Citroen SM injector points

Hi Graham,

well using a dwell measurement for trigger contacts is also something that I recommend in my compendium. I can tell you that the ECU ONLY looks for falling edge of contacts. So as long as you do not have overlaps in closing contacts, The ECU does not care when contact opens and for how long. However if contacts are unevenly worn it could happen that they overtake each other in 8-cylinder trigger contacts.They only have 90° distance. I have only seen such a case once.

Volker,
I only know from my own experience and that of others on the forums that worn trigger point rubbing blocks CAN cause a rich mixture. I also understand why, in theory, that should not be the case.

In practice, something does cause it. The Porsche forums have talked about trigger point bounce. Perhaps as the rubbing blocks wear (and closing angle increases from 100deg to 150deg and more) there is less spring pressure and therefore the points start to bounce? Just a theory of why that could happen. But if it did, the triggers could be opening and closing several times in half a revolution. What would the ECU make of that?

Don't want to prolong this. Just passing on my experience

Howdy all,

Thanks for all the feedback.

I had an hour to spend on the car today with my gas analyser.

Note: I am using a Sun MGA 1200 analyser re-calibrated 6 months ago. No doubt the exhaust system will contain a fair bit of combusted crap after the car had run excessive rich for some time.

At idle, I could not get the CO reading at idle to go under 7/8%.

Smokes a bit, some miss on acceleration. Oddly enough I substituted the MAP sensor from a late MB unit, and the thing actually ran better and a little leaner – even at idle? (a little spurious and unconvincing result though)

I appreciate the link to ECU pin chart from Nordfisch. As suggested I’ll take all readings and report back.

Regards,
Rhys

Hi Rhys,

not being able to go below 7-8% means a far too rich mixture. Assuming that you made sure that idle-run contact reaches ECU, this could be caused by

• false air
• injectors leaking ar wrong spray
• broken ECU
• engine temperature sensor wrong or disconnected
• ignition timing is said to have no effect on CO
• MAP sensor (if not completely leaking should not effect CO in IDLE-RUN as well as it is superseded by potentiometer

So yes, you should always measure on ECU connector as Norbert pointed out. That's what I made appendix B for.

Hi Graham,

I did have the experience of what happens to a worn trigger contact during this year myself. 2 of 8 cylinders that were still running when cold, ceased to work when engine got warm.

When plastic cams get worn, the cam on distributor axle will not be able to push them open any more. Should one have some dirt or whatever on either axle or plastic cams, then it would be possible that they open and close several times: i.e. bouncing. That would not really confuse the ECU. It has a Flip-Flop reading these contacts and normally interpretes each closing as a signal from T.C. But it would need more than one contact to bounce as the flip-flop needs one bouncing contact after the other to change its state.Just one bouncing contact would not create that effect. Only if we would see bouncing on both contacts in alteration, it would create an additional injection pulse and it would also wrongly interprete engine revolutions. A flip is like two push buttons. Pushing one button sets the output, pushing the other resets output. In between you can push the first button as often as you like.

On the otther hand if we would come close to 180° closing angle, the 2 contacts on a 4/6 cylinder might overtake each other. However there is a huge tolerance on this injection timing. Fuel just waits before it gets sucked into cylinder.

As a summary: A bouncing would only create additional injection pulses if you had it in such way that it would be bounces in alternation on both contacts. I would see that as rather unlikely.

An 8 cylinder is actually like 2 4-cylinders in parallel, both in T.C. and in ECU.

Hi Volker,

I am proceeding with checking values from the ECU terminal block.

I’ve dismantled an old (Volvo) ECU to manufacture a break-out box as I find the multimeter leads on the ECU terminal contacts leaves some margin for error.

In the mean time, I’m having some difficulty with the pin out sheet with regards to pin 17 (the idle contact on throttle switch). The instruction is to measure against pin 12, but it would seem to me that pin 14 would be the correct pin. Pin 12 is indeed the central distributor trigger point, but with the ECU disconnected, their paths should not cross? (poor extract of Citroen wiring diagram attached).

Do you agree - or am I seeing this wrong?

On a separate issue.....

I understand that the idle switch activates the ECU to run on lean idle mode independently of the MAP sensor. When I remove the MAP sensor of an idling Volvo P18000ES at the workshop (both vacuum or electrical connector) it dies. What is the relationship of the MAP sensor to the idle circuit?

Cheers,
Rhys

Hi Rhys,
following the ECU-pinout to be found here at the compendium pin 12 is the ground contact for both sensors - wires connected inside the wiring harness.
Pin 14 is the connecton for the absolute pressure sensor.

Your car doesn't have a full-load-switch.

Be sure to use the correct wiring diagram.

The wiring differs from car model to car model

Regards
Norbert

Mmmm bit of a correction here –

Pin 17 is measured against pin 11 (14 will earth – but not on idle)
Pin 14 (the full load switch) should be measured against 11, but this should be 0 ohm, not infinity (as per instructions) – as the switch is normally closed, and opens on idle vacuum.

Cheers,
Rhys

Sorry Norbert,

The correction is to my notes - not yours.

My SM does certainly have a full load switch.

Wiring diagram should be correct....there are few versions available.

Cheers,
Rhys

Hi Rhys,

I have taken my information from the Bosch workshop manual. According to that Citroen DS and SM differ. On SM you should measure pin 17 against 12, on DS pin 17 against pin 14. Connection of absolute pressure switch differs also: It is pin 2 on DS and pin 14 on SM.

