What's the idle valve signal look like?
 

Rough idle on my 3.2 - surging and dying.

I've been through every test on the engine switches, sensors, fuel pressure, vacuum, AFM, etc., all to spec. Only possible exception is the crank speed sensor, which is showing a perfect trace during cranking but only 2V versus the 2.5V minimum. HOWEVER... the DME test plan says that 2.5V is with the engine running, and I'm only cranking at 200revs. I can see amplitude is increasing with revs so I'm confident it's within spec, at idle speed.

Swapping idle valve makes no change, and idle valve is vibrating at idle.

So I pulled the plug on the idle valve and scoped the signals with the ignition on. 12V centre is perfect. One end wire is giving a square wave with 50% nonzero and the other end wire is giving me nothing - 100% zero.

Does anybody know what these signals to the end-pins of the ICV should look like, and does this sound like one of the signals is missing? I expect there should be a square wave on both the open and close terminals, not just one of them.

Others will probably shime in and confirm or not, but I think you should indeed see pulses on both sides. So if one side is dead, this might be the sign of a dead transistor inside Motronic. One of the ON588 Darlingtons referred as T560 or T561 on the schematic.

I have a scope and an 88 Carrera. If I find time today I'll check it and report.
What's the frequency of the square signal?

I didn't catch the frequency and forgot to take a photo. Actually my daughters appeared and asked me to take them to play tennis and how could I refuse so I packed up in a rush.

But at a guess based on my memory of the scope settings and how many squares on the display I'd say it was maybe 50Hz.

But it was 50:50 on and off if that makes sense. This was with the car idling and the plug pulled off. If I get a chance I'll check again tonight.

I have to completely remove my seat to get to the DME otherwise I'd check it at the DME and look for any visible circuit problems.

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Hi, I scoped both together versus the engine as ground, it shows 0.5V pulses on one and not on the other. I was hoping to check it at over- and under- revs, to see if the other started showing a signal but before I could the engine speed started fluctuating wildly. I suspect the ICV piston was oscillating within the valve.

https://uploads.tapatalk-cdn.com/202...687bff1eeb.jpg

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the idle control should have a continuous 12V power supply and then two pulsed grounds controlled by the Motec. pin numbering at the valve for some reason is 3, 4 and 5.
Pin 4 should have constant 12V controlled by the DME Relay.
Pins 3 and 5 should be ground, supplied and controlled by the Motec.

When you are adjusting the base idle on the system it has you place a jumper between pins B-C in the round test plug located in the engine compartment electrical panel. My understanding is that when the jumper is in place it should drive the idle valve to the center position by supplying a 50% duty cycle to ground on the 3 and 5 pins at the idle control valve. for your testing purposes you may want to jumper pins B-C then use your scope to evaluate the signal. this will eliminate any engine performance issues from the idle control signal and let you evaluate the Motec independent to engine running condition.

Normal! One side needs to see the drive signal from the other side when connected, i.e. the signals are 180 degrees out of phase.
One transistor gets its drive signal from the other side. A better test is to back-probe the connector when connected with the key-on.

Read here for a idle valve test; Idle Valve Testing, post #19, http://forums.pelicanparts.com/porsche-911-technical-forum/360766-bench-testing-carrera-idle-control-valve.html

I had this on a 964 (same as a 3.2L) engine once. What finally saved me was when I had enough guessing and just bought a Air Fuel Meter, then welded a O2 sensor bung to the cat converter and plugged it in. The Air Fuel meter told me that I was running way too rich, and I adjusted the Air Flow Meter to bring it down and then it was perfect.

Stop the guessing.

What does "stop the guessing" refer to? To my eye, I'm working systematically through the DME test plan.

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Ok thanks for that suggestion. I was reluctant to pierce the insulation if unnecessary due to future wire corrosion.

Re the valve, I should have originally stated that I already bench tested the icv as working and also switched for another unit as per the DME test plan.

