Trigger Points on old D-Jets

[Graham]

Decided to start new thread because I feel I have trigger problems and maybe others do too.

What I have found, is that my car runs reasonably well after a cold start. But after it gets to temperature, it starts to stumble and run unevenly. CO level increases as engine heats up even more (while testing at idle).

I have two sets of 1972 vintage trigger points. They both do the same thing.

How I test this, is to pull out the distributor and connect a spare plug. I rotate the distributor by hand while measuring from centre pin to each other connector with a needle type ohmmeter. It looks like the contacts should be open for something just under 180deg. When I test the cold trigger points they all do this, but when engine is hot and stumbling, one set of contacts does not.

I suspect that the fiber sliders have worn and that this makes the operation of the points very sensitive to just how they are inserted and clamped as well as, it seems, to temperature.

There is no adjustment on the points.

Possible solutions:

Buy a new set. Problems - Cost ~$500 and only available with pigtail (used on '72/early '73 engines?)

Modify existing - It may be possible to bend the springy piece outwards or the fixed part inwards and thereby get the fiber slider to move closer to center. This should increase the points gap.

What would be interesting to know, is what the distance between the slider contact surfaces should be for a new or known to good set. Mine measure 0.912" on one side and 0.920" on the other. Maybe there is a better way of measuring this - perhaps points gap with known diameter rod between the contact points (about 24.9mm diam)?

Questions
- Anyone know a source for a set of male/female plugs to match the pigtail on later distributors?
- How much of 1 revolution points are supposed to be open.
- Any other good ideas or comments?

[Graham]

I did some testing tonight.

I found the points are kept fully open for about 180deg. On each side of that there is a ramp and they are fully closed for about 90deg. Because of wear the points sometimes stayed closed for more than 90deg. My angle measurements were rough, but it looked like the range was 90-110deg.

Using a spare distributor and a dial indicator, I found that the cams that operate the trigger points move the points by 0.040" . At the maximum point, the cams are 0.942" diameter. at 90 deg to that, when one slider is in the middle of the lowest part of the cam and the other on the highest part, the measurement is 0.902".

The gap between the two sliders on my best set was 0.909". This provides a small clearance and keep the points closed for the minimum angle. The other set had gaps of 0.926" and 0.913". This means that those points would stay closed for a larger angle before opening.

I don't know how this affects performance but thought I should document it somewhere!

[Chas H]

I think as long as the points open and close and send a signal to the control unit you're good to go. IIRC 2 injectors squirt at the same time so injector timing is not that critical.
Have you cleaned the 2 major grounding points at the right fender?

[Graham]

Originally Posted by Chas H

I think as long as the points open and close and send a signal to the control unit you're good to go. IIRC 2 injectors squirt at the same time so injector timing is not that critical.
Have you cleaned the 2 major grounding points at the right fender?

I have checked the ground connections both next to battery and on top of intake as well as just about everything else on this car!

It definitely looks like it is the trigger points that are one of the main problems with my car. If one pair of points are staying open for 350 deg rotation instead of 180 deg, this must throw the ECU off and cause it to fire the injectors incorrectly. I don't believe the trigger points actually fire the injectors, I believe they just tell the ECU that they CAN be fired and then the ECU determines how many injector pulses to send based on other inputs. If ECU is being told it can fire those injectors at wrong time, it fires fuel into the intake manifold that probably results in rich/rough running and other problems.

Or so I have read! I really don't know exactly how all of this works

[Chas H]

Originally Posted by Graham

I have checked the ground connections both next to battery and on top of intake as well as just about everything else on this car!

It definitely looks like it is the trigger points that are one of the main problems with my car. If one pair of points are staying open for 350 deg rotation instead of 180 deg, this must throw the ECU off and cause it to fire the injectors incorrectly. I don't believe the trigger points actually fire the injectors, I believe they just tell the ECU that they CAN be fired and then the ECU determines how many injector pulses to send based on other inputs. If ECU is being told it can fire those injectors at wrong time, it fires fuel into the intake manifold that probably results in rich/rough running and other problems.

Or so I have read! I really don't know exactly how all of this works

You can measure the height of the rubbing blocks with a micrometer to identify a worn part. Since you have 2 trigger assemblies maybe you can swap a good trigger for a bad.
As I remember, the cams operating the trigger points is a simple off center circle and only needs to give the control unit a signal every revolution.
But I think your problem is else where.

[Graham]

Originally Posted by Chas H

You can measure the height of the rubbing blocks with a micrometer to identify a worn part. Since you have 2 trigger assemblies maybe you can swap a good trigger for a bad.
As I remember, the cams operating the trigger points is a simple off center circle and only needs to give the control unit a signal every revolution.
But I think your problem is else where.

I tried to measure the rubbing blocks themselves, but it's a little difficult to get at them. I measured the distance between the blocks and obtained:

Original distributor - 0.926" and 0.913"
Spare Distributor = 0.909" and 0.910"

Original has worn, especially on one set. Interestingly, it has no lube pads and you a see it never ever did. Also no plastic covers on lower points.

The cams are really ~180 deg of a 0.942" circle and ~90deg of a 0.862" circle with ramps between them. The points are held open by the larger diameter sector and begin to close near the bottom of the ramp down to the lower diameter. Just where depends on the amount of wear. They stay closed for 90deg approx.

I am testing my points on the bench and finding that one set of contacts on each set of trigger points stayed closed much longer than 90deg - one of them for 350deg! Hard to see why. Maybe I should check internal wiring for shorts.

The spare set now seems OK - One set of contacts were mis-aligned. I will put them back in car in morning. It will probably run OK. But we will see for how long!

Thanks for input