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TACH PROBLEM- help me please

1410 Views 9 Replies 5 Participants Last post by  TitoElSpicco
okay guys, I´ve got a 1991 escort GT and my tach isn´t working. The thing is, it works SOMETIMES... usually it wont work, but every once in a while when I´m driving, it will kick back in.

Okay, I don´t know exactly how everything is hooked up, but I´ve pretty much torn down half of my dash and console and pulled apart a good portion of the right side of the engine bay looking for the problem and found nothing.

Here´s what I have done thus far:
1.Checked the diagnostic module for any corrosion. Checked out AOK.
2.Checked the ignition coil for damage/corrosion- checked out AOK.
3.Checked the ignition module for corrosion/damage- looks fine from the outside, but I´m suspicious about whether or not it´s working. I don´t know exactly what its function is.
4.Checked out tach cluster. Checked out AOK with no corrosion and all connection points clean and contacted.
5.Checked out ECU- NOT SURE. Looks fine from the outside, but I most of the time the "check engine" idiot light is on in my cluster, and I have a feeling that this is the reason why my tach may not be working, because my EDU is about to shit the bed.

Here is a diagram of the ignition system and tachometer:


This baffles me because all the parts and wires appear to be in-tact and with no rust or corrosion. I think the ECU went bad, but from the diagram, it seems like the ECU may have nothing to do wit the tach.

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Okay, well I´ve figured out that it´s NOT a problem with the signal lead or the ground lead, so apparently it´s a problem with the 12V input lead.

Even though I haven´t tested the 12V lead yet (still have to put the battery back in the car and hook up the ignition system again), I am assuming that this is the problem because continuity of ground and signal wires are all good.

Anybody know what I can do once I know for sure that the 12V input is dead? From where can I splice in a new 12V input?
Okay, i got it fixed... sortof.

The tach works, only it doesn´t read correctly and it reads in negative rpms. In other words, it is totally fuxored.

When I rev the engine, instead of going clockwise in a linear fashion, it goes COUNTER CLOCKWISE in a very non-linear fashion. The first 3000 rpms are all within about 10 degrees of eachother, then above that, the whole thing goes nuts and does the entire rest of the 350 degrees on the last 3000 rpms.

Anyway, I got all the cables working and everything, but it´s the gauge itself that is the problem, so if anybody has an extra gauge lying around, I´d like to buy it.
ArgH! I know some of you guys have a shop manual- throw me a fucking bone!
My tach does the same thing. It reads 3100 rpm all the time while the car is running. It started working the other day, but only till I got in it and started it again. Anyway, I posted a similar question a few months ago and never got much response.....so ...good luck.
Anybody have a shop manual?

does anybody know what the ignition module does?
Siragen, man, here´s the description my Chilton´s gives for the ignition module:

"The PCM then signals the ignition module, telling it when to fire the coil. The module controls the current through the ignition primary winding by turning the current ON between firing points to build up a magnetic field around the coil windings. It then turns the current OFF upon a signal from the pulse generator and the pickup coil. Once the current is turned OFF, the field collapses and a high voltage pulse is transmitted to the central terminal in the distributor cap, and through the rotor to the distributor cap terminal for the respective spark plug to fire."

Hope that helps! :p
does it say anything about the respective voltages of the signal wire to the tach?

The problem with my tach is that it is recieving a signal that DROPS IN VOLTAGE AS RPMS RISE. Intuitively, it seems like the voltage should increase as rpms increase, but my voltmeter is telling me that the voltage is dropping.

Also, the gauge needle works, only instead of reading positive rpms, it´s reading negative rpms. In other words, the needle moves counter clockwise when it should be moving clockwise. Bizarre.
Sounds like uve got some wires backwards somewhere... that would cause your tach to go backwards instead of going forward... hope that helps
shop manual or not, your tach is farked.

If it has power and the correct wire is going to the neg. side of the coil, it should read the correct rpms.

.. the signal voltage doesn't go up and down, it goes faster and slower. It fires once each time every cylinder fires. That wire seems to be working fine.

What I'm trying to tell you is you need a new tach.
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