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Thanks. Worked a dream. Looks like I could be honing in on the problem. The icv bypass wire did indeed switch it to 50% duty cycle, but the dead terminal continues to be dead. I'll confirm continuity of the wire from the ecu next and assuming that's good, test the ecu output transistor.



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Here is the part of the diagram that can be of interest for you to troubleshoot these ICV output signals.

ADV11 is control signal from processor.
Both signals are output on pins 33 and 34.

So as stated before, one transistor (here T561) gets its control from the other transistor (here T560).

Now you need to find out why you only get one of these and not both. May be T561 is shot. http://forums.pelicanparts.com/uploa...1688315892.png

Again, you're failing to understand the signals. You only see a pin 34 signal (0.50V) of T560 because of the base to collector junction of Darlington transistor and signal thru R560 from the logic. The pin 33 Darlington transistor does not have the same base signal as the pin 34 Darlington. Both outputs (pins 33/34) NEED pull-up resistors (or the valve) as mentioned before to properly check the idle valve outputs!!!!!


http://forums.pelicanparts.com/uploa...1688336489.jpg

Study the schematic in post #11 to understand your scope images.

You basically answered your own question, initially! T560 needs a pullup resistor or the valve, which will provide the necessary base drive for T561 resulting in a waveform like T560 (.50V square wave).
He just needs to add two 1K resistors to simulate the valve at the connector to properly test both idle signals properly. It's not that difficult

Yes I did understand your point about the resistors the first time you stated it. I will deal with that. Thanks for your help.

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I did a little more on this today - ordered a couple of 1.5k ohm resistors as suggested, popped out the passenger seat, confirmed continuity of the pin-34 wire, and confirmed continuity to the transistor on the board. Inspected the board and pins for cracks and overheating, especially around T561. tbh I was disappointed the wire was continuous because I was hoping that would be the fault - hey ho, you can't win them all.

I attempted a test of the transistors in situ. T'internet said to diode test from base to collector and base to emitter, with positive on the base, and look for about half a volt drop. Well that didn't work with my multimeter, which was completely unresponsive - until I double-checked my assumption that the base was in the middle as per the circuit diagram and discovered it's at the end.

So with the positive on the base, it showed .545V and .619V drop across the two pins on T561 and comparable on T560 too. So all looks good there tbh, although to read too much into that basic test, and with the transistors in situ in the circuit, would be guesswork in my view.

All in all, having inspected thoroughly, tested a good many circuit board joins for continuity and checking resistors etc., I was unable to identify any defects so next step is to repeat scoping the ICV signal across reference resistors when they arrive.

Actually, the key test is from the collector to the base/emitter, collector positive, using the ohms scale (ECU not powered).
The transistor typically fails collector to the emitter.

Thanks for the advice. I can't really test ohms in situ due to the other components in circuit. The two transistors were giving different readings to each other across the other pin combinations. One reading was suspect but I'm not about to work our the impact of the other circuitry.

If I remember rightly, one was unchanged by reversing polarity which was the reading I found suspect.

Anyway, I fabricated the load resistor circuit this evening out of the resistors and a spare sensor plug so can retest the circuit at idle next.

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Can't find the datasheet for the ON588, but internal schematic shows a diode between C and E, so that may impact what you are trying to ohm.
I don't like checking resistance of components in situ, too risky for sensitive parts (probably not here) and a serious risk of wrong results because of other components interfering with what you are trying to check.
When I was a teen and diode mode was not yet available on multimeters, I used to roughly check NPN transistors by ohming between B and C and then between B and E in one direction, then in the other (swapping black and red probes). One direction would yield partial continuity while the other would show a very high resistance or even infinity. This was a rough indication of whether the transistor was more or less ok or completely shot. In many cases it seemed ok but was still shot for linear applications like audio.

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

http://forums.pelicanparts.com/uploa...1689091125.jpg

Because you are using 1.5K resistors, & not 22 ohms per side of the actual valve to simulate the idle valve, the scope signals will not both rise to 12V;

http://forums.pelicanparts.com/uploa...1689125584.jpg