Rev Counter

[Guest oldman]

I have next week off work will get home tune to have a look

[tractor]

Have you spoken to home tune and told them wot the car is they may get cold feet

[Guest oldman]

Not spoke to him yet but he sorted my bond bug out he is old school

[Guest oldman]

Home tune sorted 2b out a bad connection on a multi plug and socket buried behind engine and it was one I had not touched well pleased now on with tidying up rest of car

[Guest mcramsay]

Do you know what connection it was? It wasn't the live to the fuel pump was it?!

[Guest oldman]

No mr mcramsey fuel and spark ok must have something to do with the injectors

[Guest oldman]

A chap on here told me which wire feeds the rev counter how do I test it is there a voltage there or is it a pulse cheers

[Guest mcramsay]

You will have 4 wires, a live, an earth, signal and illumination. Strip the end off each cable and then set your multimeter to read continuity ( it should beep if you touch the end of the probes together) , now put one probe on a good chassis earth or battery negative terminal, with the other probe touch the copper of each cable one by one,when the multimeter beeps that identifies that cable as the earth. Now set the multi meter to read voltage. And carry out the above test on the remaining 3 cables with the ignition on but lights off. When the multimeter picks up 12 volts that is your 12v switched live, now go again with ignition on and lights on with the remaining 2 cables , one will read 12v on the multi meter. This is you illumination to the gauge. By elimination the remaining cable is the pulsed signal.

[Guest oldman]

thank you mr mcramsey

[Longboarder]

I would be very wary of testing an output from an ECU with a multimeter. ECU's are sensitive beasts and if you connect up a test meter to its wires it can damage the ECU.

The output is essentially a pulse in time with ignition events. Most rev counters pre ECU's expect a pulsed earth as most ignition systems have constant live feed to the coil and switch the earth. So if you were reading off the coil it would be a negative. I can't guess what system the EEC uses in its signal wire. I can only suggest you connect it to the signal input of the rev counter, wire up the rest of the rev counter as above and try it. It will either work or not.

 

Nigel

[IanS]

Modern digital multi meters in voltage mode (as opposed to current mode) have an input impeadance of 10Mohms (i.e.bugger all).

This will not damage any ECU provided that the 0V (earth) connection is made first to clear any static charge.

In continuity mode it will output a limited current (it then measures the volts to give resistance) which will be below 5v and below 1mA. This is safe for any external connection to an ECU.

[Guest mcramsay]

To test the tacho signal wire propoerly you need an oscilloscope, but you are only trying to identify which wire is which, so there is no need to go down that route. Look on your other gauges for 3 wire colours that are consistent in all the gauges.... a good builder would keep wire colours the same! 3 of the same colours used on each gauge would indicate power earth and illumination

[Guest oldman]

Thanks to the both of you looks like the gauge is from pinto engined car only 2 connections I have taped up wires in the loom will check those out