Help with electrical question Please

[gtech]

Really pulling my hair out with this one.

Rev 1 NA...Tacho stopped working, so here's whats been done:

1. All connectors removed/checked and reconnected.

2. Continuity check the black wire from ignitor to tacho head - OK

3. Feeds and Earths checked at Tacho head - OK

4. Tacho head replaced - NO CHANGE

Basically I dont have a full understanding of the MR2 tacho signalling.

If I put a volt meter on the BLACK wire from Ignitor to Tacho, this read 14.x volt, and drops slightly as the car revs. This reading is also confirmed at the connector on the Tacho....Start the engine and that voltage drops to about 3.85 volt (connector plugged into the binnacle).

I cannot work out where the signalling is initiated, goes through, ends up and then causes the RPM to go up and down.

Totally desperate for help on this one as its really getting me down now, and obviously have to be careful through the gears.

I might add that ALL other electrical items are functioning fine, no problems starting, ticking over or general running. So basically I am open to suggestions PLEASE

Thanks

Graeme

[MitchB]

Wow, fun.

Ok, the tach signal comes off of the igniter, and you found that. The whole thing is very similar to a MKII, like mine. The black wire gets the tach signal from the igniter and it goes forward to the instrument cluster. This signal would also go to the A/C amplifier to tell it that the engine is running and it's OK to turn on the A/C compressor.

The tach is not concerned with the voltage so much, but more the frequency of the pulses coming from the igniter. What you can't see with your meter are the voltage spikes from the igniter that are higher than 12 volts. (You would need an oscilloscope to see these.) The tach doesn't care about too much below about 20 volts.

Now, to test it, and you'll think I'm nuts. (Hey, I *am* an Electronics Engineer though.) You need to disconnect the wire from the igniter to the tach as close to the igniter as you can. (I know there's a connector there somewhere, but I don't know that engine bay too well, sorry.) You need to pull the terminal from the connector by releasing it so you can plug the connector back together to connect the rest of the wires. You could cut it if you absolutely have to and splice it later, as some terminals are VERY tough to get out. And, some are cake.

Once the wire is free, you need to get about 10' of a piece of insulated scrap wire to use as a jumper wire. This can be almost any gauge wire, 18 Ga. is the most common. You will need a set of battery jumper cables as well.

Take a friend's MR2 and park it so that you can get the battery jumper cable between the two cars. Ground the two motors together with the jumper cable.

Connect the jumper wire from the friend's car at the tach terminal of the connector. Just strip the wire and jam it into the back of the connector, make sure it's got a connection. Hook the other end of the wire to your tach wire that's going up to your dash. (You can use your voltmeter to verify that you've got a decent temporary connection.)

NOW, turn on your ignition key, the car does not need to run. Start the friend's car. Your tach should go up.

Yes, it sounds goofy. BUT, you need a signal, and the best place to get one is from another identical car.

SO, if it doesn't work? You might have a bad tach ground or no +12 Volts at the tach. It does require power to make it work. Three wires total.

[gtech]

Hey MitchB,

What a fantastic explanation......thats all I wanted, as the voltage from the signal wire was totally screwing with my head, been many years since I did any work on cars, wow they certainly made it difficult with the Tacho signal.

And yes its a goofy idea to check its working, but its bloody brilliant, would'nt of thought of that at all.

Fortunately with grit determination I found the fault last night, when I started measuring other voltages at the instrument panel I found that on IG+ there was no voltage at all, so traced the wire to the kick panel on the right side and found out the connector plug had come adrift.

Annoyed me as I had checked every wire on the left side as that is where the BLACK was, never thought of the other panel.....so now I feel stupid and annoyed with myself

Many many thanks for your advice, and to all the others who helped, very much appreciated and I shall now cut n paste your explanation into my book of tricks.

Thanks

Graeme

[MitchB]

Ah yes, support of my favorite creedo: I fixed it by doing the last thing I did. (Much like one of my other favorites, when someone asks where you found it, "in the last place I looked".)


> I found that on IG+ there was no voltage at all

So, I guess you really DO need all three wires to make it work.

[gtech]

Sorry but I dropped a boo boo.....Sorted the IG+ thing and STILL NO Tacho. So measured the voltage at the black wire at the head and it fluctuated between 12.09 & 13.29 Volt.

Measure the voltage at the IG at the diagnostics and it never dropped below 13.59

Cut the black wire at the head and joined it to the cable going to IG on the Diag socket and all works well.

I guess that somwhere in the circuit there is a drop on the wire feeding the head from the ignitor.....but where would be like looking for a needle i a haystack :-)


Graeme