Ignition circuitry with MoTeC

Number 7 · Post by **Number 7** » Sat Sep 24, 2016 9:56 am

Hi all, I'm trying to troubleshoot the 2 Group ECU in my TVR 400SE. Its Rover V8 based, with a locked distributor providing the trigger to the ECU. Been running fine since installation 16 years ago. However,a few weeks ago, the ignition light came on and remained on whilst driving. The tacho started jumping around as well - 2 to 3 K up and down - engine was behaving normally though, although the voltmeter in the car was reading slightly higher than normal. I replaced the alternator which cured the warning light but not the tacho. I had the tacho rebuilt, although that's probably just a symptom of the underlying problem. Car now will not start, and there is no spark. Replaced coil and ignition amp (the latter twice), and checked wiring continuity. Trouble is, my wiring doesn't seem to follow the pin guide from the manual, e.g. there's supposed to be a live feed from ignition to the amp (module in the diagram)and I think normally to the coil (although that's not shown). There is no voltage at the coil when ignition is on. There is 5V at the amp end of both wires from the ECU pins 12 and 24, but as soon as the amp multiplug is inserted into the amp itself, this voltages drops to nothing. Totally confused! I've also tried to test the dizzy magnetic pickup (Lucas 35 type), and the resistance across the pickup coil is 3,325 ohms. The only reference online I can find for this spec is 500 to 1500 ohms, so I'm not sure if mine is too far out to even work. Any gurus see anything obvious? The diagram at the foot of the image is my circuit on the car.

DaveEFI · Post by **DaveEFI** » Sat Sep 24, 2016 10:22 am

Dunno your ECU, but pretty well all need a reliable tach signal - in this case from your locked dizzy - to both inject and spark.
Is there tuning software with this set-up that shows engine RPM? If so, does it show cranking RPM?

If that's not possible, a 'scope is the best way of looking at the signals down the chain.

However, I would expect to see 12v at the coil with the ignition on - the amplifier merely grounding the negative. But without a full schematic, have no idea what sort of protection circuits your device has. It's possible it cuts that 12v when the engine isn't running - ie no tach signal.

Number 7 · Post by **Number 7** » Sat Sep 24, 2016 11:37 am

Interesting point about the cranking rpm. I've looked at the ECU with it static, so I'll try when cranking and see if I can see rpm.

Number 7 · Post by **Number 7** » Sat Sep 24, 2016 12:25 pm

OK, the ECU reads 150 rpm when I'm cranking (also noted injector dwell time registering). Does this mean that the dizzy pickup and feed to the ECU is working then?

DaveEFI · Post by **DaveEFI** » Sat Sep 24, 2016 1:21 pm

Yup Looks like your tach signal is just fine. Which is very good news.
Is there 12v at the coil positive when cranking?

Your main diagram shows 12 being fed to the the ignition module from the same feed as to the ECU (after the relay)

But the hand drawn one at the bottom doesn't show a 12v feed?

Number 7 · Post by **Number 7** » Sat Sep 24, 2016 1:28 pm

Nope. 1V when ignition on and still 1V when cranking.

DaveEFI · Post by **DaveEFI** » Sat Sep 24, 2016 2:12 pm

What are the connections to the ignition module?

Your drawing shows 12 & 24 to the ECU - which I'd guess won't be 12v as the main diagram shows 12v as a separate feed to its module.
And a ground, and the two to the coil. But no 12v?

If you only have 5 connections at the ignition module including ground, I'd guess the 12v was applied at the coil? This is likely how the original dizzy was fed.

Number 7 · Post by **Number 7** » Sat Sep 24, 2016 3:10 pm

The amp (module) multiplug gets 2 x 5V supplies from the ECU, but as soon as that plug it connected to the amp, the voltage disappears! There is no separate 12V feed to the amp in my set-up, hence the confusion with the MoTeC diagram, which says there should be. No 12V at the coil either. The only thing I haven't shown on that diagram is the tacho, which before its rebuild with different internals was connected to the coil +. The supplier has told me that the new tacho requires connection to the coil -. I was assuming that this link between coil and tacho was merely the signal to trigger the tacho. Tacho has a separate 12V feed to it.

