What's the idle valve signal look like?

[wazzz]

Originally Posted by mysocal911

Careful use of a VOM won't damage transistor checking! Here's the replacement idle driver;

Thanks for the tip!
Do you know if TIP12x and ON588 have comparable internal resistors? Have you had the opportunity to measure ON588 resistors from outside? If so, what are their values? Same as for TIP12x, i.e. 7 kohms and 70 ohms?

As for the test of Motronic output signals with pull-up resistors as a substitute for the ICV, why not use 150-ohm resistors? This would be closer to the actual 22 ohms and would only require 1-watt resistors, or even less, considering a 50% duty cycle. Only a suggestion. Maybe there's a good reason to use 1k or 1.5k resistors instead.

[mysocal911]

Originally Posted by wazzz

Thanks for the tip!
Do you know if TIP12x and ON588 have comparable internal resistors? Have you had the opportunity to measure ON588 resistors from outside? If so, what are their values? Same as for TIP12x, i.e. 7 kohms and 70 ohms?

A TIP120 is a TIP120! You always go by the datasheet values. Unless you have a very very critical design, do you start to measure every little semiconductor parameter.

Originally Posted by wazzz

As for the test of Motronic output signals with pull-up resistors as a substitute for the ICV, why not use 150-ohm resistors? This would be closer to the actual 22 ohms and would only require 1-watt resistors, or even less, considering a 50% duty cycle. Only a suggestion. Maybe there's a good reason to use 1k or 1.5k resistors instead.

The lower the resistor value, the more you need to consider its power rating. A value near 1K is an easy first choice and available, and it provides the needed results.

[RobFrost]

Originally Posted by wazzz

Thanks for the tip!

Do you know if TIP12x and ON588 have comparable internal resistors? Have you had the opportunity to measure ON588 resistors from outside? If so, what are their values? Same as for TIP12x, i.e. 7 kohms and 70 ohms?



As for the test of Motronic output signals with pull-up resistors as a substitute for the ICV, why not use 150-ohm resistors? This would be closer to the actual 22 ohms and would only require 1-watt resistors, or even less, considering a 50% duty cycle. Only a suggestion. Maybe there's a good reason to use 1k or 1.5k resistors instead.

The scope only draws a tiny current and it measures the voltage, so it can theoretically measure across an infinite resistance. It's generally whether the resistance is too small you would worry about, so as not to draw more current than the circuit can handle.

Although the icv may have lower resistance than 1.5kohms in a dc test, it's a coil and its supplies are switched and therefore AC. This brings the inductance of the coil into play and therefore its impedance at that frequency would determine the current flowing rather than its pure resistance, and its impedance will be higher than its resistance giving a lower current. I haven't calculated by how much but a using higher resistance in its place can help make up for the discrepancy.

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[mysocal911]

Originally Posted by RobFrost

The scope only draws a tiny current and it measures the voltage, so it can theoretically measure across an infinite resistance. It's generally whether the resistance is too small you would worry about, so as not to draw more current than the circuit can handle.

Although the icv may have lower resistance than 1.5kohms in a dc test, it's a coil and its supplies are switched and therefore AC. This brings the inductance of the coil into play and therefore its impedance at that frequency would determine the current flowing rather than its pure resistance, and its impedance will be higher than its resistance giving a lower current. I haven't calculated by how much but a using higher resistance in its place can help make up for the discrepancy.

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Actually, the ICV windings are essentially resistive, i.e. about 20 ohms per windings. So the Darlingtons just see a peak current of about .60 amps.
When using a scope with the valve connected, the signal is a pure resistive square wave, i.e. no inductive component.

[RobFrost]

Originally Posted by mysocal911

Actually, the ICV windings are essentially resistive, i.e. about 20 ohms per windings. So the Darlingtons just see a peak current of about .60 amps.

When using a scope with the valve connected, the signal is a pure resistive square wave, i.e. no inductive component.

Just to square up our mutual understanding, every coil has an inductance including the icv (in fact including any simple wire) and at the 10khz at which the circuit is switching, it requires an inductance of about half a henry to add 30 ohms to the impedance. So I assume your claim is that the inductance of the coil is much less than half a henry, correct?

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[wazzz]

10kHz? Are you sure?
Your scope pic posted earlier shows 10ms/div. Signal period appears to cover 1.2 div, so that would be roughly 80 Hz.
As for the inductance that comes into play, don't really know about that, but the ICV behaves more or less like two small DC motors being pulsed.
Anyway it is not AC, I mean no sine wave here.

[RobFrost]

Right the plot thickens...

Is I mentioned I had a spare socket so I set the scope up across 1.5kohm reference resistors and it makes sense to use the 12V common as earth and read the switched earths as negative voltage.



Initially baffled myself having one of the probes set to 10X.



Anyway, I did a variety of tests with the engine running and not running, with the icv valve bypass pins bridged and not bridged, and with the throttle adjusted to high, low and normal revs.

First thing that's apparent is that at all times, one terminal is getting the full -12V, I.e. switched to Earth while the other is getting much less, I.e. switched only to circa -7V. I switched the probe leads to confirm the difference was in the car.

