Motronic 3.2 debugging idle
 

Hi,

Question for the Motronic ECU crowd !

I have a 1985 Motronic on the test bench (std chip) that allegedly has a 'rough idle' & 'shakes to a stall' problem. I will only be able to test it in the car next week.
Apparently the car is driving around 'no problem' with another Motronic DME.

Doing the checks, the DME seems to respond to all inputs in a normal way (speed signal, reference signal, CHT, IAT, ...).
At 800 rpm, it holds a very stable pulse towards the coil at the required frequency and a nice injection pulse.

However, when I vary the rpms a bit (speed signal), there is almost no reaction to be seen on the ICV output. The duty cycles slowly changes a bit (max 10%).
I can get an 'immediate' response if I change the CHT input from cold to warm for instance so the driver circuit works.

So is the assumption correct that the ICV is not used to regulate idle ?
Or should it react promptly to changes is rpm (700 - 900 variations) ?

I would appreciate any pointers on how the idle regulation circuitry actually works on these Motronics.

Kind regards,

Frank

https://uploads.tapatalk-cdn.com/202...56e3c43894.jpg
Photo of my Motronic ECU tester for attention ;-)

Can anybody explain how the idle gets regulated by the motronic ? (1985 std chip)

Frank


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The ICV is used to regulate idle, that's its job. The ECU feeds a pulse-width-modulated signal to the ICV to control the idle speed. You might need to adjust the AFM gate spring position in the airbox thingy. By the way, it might be more informative to measure the Air Fuel ratio using an AFR meter.

I can see that the ICV is a block wave signal. I was expecting it to vary quickly when I deviate from the idle rpm (800) to compensate. However, I can only see very small/slow changes in the duty cycle (idle switch connected to GND).

I was wondering if I can check anything in the dme itself ?


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Make sure your closed throttle contact switch is making contact. It’s the two wire connector.

If the idle contact is open for any reason, the ECU will not control the ICV because it thinks the throttle is open.

The ecu is off the car and I simulate the switch by putting pin 2 to ground. It does react immediately to the idle switch on / off but only seems to regulate the ICV on very large devitions of the rpm.
I traced the signal back on the board to the processor and this is indeed how the processor outputs the signal (in the 5v domain).

It seems to act as a slow PID controller, I was expecting a fast-following controller ?
I can vary the output more by simulating a cold cyl head temp than it reacts to the rpm changes (600 -1100 rpm)

Frank


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When I do this on my DME simulator I look for are a couple of things. First, with the idle switch engaged and WOT not engaged verify that fuel pulses cut out above something like 3000 RPM. This verifies the idle switch is logically correctly recognized by the DME software. Idle and WOT switch inputs are internally wired into a resistor network to one of the 8 A/D converter inputs. Theoretically, if the A/D converter has an issue or the resistor network is wrong the DME might misinterpret your input on pin 2 as another condition.

Next, if you watch an ICV while setting the simulator to around 880 RPM and AFM input near lowest reading you will see that the ICV reacts fairly quickly to slight variations of the RPM thereafter. However, this only happens in a small band around the setpoint RPM. Once you increase RPM it closes and when you exceed a certain RPM the ICV snaps back to a different more open position. I assume that is so it can "catch" RPM dips when you close the throttle body by letting of the accelerator.

And as you correctly identified the output is a differential PWM signal with about 100 Hz. So in other words one output is the inverse of the other. That signal is also used as watchdog input. With this heartbeat absent the MCU resets.

Ingo

Ingo,

many thanks for the pointers !

I repeated some tests :

1. The IDLE switch is recognised and works as described by you. Under 'idle' the injectors cut out at about 1600-1700 rpm

2. I checked all the inputs to the A/D converter (Idle, WOT, IAT, CHT, AFM, Volts, ...) - pins 1-4 and 26 - 28 on the A/D and they all have correct and stable values at the 'input' and react to the simulated inputs.
Except the altitude box - not connected / not simulated for the moment.

3. When I run through the RPMs 550-1300, Idle switch on (GND) I get this response (ie nothing) : see short video : https://youtu.be/cNqamH6q1a0

However, I can vary the pulse width significantly by changing the CHT (?)
(Since I don't have a spare ICV, this is by using a LED on the output - pin 33 in this case)

Flip the idle swicth off and it goes to a 'standard' duty cycle immediatly (exact value depends on CHT)


I don't quite understand your final comment :

"That signal is also used as watchdog input. With this heartbeat absent the MCU resets."

The only input to the ICV circuitry that is rather isolated is the ADV11 output from the uP ?

Kind reagards and thanks for sharing these insights already !

Frank

UPDATE : I see what you mean, the Motronic checks the presence of the ICV output signal internally through IC S890 and resets the CPU if required.

