91 vw gti tach output

[mk2_20v]

Hey guys I've got a 91 Gti with a aeb 1.8t ls2 coils in wasted spark and can't get the tach to work with the output on the ms.my installer tried several fixes including the relay coil and nothing has worked.I'm open for any input on a known way to get the factory tach working

[2swe]

AFAIK mk2 rev counter needs spiky signal from coil negative terminal, so relay coil is the way to go.

keep the transistor and coil far from MS case, otherwise you will see a lot of noise..

[PSIG]

Hey guys I've got a 91 Gti with a aeb 1.8t ls2 coils in wasted spark and can't get the tach to work with the output on the ms.

From what I understand, the Mk1 and Mk2 use high-voltage (coil negative) triggering, and the Mk3 uses low voltage triggering. That means to test it, you should be able to run your RED/BLK tach wire directly to one of the coil ground wires and get a 1/2 rpm reading in waste-spark. If so, use the diode wiring in the Extra manual on only two of the coils (1&3 or 2&4). Verify you really have a Mk2 tach that needs a high-volt signal before you begin sending zaps to it.

David

[mk2_20v]

We tried the relay coil with no luck,I will give the two coil diode mod a try.thanks for the replies

[mk2_20v]

Had a talk with my installer and he said the diode method won't work being the ls2 coils are 5v pull up.what say you guys?

[Paul_VR6]

He's right.

I have had good luck with the transistor in the manual and relay coil (40a).

[mk2_20v]

He tried that years ago when it was installed and said it didn't work but I'll give it a try myself,thank you for the input

[PSIG]

It's not how the igniter is fired that makes a tach signal, it's the power spike ringing through the coil on firing that does. All coils spike at both the (+) and (-) terminals, but you don't also want the 12v on the (+) terminal. Terminal "A" of the LS2 coil is ground, as in this diagram. Both the primary and secondary windings apparently ground through that terminal. You should get a nice inductor ring (voltage spike) to send to the tachometer from there. I have not tried it myself, but know a few that have and say it works well and I sure hope it does, as I'm planning to use it on some LS3 coils. I'd say to give it a shot.

David

[mk2_20v]

Alright I'll give it a try,I just didn't want to mess something up

[PSIG]

I'm not suggesting you do anything you are not comfortable with, nor risk anything by the information presented. As I have indicated previously, it is your choice on how to proceed, as the choices are inevitably yours. If you need a high-voltage pulse to your tachometer, this is normally sourced at the coil ground. The coil ground on an LS2 coil appears to be terminal A according to the link posted. If all that is accurate (and it is up to you to verify any information), make your own choices from there.

David

[jsmcortina]

Terminal A connects to ground, so it will erm... be grounded and can't be used to drive a tachometer.

James

[PSIG]

Terminal A connects to ground, so it will erm... be grounded and can't be used to drive a tachometer.

James

James, the 'tach adapter' modified relay coil has worked many times before for myself and others, and would seem to be the method of choice for the OP's project. As to the ground wire pulse pickup - I would generally agree with your statement (ground = 0), but my interest here is the success others have had in "capturing the pulse on its way to ground". I'll contact one of those that have used the method to find exactly what the parameters were, or do my own tests on an LS coil, and post the results.

David

[Paul_VR6]

The input resistance of the vw tach is too high for that. Also only connected to one coil will not be enough pulses to read correctly.