Tchaps

Hi all,

Car has '94 wiring, so originally tach signal came from the coil packs and used the stock ECU for the 12V pullup. Tach worked.

Then went to MS2 V3 a while ago, used a resistor in the diag box to provide the 12V pullup, still using the coil packs for the tach signal. Tach worked.

In my infinite wisdom I am looking at different coil packs to use in the future, that will not provide the tach signal. So I decided to get the MS2 V3 to provide the tach signal.

I followed the instructions on the MS hardware manual, I jumped from S12C to JS9 to provide 12V to the IAC pins. I then wired js1 directly to the IAC1b pin on the DB37, then that pin on the plug to the black and white tach wire that used to connect to the stock ECU.

I removed the resistor from the diag box.

Black and white tach wires at the coil pack plugs are cut.

Nothing else is connected to J0 to J3, only J1 is used.

Tunerstudio is set to use 1ac1 for tach output. Nothing else is set use those pins.

Tach doesn't work.

My understanding is that J1 / IAC1 could provide the 0-12v square wave signal the tach needs without a pullup in the proto area.

I checked that the wiring has continuity, and that S12C was providing 12V and both seem ok.

Any ideas anyone?

Thanks, Tom

Tchaps

Quick update. I double checked wiring, resistance from 1AC1B DB37 pin on the plug to the IG- pin in the diagnostics box is 0ohms, so definitely got the right wire.

With ignition on but not cranking IG- is 12V.

I tried to use my PC oscilloscope to read the signal at IG- when cranking but just got a load of noise. Not sure if that is reliable or not because I have never had to use the scope before. I think maybe it was me now being able to set the adjustments correctly, but maybe not.

Here is a diagram of my wiring:

I think maybe I should have used pin js0 (1ac1a) instead of js1, I know the IAC pins work in pairs, one high with the other low. I wonder if I have an inverted signal that the tach can't read.

Tchaps

Update.

So I ended up finding a thread on ms extra that seemed to indicate that the pullup tach circuit was still required when using the IAC pins.

I built the circuit using a 2n2222a transistor and 1k resistors as shown in the MS manual, still using js1 as the activating pin. When I checked the V at IG- and it was 0V with ign on, which is what I would expect now the tach circuit is in place in the MS. But when cranking there is still no tach.

I then swapped the activation pin to js0. With ign on IG- is 12V, which is expected. I understand this is also the way round the stock coils provide the signal, 12V until the coil grounds the signal on an ignition event to pull the signal to 0V. Still no tach on cranking.

Is it possible I have fried the tach when I connected it directly to the IAC pin without the pullup circuit?

Is there a way to check this? Expected resistance across the tach etc?

Or am I missing a really obvious trick here?

I tried to scope the signal from the MS to isolate that as the problem source but of course it didn't seem to want to play ball.