Correct! The early Intel processors lacked the logic that could monitor whether the processor was reading its instructions and was operational after a power-on.
If not, it was continually reset until "up & running". Most all processors now days have a internal watch dog timer which does that function.

Thanks,

Learning so much here.
The icv signal is present, I monitored the reset circuitry and it is not resetting the cpu as far as I can tell (unless the reset are very short pulses).

I do see an immediate (moderate) reaction in the duty cycle and as Ingo said, once idle gets high it goes to a fixed position to anticipate throttle closure. So far so good.

What I do notice is that the pulses towards the injectors are ‘reset’ once every 2 seconds to an extremely long injection pulse (as if afm is full open/wot) and then they decrease step by step until it hits a certain length again and it ‘resets’.

Also clearly audible in the injectors.

So if it is providing a varying injection pulse with stable inputs, that would probably be the cause of the ‘unstable idle’ and it is partly masked at higher rpm?

I am going to see the car now and test all the sensors ‘live’.
Pretty sure it will be rough with this unstable behaviour, I will post a video of this behaviour afterwards.
Also noticed someone has been ‘in here’ before as one of the transistors in the injection circuit is non-std.

Frank


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Are you referring to T402? This Darlington transistor can have a hard time driving the six low-impedance injectors wired in parallel. It may overheat. I think the original RBDT65A is NLA.

Following this very interesting thread, trying to learn and understand how the IDE works. However I only have the schematics for the 944/951 version of the Motronic IDE.
Would anybody here have a decent readable jpg or pdf copy of the 911 version that they would be willing to share via MP?

Motronic 3.2 debugging idle
 

Send me a PM for the schematics

No problem finding most of the original transistors here.


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PM sent. Thanks.

Not really, unless the injectors are over-voltaged, e.g. bad alternator.

An actual IDE; https://www.silabs.com/developers/8-bit-8051-microcontroller-software-studio

Silicon Labs IDE

"The Silicon Labs Integrated Development Environment (IDE) is a complete, stand-alone software program that includes a project manager,
source editor, source-level debugger and other utilities. The IDE interfaces to third party development toolchains to provide system designers
with a complete embedded software development environment."

Very, very minor differences!

Actually, I do have a copy of the 911 Motronic schematic, but its low resolution does not allow to compare component values with that of the 951/944 version. So whatever minor differences there are, I'd like to identify them in case I need to stick my nose in my 911 88 Motronic.
So if you have a good copy of the 911 version and are willing to share it, please don't hesitate. :)

Think about what key function changes between a four and a six cylinder engine ECU.
You can always buy a 944 DME ECU on eBay versus relying on a marginally accurate schematic. They're cheap!

That would have to be the S400 (0127) circuitry, including the Darlington. I have already checked this part. As far as my eyes can see it (as some component values on the 911 version are unreadable), I can't find any difference between the two.

Or don't have any values indicated.

OK, back to the original problem ;-)

First we ran the car with another (known good) DME - no issues, clean idle, good injector waveforms, nice acceleration.

With the problem DME, idle was indeed rough but the ICV was regulating as Ingo said in a relatively small band around 800 rpm. However, the idle quickly got 'out of control' sweeping to 2000 rpm, dropping to 500 rpm and always eventually stalling. (Way too wide for the ICV to control anything)

Checked and known working : Idle switch, ICV, stable Vcc on the board (both 13.7 Vbat and 5.05V on the 5V level), O2 disconnected, Vref stable on the ADC, all inputs stable on the ADC, ref & speed pulses stable and correct on the uP.

When looking at the injection pulse (pin 1 uP) and the resulting 'peak & hold' output with the whole injector chain connected (I couldn't replicate that at home) the problem became clear : the injection pulse jumps up and then shortens, then jumps up again.

With 3 injectors connected now I can replicate the problem on the bench .
Video of injector pulse (pin 14) here : https://youtu.be/bQrVtaFdOKA
Video of output on pin 1 of the uP here : https://youtu.be/g_ePbtH7nv4

The uP pulse comes through correctly on the R421, although through ADV12 on this board (older board than the schematic). However T405 has been replaced by a 2N3019.

http://forums.pelicanparts.com/uploa...1641488531.JPG

Another thing I noticed, even with 3 injectors running, I do not see the positive peak on the scope at closing (first pic below) as I do on my good running car (last pic below).

http://forums.pelicanparts.com/uploa...1641488849.JPG
http://forums.pelicanparts.com/uploa...1641488849.JPG

Any help in this area is greatly appreciated !

Probably gonna check my stash of electronics and see if I can find a BC639C to make
it closer to the original again.


Kind regards and thanks for helping me along this quest...

Frank