to clarify....

We're talking about the actual signal wire here, not the shield, correct?

What kind of signal should I be seeing?

I thought it was just a DC trigger-- on/off/on/off

When cranking I have no DC pulse/trigger at all on this wire.

Incidentally, I have spark at the plugs but it is weak-ish.

For giggles swapped known good GM ignition module...no change

Coil (older DAC6093) has .9 to 1.2 ohm primary resistance which is out of spec...but I'm not sure if that ties in with lack of pulse on the trigger wire

BW, the trigger wire has good continuity from engine bay to Pin 18. The connector was shabby so I changed it....but nothing changed as a result nor did I expect it to

Any other thoughts?

I'm mulling the zener diode inside the amp



Have not done this yet

Cheers and thanks
Doug

 

That's a single coil, right? Should be 0.5 ohms then. I have had a bad coil and changing it allowed the car to start.

You should have AC from the pickup to the module, it knows when to fire by the zero crossing on the AC signal.

You've got it right based on the Lucas diagram here: V12 IGNITION SYSTEMS / AJ6 Engineering

 

Yes, single coil.

I will replace as it is out of spec and I have weak spark....but I'm not sure if this enters into lack of signal on the ECU 'trigger' wire.



To confirm, the EFI 'trigger' as shown in the linked diagram, is DC, on/off/on/off?

Cheers and thanks
DD

 

I'm not sure, but I think so.

 

Yes and No. AC is in effect alternating DC and a DVM on DC Volts range will not be able to respond quick enough to read the signal. The tach signal does not change in voltage level, it changes in frequency as RPM rise. I have scoped the signal on my MegaSquirt and its a Squarewave (almost) 0 to 12volts with the positive being slightly longer. If the reluctor in the dissy has equal tooth to space ratio then you will have a squarewave. If the teeth are narrower and tapered you will see more of a spike.

What is the Ohm reading of the centre conductor?

You could temporarily disconnect the Zener, it provides spike protection but the engine should still run without it.

Thinking outside the square here, is the ECU good?

 

The reluctor is a Lucas V12 has scalloped teeth, not square teeth like in typical MS applications. It's usually called the star wheel, because it kind of looks like points on a star. A scope trace has a rising slope to a peak, then sharp fall. Definitely not a square wave!

Reluctor photo on this page: Replacing the Vacuum Advance

 

If you read what I said I scoped MY Megasquirt and it gives a squarewave because the 36-1 wheel has square teeth.

 

Doug,

Have you checked/replaced the hall sensor in the Dissy?

 

Doug,

Just some old thoughts.

You mention changing the module, no change.

I am thinking of the resistors INSIDE the white rubbery blob insid ethe amp. One is for the tacho, the 2nd is for the Pin #18 signal wire.

This is what I wrote a very long time ago.

Maybe a tad helpfull.

HEI V12 Resistors explained.doc

 

It's my completely unprofessional opinion the the shielding on this lead provides isolation from more than just the HT discharge RF noise. On my XJ6 ( longer and more exposed dizzy-to-amp run ) I noticed that the idle became erratic at the same locations as I drove the same route repeatedly dozens of times. Suspicious, I shielded the wires from the dizzy to the amp and drove the route again. Steady the entire route. I removed the shielding and drove the route. Erratic at the same old locations.

IMHO the leads in this system are suceptible to RF interference. Because the signal is relatively weak ( it requires amplification ) strong RF signals can introduce irrelevant charges that confuse the ECU.

 

AHA, UFO's and Jags, not a match made in heaven.

VERY interesting search and test there.

My HE does NOT have a coax until the bulkhead connector at our RHD gas pedal point. My Ign AMP is out the front, near the 2nd coil, and standard automotive wires run up the RH inner guard channel, and those needed go behind the brake booster, and enter the engine from the scuttle panel top lip, LH side of centre. NO issues for 15 years now.

 

Ok, so it appears that, without an o-scope, I can't verify that a 'trigger signal' is being produced. Is that correct?



From the engine bay to the trunk it's .1 to .2 ohm


Hopefully.

It was good a couple years ago when I took it from the (running) donor car....and it's been carefully stored since. I don't know any way to confirm it is good or bad...other than eliminating other possibilities

Cheers and thanks
DD

 

No and no.

Should I?

I do get (weak) spark at the plugs which, to me, suggests the ignition system, including the hall effect sensor, is fundamentally operational.

If the sensor was degraded in any way could that account for no injector pulse?

Cheers
DD

 

Far out !

I check that out, Grant!

Cheers
DD

 

Very helpful...but no joy.

Spec is 6.8k ohms, mine is 6.78K ohms. Doesn't get much better than that

Cheers and thanks
DD

 

FWIW I measured the resistance of the pick-up coil and it is spot on at 3.3k ohms.

I didn't check air gap. Do you think I should?

Cheers
DD

 

Can't hurt, should be around 0.015"-0.020" off the top of my head.

 

Time to tame that V12.

Find, borrow, steal some speaker wire.

Splice it into the Pin #18 wire AT the ECU plug, thread it over the car roof, and attach the other end AT the amp connector for that THIN wire.

Give it a try. This has woken one up for me a very long time ago, and I eventually found issues with the coax behind the brake booster (RHD car). A new coax all the way to that area then fixed the car.

If ya got spark at all, the module is working, as it is really just an electronic contact points replacement. Quality of spark is mostly coil related.

Reluctor gap is worth checking, if you have a plastic feeler strip of 0.016" or thereabouts. I have used my good eye forever, coz I am cheap.

If that module is triggering the coil, and it appears it is, then there is a pulse available for that wire.

I will keep thinking.

 

The ECU has noise filtering and signal conditioning to reduce RF interference. The shielded wire helps in reducing common mode noise and only being grounded at one end reduces capacitive coupling to the centre conductor.

 

Yes if the gap is too wide the signal level will be low and the ECU may not trigger.