DaveEFI · Post by **DaveEFI** » Sat Sep 24, 2016 4:19 pm

The 5v signal is usually a low current one which switches the high current coil driver. That driver normally grounds the negative of the coil, with the positive having 12v when the ignition is on. As I said, that +12v is normally connected at the coil, as it makes no sense to route it through the amp, as it's the coil which takes the current.

The rev counter merely takes a 'sniff' of the signal at the coil. It should make no difference if it is connected or not - unless it was faulty and shorting things out.

Once you've sorted it, spend some time drawing out your circuit diagram as it exists, including wire colours.

Number 7 · Post by **Number 7** » Sat Sep 24, 2016 4:32 pm

Thanks Dave.

stevieturbo · Post by **stevieturbo** » Sat Sep 24, 2016 5:10 pm

As has been covered.

The amp will receive a 0/5v signal from the ecu ( might be 0/12, either way it will be a simple hi/lo switched voltage ).

You should be able to test this with a meter, dwell meter, LED, etc etc etc during cranking to see if there is some sort of signal.
That will at least let you know if the ecu is attempting to fire a coil.

As you have a stable rpm then the dizzy/trigger itself should all be ok, and presumably you have fuel being injected...which again suggests the crank trigger ( ie dizzy ) is ok.

If that's ok....then you need to do the same for the coil itself which would verify if the amp is working ok, or not.

DaveEFI · Post by **DaveEFI** » Sun Sep 25, 2016 8:52 am

Number 7 wrote:Thanks Dave.

If you don't get anywhere, post pics of the ignition amp and coil showing the wires that are attached to them.

Number 7 · Post by **Number 7** » Sun Sep 25, 2016 6:55 pm

Further test: After ensuring continuity between the ECU pins 12 and 24 to the amp multiplug (the thin brown and thin black wires visible at the multiplug), there was 1.6V steady at the brown and 0.03V steady at the black, both whilst cranking, at the multiplug end but without being plugged into the amp.
The thicker black wire at the multiplug is the earth, the purple goes to the coil positive and the white goes to coil negative. Aside from the suppression capacitor, the only other connection to the coil (negative) is the link to the tacho - this is the insulation covered wire to adjacent to the king lead. As previously mentioned, the internals of the tacho have been replaced, and it was at that point that the car no longer started. The only wiring difference between old and new tacho is that previously the signal was taken from the coil positive - now it is required from the coil negative. Is there any possibility that this wire was previously supplying 12V to the coil as well as taking a signal to fire the tacho, because that is really the only change that has been made?

stevieturbo · Post by **stevieturbo** » Sun Sep 25, 2016 7:09 pm

I would be surprised if any tacho was taken from the coil positive.

It's just a positive...12v, this never changes.

Old tachos were taken off the trigger wire ( some may call it a ground...but it's not just a ground....as it goes off/on etc )

If the only changes that were made to the car was the tacho and it's wiring....then simply completely disconnect it and see if the car runs.

And as you were given the wiring diagram over on Motec, why not feed the tacho via the amplifier as it's designed to ?

Exactly what tacho did you have...and what do you have now ?

Number 7 · Post by **Number 7** » Sun Sep 25, 2016 7:20 pm

stevieturbo wrote:I would be surprised if any tacho was taken from the coil positive.

It's just a positive...12v, this never changes.

Old tachos were taken off the trigger wire ( some may call it a ground...but it's not just a ground....as it goes off/on etc )

If the only changes that were made to the car was the tacho and it's wiring....then simply completely disconnect it and see if the car runs.

And as you were given the wiring diagram over on Motec, why not feed the tacho via the amplifier as it's designed to ?

Exactly what tacho did you have...and what do you have now ?

7

Ha! That's TVR for you. Always expect the unexpected. It was definitely originally fed from the coil neg - even the company which replaced the internals said they were surprised it worked! Just to clarify though, the tacho has its own 12V ignition switched supply. Car doesn't run either with or without tacho connected. Take your point about connecting the tacho to the amp. Just a bit reluctant to try that without really understanding whats gone wrong. Couldn't tell you the difference between old and new though.