1.0 ohms to get the higher output. Putting a higher resistance coil will reduce the current flow needed to get the higher output. I would get the proper coil for it.

[zed]

More info: in South Africa, my Hardbody came stock with an L18 (rated 70kw with V912 head!) with points ignition. I replaced the points with an Ignitor ignition module, and 40kV coil as recommended. It would be a pity to downgrade the ignition just to be able to have a tach? Is there no way I can 'protect' the tach? thanks Mike for the reply

[datsunaholic]

You could put a resistor inline with the tach feed. Not knowing the current draw of the signal wire, I think you could start with 100Ω. The tach only reads pulses, so as long as the pulse is strong enough the tach will read correctly. If it doesn't read, you can lower the resistance (smaller number).

 

The big issue though it to make sure things are grounded. The tach jumping around is a sign of transient spikes, probably because the condensers that the points needed aren't being used anymore. Make sure the chassis and dash grounds are good, and maybe add more.

[zed]

thankyou for this advice! going to give it a try

[zed]

this is frustrating project. Installed the tach after re-making my grounds - they are clean and solid with extra grounds from dizzy and body. Soldered a 100 ohm resistor into the tach trigger wire - needle was bouncing. Tried two resistors - showed a lower rev reading on the tach, less bouncing, but still there. I read on an MG website to try connecting the tach directly to the battery - improved, but still bouncing. Tried a new voltage regulator on the alternator - small improvement, but still not right. I reckon the coil puts out too much voltage to control

[datzenmike]

The 720 and later 200sx uses a 2.2K ohm resister in line. That's 2,200 ohms. At least one watt heat dissipation though. Nothing to loose by trying it.

[ggzilla]

I reckon the coil puts out too much voltage to control

 

Tachometers don't run off the coil output voltage. They run off the the coil input signal (coil negative terminal).

[zed]

 

 

Tachometers don't run off the coil output voltage. They run off the the coil input signal (coil negative terminal).

I understand this, but from what Datsunaholic says about transient voltage spikes, and the fact that the OEM Hardbody coil is only 15kV, I suspect the bigger coil. Thanks for all the advice/opinions. Will try Mike's suggestion

[ggzilla]

I'm pretty sure the OEM hardbody coil is a lot more than 15kv. The 720 coils are higher than that.