FredR

I have a slightly erratic idle at the moment. The ISV is probably well past its sell by date given it is the original item that came with the motor when new in my late S4. At the moment I am preparing for my annual maintenance and I will be removing the inlet manifold for a number of reasons.

I have a spare ISV in my parts box that I took from the original GTS motor that was TBF'd before I purchased the poor thing. I would like to know if it works and in this regard wondered if anyone has a simple procedure to prove whether such is likely [or not]. All I could find in the WSM was test point 6 which is an in-situ test that requires test leads and presumably a scope - I have neither.

All I know about the ISV is that it has a stepper motor and presumably some ind of controlled input moves the shuttle in small increments to either open or close it as needed to maintain the set point rpm.

Rgds

Fred

StratfordShark

Mrmerlin

Fred read through this thread .

this unit is magnet operated,
its failure mode is for the shaft to corrode then bind up the rotor or for carbon to accumulate on the rotor vane and bind up vane.

Adding 12V will run the vane to full open

https://rennlist.com/forums/928-foru...-it-works.html

Bertrand Daoust

I did like Stratford did.

Used a 9V battery to open/close it several times while spaying some WD 40 in it.

Working perfectly. For now...

FredR

Gents,

Thanks for the inputs. Stan's thread is familiar [ I added a few cents worth in it]. The main thing is that I did not want to plonk voltage on the terminals blindly in the hope it might move and blow something because I was a dumb ***. I take it nothing will blow if one reverses the polarity by accident?

Given the detente position is slightly cracked open [a position that gives a reasonable idle if the ISV is not working] I can understand how adding voltage drives it open- i.e. more voltage equals more magnetic field equal more movement but I am at a loss as to how it actually reduces the detente position unless it reverses the voltage somehow. 3 phase power stuff I am modestly OK with but square wave electronics etc I throw the towel in without a fight .

StratfordShark

danglerb

Bad idle seems like the ISV always ends up being replaced. What I miss for testing is a noid light type thing that would show the control signal is good.

worf928

the flyin' scotsman

Bench test has the intake off so the test through the ECU connectors is less intrusive.

StratfordShark

FredR

That's the idea- plan to put some juice on the thing tomorrow. Still a bit puzzled as to how it reduces the aperture from the default position if the rpm's are running higher than the set point. Logic says it has to close the default position some and based on the descriptions of "modus operandi" the only way I can visualise that happening is if the voltage polarity is reversed to shove the shuttle in the opposite direction.

worf928

Why?

Based upon my observations of the ISV with the SharkTuner, the LH in general is sending a ~50% duty-cycle square wave to the ISV at idle. Furthermore, based upon my observations, at idle the throttle plate is 'so closed' that the ISV is what feeds the vast majority of the air required to keep the motor running at idle.

FredR

Dave,

As I can tell the main butterfly is totally closed when your hoof is off the pedal, there is an adjuster stop on the main butterfly but I could not fathom out why it is there and more to the point, looks as though it is not intended to be adjusted, not by the owner at least. The shuttle that moves in the ISV is a sliding [variable] aperture. As I understand its default position has been carefully calibrated to allow approximately the right amount of air into the motor to support an idle with the a/c switched off. My assumption is that under normal circumstances, if the rpm's drop such as when the a/c is switched on, the shuttle opens in whatever steps are needed to compensate the drop in rpm's. On the other hand, rightly or wrongly, I have also sensed that the ISV will try to compensate if the rpm's are racing a little and to do this it logically has to close relative to the default position. Now the question is if this happens how does it happen functionally speaking or alternatively, is this something that the system simply cannot do and in reality the ISV can only open from the default position?

Nice write up by the way

worf928

I do not know the origin of this understanding. It contradicts my observations and what I seem to recall has been posted by Speake and Corenman.

It is easy to test though: Take a well-running '87+ and disconnect the ISV when the engine is idling. I will bet you that it will die, or idle very, very low at best.

On '87+ the ISV can only open. If you give it a DC 12v signal it opens fully. With a 0 volt DC signal the return spring will drive the aperture to its non-adjustable stop. If I recall correctly, based upon what has been posted by Speake/Corenman, the LH sends a 12v (AFAIK) square wave to the ISV. The duty cycle of the wave is what determines the average opening size of the ISV's aperture. The LH manual is devoid of information on this topic for '87+.

Now, on the other hand, ISVs for the S3 engines operate differently. They have three leads, and two springs. Changing polarity of the control signal can either close or open the aperture. I work on very few S3s, so their guts are not burned into my brain: If I recall correctly, their default position is roughly 50% open.

Perhaps this is the basis for you understanding?

Based upon my observations and pretty-extensive black box testing, the primary purpose of the ISV on 87+ is to ensure that the engine doesn't die when, at idle, the A/C is turned on and, for an automatic, when coming to a stop with the transaxle in gear. And for this latter behavior it requires the coding plug to be correct. So...

Do not impute too much 'smart' to to the control loop. The OE control code for the ISV is, based upon my black-box testing, very stupid.

It has a very limited range for adjusting. If this was not the case, a small intake leak wouldn't cause idle to increase. And the O2 loop always 'wins' (within the adjustment range of the 02 loop.) Or, in other words, I've seen no indication that the 02 and ISV control loops are integrated and adaptive such that they will drive aperture and fuel pulse to a stoichiometric idle at an RPM set-point. The ISV operates within a limited range and the 02 loop will change fuel pulse to drive to stoichiometric regardless of RPM or ISV operation.

For S4s yes.

Thanks.

FredR

Dave,

The ISV does support a stable idle if the cable is disconnected [ask me how I know!], the motor is fully warmed and the a/c is not driving.

Your post induced me to think a little more then a light came on! I took a look at my spare ISV and what I noticed is that the narrow slat that air passes through when at the default position is at the opposite end of the travel range compared to when the shuttle is fully open. Needless to say the shuttle is on a spring loaded shaft that drives the thing to the default [no signal]. Thus when there is no signal air passes through the narrow slat to give some kind of idle if the ISV is non functional. Once the shuttle starts to travel it seems to blank off all air flow but my guess is that as the control signal is applied it rapidly travels to a point where the main aperture is open and the control signal then modulates around that operating point opening and closing as necessary to achieve the set point required- does that make sense? It seems to answer how it might control a situation where the idle is too high. Thus more voltage more movement- as to how this manifests electronically I have no idea- I am somewhat clueless when it comes to electronics and circuit design.