Next, with the engine not running, we get perfect 50% duty cycle as seen here:




Now bear in mind both the icv bypass connector and the throttle closed position switch are both definitely working, because the engine responds noticeably to both (in fact i can fix the surging by disconnecting the throttle closed valve). Also you can watch the motronic adjusting the timing with a timing light, when the throttle is closed.

However the duty cycle doesn't respond to engine speed. It stays at these percentages whether you manually bypass the throttle to get 500 revs, 880 revs, or 1100 revs:



My expectation was that the pulses would widen and narrow with engine speed.

This is perplexing because the engine speed sensor is within reference values and I scoped it at cranking speed.

So unless you have other suggestions I think the next steps are to recheck the flywheel speed sensor, then make sure its signal is getting to the ECU, and finally it might be a circuit defect in the ECU itself not reacting to engine speed.

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[RobFrost]

Originally Posted by wazzz

10kHz? Are you sure?
Your scope pic posted earlier shows 10ms/div. Signal period appears to cover 1.2 div, so that would be roughly 80 Hz.
As for the inductance that comes into play, don't really know about that, but the ICV behaves more or less like two small DC motors being pulsed.
Anyway it is not AC, I mean no sine wave here.

You're right I misread the scale so I'm out by a factor of 100. So we would need the inductance to be much less than 50 henry for the impedance to be very close to the resistance.

Square wave is still AC, in fact it has higher root mean square power than the same sine wave by a factor of sqrt 2.

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[wazzz]

Responding to your post above with new scope pics.

First, the difference in voltage (7 volts vs 12 volts) was expected, as explained and illustrated by Dave on previous page of this post. That's because you are using high load resistance values and not real low values, which would require 5-watt resistors.

Also, I don't quite understand what you are trying to test or do. If you disconnect the idle switch on the throttle or bypass the ICV by jumping terminals B and C on test socket, in both cases the Motronic stops adjusting duty cycle. It will stop controlling the ICV. My guess is that it will keep sending two 50% duty cycle opposite signals. And that's what you see on your scope.
If you want to see the Motronic in action, i.e. adjusting the idle by varying both signals duty cycle, you need to plug everything back with you scope probes connected, run the engine, keep it idle (idle switch connected and closed), and then make the engine load vary, like maybe switching on and off the AC or high beams or both and then you should see the ICV signals duty cycle vary, as the Motronic is trying to compensate the extra load from the alternator by adding more or less air to maintain the idle speed at nominal.

[RobFrost]

I'm seeing 50:50 with ignition on and engine not running, and about 70:30 at all times the engine is running, whether bypass is in place or not. When the bypass is removed and the motronic is adjusting idle with ignition timing only, it's still 70:30 whether revs are adjusted to 500, 880 or 1100 plus by manually allowing air to bypass the throttle.

Are you able to confirm you can see the 70:30 duty cycle it operates at, at all states, while running? It's the last image.

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[RobFrost]

Righto, as icv duty cycle isn't responding to engine speed I decided to double check the speed sensor at idle speed. You may remember I had 1.5-25V at cranking speed, and assumed this would amount to the required 2.5V minimum at idle.

Also checking at the ECU now, to rule out loom issues - pins 8 and 23.



And got a healthy signal over 5V with no significant interference.



Then I spotted a schoolboy error, any guesses?



So it turns out the duty cycle results are all meaningless and I need to repeat the original tests!

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[RobFrost]

So if you said the throttle closed valve is unplugged, you were right. Now, repeating tests with it connected.

Now here we are the same with icv bypass circuit in place:



And now it's clearly responding to idle speed, seen at low, normal and high idle speeds:









So I think I can finally rule out faulty icv signals as a possible cause of the rough idle.

In my view this now points squarely at the missing O2 sensor as the only parameter which is not how it should be - unless anyone has a different point of view?

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[RobFrost]

Here are some reference measurements of the icv duty cycle from my car - any pointers as to whether they're correct or not would be appreciated. Bear in mind mine is a 1988 so target idle is 880 rpm.

1200rpm and over 72%
900rpm 60%
840 48%
820, 700 and 650 rpm 43%

That operating range of 72%-43% seems narrow to me, but that's what it measures as.

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[wazzz]

Not sure to understand you setup, but since it appears that one signal is 12 volts in amplitude and the other one is 7 or 8 volts, that means you are testing with the two 1.5k resistors in place of real ICV. Therefore this is open loop. You are varying the rpm and the ECU is trying to maintain it to 880 by changing the duty cycle, and it has no effect since the ICV is not plugged in.
But at least you see correct signals of the proper shape and "phase", with a varying duty cycle showing that the ECU is responding correctly.
It seems to me that you can leave the ICV and its control alone, they are doing well.

[RobFrost]

Yes, that's right. I was testing the icv signal was responding correctly to engine speed.

The latest news is... I fitted an O2 sensor and the car is now running like a dream. I knew no O2 sensor made it run rich, and some said Euro cars were shipped with no O2. Mine certainly hand no O2 sensor, but fitting one has fixed the idle problem.

It feels so great to finally have fixed this.

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