Regarding full-load or absolute pressure switch: that closes when engine is in full-load (no vacuum) or stalling (also no vacuum).

Hi Rhys, Hi Volker
I try to follow your explanations but still have problems in understanding.

- Rhys said the car has a full-load-switch. So says his figure from the citroen-manual ???
(or is this just a problem in using the term 'full-load-switch' instead of 'absolute pressure sensor'??
a car equipped with full-load-switch must have a TPS with 5 (connected!)connectors.

- Following the explanations of Volker, my linked measuring-plan seems to be o.k.

But an error in the Bosch service-instruction at this point would be not the first one we detect...

Please, Rhys, stay tuned with us and tell us your results.

Regards
Norbert

Hi Norbert,

well the absolute pressure switch in a way is nothing else than the full-load switch in TPS: In Citroen and early VW type 3 we had the full-load detection via this absolute pressure switch, in late cars this was done in throttle switch via a contact. Therefore absolute pressure switch is often also called full-load switch. In principle you could replace the absolute pressure switch by a proper throttle switch with full-load contact.

I have no idea why Citroen even in SM held on to absolute pressure switch and its failures in high altitudes.All other SM engineering strives for the most recent possible solution. VW type 3 is the only car that was available with all 3 kinds of full-load detection: Absolute pressure switch, full-load diaphragm and full-load contact.

And now I notice that I wrongly marked the 0 280 100 036 MAP sensor as a type 2. Of course it does not have a diaphragm. Will correct that. Therefore Rhys, I am 95% sure that is works correctly if it holds vacuum and potting has never been removed.

Hi fellas,

I use the term "full load switch" - as this is how it is referred in Citroen documentation. It is the same switch you refer to as absolute pressure sensor (APS). The throttle switch does indeed have only four terminals.

The APS switch is normally closed but opens on 3-4 hg vacuum (tested and ok).

My current mission is to trace the earths......I find one listed in the documentation that I cannot identify on the injection harness anywhere. I'll persevere with the Bosch pin outs and discover what I can.

Regards,

Rhys

rhysaccess schrieb: ...
The APS switch is normally closed but opens on 3-4 hg vacuum (tested and ok).
...

Hi Rhys,
following Bosch and Volker it just works vice versa.
And: The highest vacuum is when the throttle is closed, at idle!

If defectice or maybe incorrect connected the switch would enrich the mixture in all conditions, but not as much at idle because then the idle switch in the TPS has more influence.

Could be a possible reason for the engine running too rich ... would have some influence at idle, too - like you reported

Just disconnect the switch and look what happens. Maybe the engine now runs normal - lean only at full load.

Regards
Norbert

Sorry to interfere again:

Rhys is correct. When we are in full-load, there is no vacuum applied. That's exactly what he alsp measures on a stalling engine as there is then no vacuum. If vacuum fals bwlow a certain level (indicating part-load pr idle-run), then ot opens. So it should show 0 resistance on a stalling engine, but not on an engine in idle-run.

Hi Volker,
I didn't ever check an absolute pressure sensor:

Is this correct: The sensor shows endless resistance when doing the measurements at the ECU -plug
- because there is no vacuum when the engine doesn't run.

But Rhys measured zero resistance in this situation, I remember???

Regards
Norbert.

Hi Norbert,

with stalling engine, there is no vacuum. That is why Rhys has to measure 0 resistance. With a stalling engine we so to say have full-load conditions. Only when the engine runs in idle, we have maximum vacuum and then Rhys should measure infinite resistance.

A bit tricky as you do have to think around the corner. Engine not running has suddenly full-load vacuum = none.

Ok,
I was just confused about the type of switch. It is switching 0 resistance (1) to endless resistance (0), not vice versa.

Regards Norbert

rhysaccess schrieb: Hi Volker,

On a separate issue.....

I understand that the idle switch activates the ECU to run on lean idle mode independently of the MAP sensor. When I remove the MAP sensor of an idling Volvo P18000ES at the workshop (both vacuum or electrical connector) it dies. What is the relationship of the MAP sensor to the idle circuit?

Cheers,
Rhys

Hi Rhys,

I oversaw that question. When you remove the MAP sensor, the ECU cannot provide the basic injection pulse any more. However, when idle-run contact is closed, it supersedes the MAP sensor pulse forming.

Hi fellas,

The APS switch is indeed counter-intuitive. Normally closed while the engine is off, open at idle (high vacuum) closed again at speed (low vacuum). Either way, mine works fine.....and been electrically disconnected while I try and source the running rich issue.

I completed construction of my break-out box today (attached) and repeated testing of the ECU connector. It is clear that the pin out instructions have some errors.

Idle switch should be 17 and 11 (not 12), throttle switch should be 9 & 20 and 11 (not 12), APS should be 14 and 11 - but 0 ohm (not infinity). I wish the Bosch details were correct....because there would be my problem....but I fear it is not the case.

I'll check my readings one more time, and post the results within the next day for some further scrutiny.

[quoteI oversaw that question. When you remove the MAP sensor, the ECU cannot provide the basic injection pulse any more. However, when idle-run contact is closed, it supersedes the MAP sensor pulse forming. ][/quote]

As the car dies with just the vacuum line removed from the MAP sensor (while idle switch is closed), the sensor cuts all injector pulses?

Thanks for the continued interest in my D-jet issues.

Regards,
Rhys