Socal_Tom

Hi guys, hope everyone is well.

I have a '88 S4 engine and ECU set up in my '84. Due to ignition system differences the tach reads 1/2 what it should because I believe it's only getting signal from 1/2 of the ignition system it is expecting to see.

With this theory in mind I bought one of Bob's TACH-ADAPT units (nicely made, and he's a great guy) to double the signal. I thought I traced the signal to the tach to the green wire on G7. So far, so good, or so I thought.

EDIT - I should probably indicate that I cut the green wire at G7 and used that to connect to/from the TACH-ADAPT unit.

When I connected it all today the car wouldn't start, and when I disconnected it she started like normal. Start it and then connect it and she dies.

I'm perplexed because I wouldn't have thought that would be possible given, and this may be wrong, that green G7 wire isn't used by the ECU.

Could someone please double check me on the signal being 1/2 of what it should be due to the dual ignition set up I am no running?

Is there a better way to get the correct signal to the tach?

Thanks!

Socal_Tom

Update – I got a multi meter on the G7 green wire and I’ve got a pretty constant 12.5 mV there.

That doesn’t change when the engine speed is Increased so (on my car at least) that would not seem to be not be the right feed or my car is wiring is further deviated from standard.

Bigfoot928

someone else may be able to shed more light. What i found on my 90 model Plug W pin 11 is for the tach, My old motec only had 2 ignition outputs so I did tie in the tach to the green wire which is only one side of the ignition output on the EZK I would unplug the doublerto take it out of the equation. It could be creating some sort of feedback into the signal for the ignition.

Socal_Tom

Thanks for the response. I did remove the doubler from the system and car starts and runs like normal now so no damage done.

Let me get a multimeter on that pin in the morning and see where that gets me.

Thanks!

Socal_Tom

Gentlemen - what sort of reading should I expect from the tach signal? Is there a normal range?

Bigfoot928

If I recall correctly it was a square sine wave signal of 12v

Socal_Tom

Good morning and thanks for the follow up. I don’t have a device to read that, is there a way to get a good reading from a conventional multimeter?

I am obviously struggling a bit here, but will stick with it until it gets figured out and I understand the schematics, as well as had a test. Thanks for everyone’s patience as I work through this.

dr bob

Tom --

There's no easy way to see the tach pulses with a common DMM. The sampling rate on a good meter is probably adequate to read the pulses, but the display update is too slow to actually pick out the values as they change so quickly. Some better automotive meters have a frequency reading that will at least tell you there's a tach signal present.

For your doubler, consider adding a resistor in between the signal from the car and the input to the doubler, so the EZK output isn't dragged down so much. You'll want to start with something like 10k Ohms, and go up or down from there. Higher if the car won't start, lower if it will start but doesn't give you a tach output.

You really want an oscilloscope to read the pulses, by the way. It will show the waveform and the amplitude (voltage), and will help you tune the input and output levels (voltages) of your doubler to get full functionality. There are some pretty good add-on USB oscilloscope modules for a laptop PC, for not much money at all. My dinosaur 'scopes were $thousands with some of the plug-in signal conditioning modules, and they do less and have giant 6" displays, compared with the <$100 and your-laptop-screen display size available now.

Socal_Tom

Thanks Doc.

Any tip on where best to pick up the signal?

I asked a related question this morning on what Porsche calls the Tach on the wire diagrams...

76FJ55

I haven't looked at the compatibility, but could you just drop an 85 tach into your cluster? I don't know if the instrument cluster back panel will accept it, but it would accept the tack signal from your 88 harness correctly.

It's been a little bit since I've looked at the tach circuit, but if it has a low resistance path to ground it will kill the ignition pulse going to the LH, so you will loose fuel pump and fuel injector pulse which would cause your no start. are you sure you have the adapter connected correctly?

Socal_Tom

Had not thought of that, clever.

I think I do have it connected properly to power / ground but don't have the right signal pick up. The Tach-Adapt is a really nice product and my 3.2 911 runs it without issue (different electronics to be fair).

76FJ55

I haven't looked at the signal generated by the EZF to the LH so can't say how it compartes to the 84 tach signal. the 84 signal is pretty standard as it is essentially attached to the ground pin of the ignition coil. the 88 tach signal is actually picked up between the EZK and LH, so it may not be quite as standard and may be more susceptible to interfering with the LH RPM signal, but I really don't know.

I'm parting my 85, so if you are interested in trying the tach to see if it will work let me know. or at lease we could compare pics of the interface between the tach and the housing. I know mine has the gear position lights which I don't think yours does, but that shouldn't be a problem, they just won't illuminate since you wouldn't have the bulbs behind them in your cluster housing, and worst case you could swap the face from yours onto mine.

Socal_Tom

Hi Simon, thanks for the note and generous offer. Can you put it aside for a couple days while I try to better understand the wiring? Thanks!