NA_VersionFC3S

In my 87 TII my tach does not work. I know now that the tach lead wire is a yellow/blue wire on the trailing coil.

I took off the trailing coil yesterday just to see if everything was ok visually. Everything looked ok to me.

My question: I found a lead wire that is yellow/blue that is on one of the harnesses right there, can I run a aftermarket tach and get the signal from this wire.


Pic 1: General drivers side with the leading coil removed.


Pic 2: The wire encircled in green is the wire I want to use to work the tach. The red arrow is pointing to the yellow/blue wire that the stock tach is sopposed to work off of.





Thanks.
Tim

NA_VersionFC3S

Also does anyone know how to check to see if the trailing coil is working properly. I had a smog pretest yesterday and they said I had WAY to much un-burnt fuel comming out the exhuast. I was thinking the trailing coil could be the problem.

Tim

RX7-RICE EATER

i hooked up a shift light(same concept), but i used the same wire but on the leading coil, which is the one on the front of the car, its also the same connector thats not connecting to anything.

NZConvertible

Yes you can, that wire is there so a diagnostic tach can be connected for servicing. But...

Before you spend any money you need to find out why tach isn't working. If the gauge itself is faulty, repacing the cluster would probably be cheaper than buying some big, ugly aftermarket one to bolt to your dash. It may even be repairable. However this may be a problem with the coil itself. If the coil isn't firing, the tach won't read.

Find the problem first.

NA_VersionFC3S

I dont think the cluster is bad. I'm gonna try my friends T-coil tomorrow to see if it fixes it.

Also about the wire on the Leading coil. Can the ECU get a reading from both the Trailing and Leading coils?

Thanks.
Tim

NZConvertible

The ECU doesn't get a reading from the coils.

NA_VersionFC3S

OK, then does the tach get a reading from botht the trailing and leading coils?

Tim

NZConvertible

No, just the trailing.

SureShot

The trailing coil fires once per rev.

The leading coil fires twice per rev.

(That's one reason leading plugs wear out faster.)

NZConvertible

Each trailing coil (there are two) fires once per rev, but the tach signal from them is summed.

The leading coil (which is a single dual-post coil) fires twice per rev.

Thus the tach signal can be picked from either coil pack, because both give the same two pulses per rev, just like a 4-cyl.

SureShot

Hmm - I didn't know they were summed - cool..

ORX705

so are you saying that you can hook up the factory tach to 1 of the trail coils or both?

i have a aftermarket ecu and running 4 bosch coil and 4 008 ignitors. can i hook up 1 of the tach o/p from the a trail ingitor or di need to do both?

i assume when you say summed you mean both, but just checking.

cheers
Felix

NZConvertible

Plug it into the little bullet connector hanging off the trailing coil harness (yellow/blue wire), which is the tach signal from both coils.