aggravation

On an S4, 1988, I believe the ISV gets pulsed voltage to regulate the opening of the valve.
Does the positive side of the voltage stay constant and the negative is the "switched side" or is it the other way around?
My car will open the valve all the way and just stay there if I plug it in, I plugged in a spare ISV and had it up on the cam cover where I could watch it. I've tried two different ISV's and two different LH ECU's always the same thing.

The reason I ask about the pulsed signal is, if the negative is the switched leg, then a short to ground could account for the constant open state...I think...

Anyone know?

the flyin' scotsman

When I did my intake R&R I tested the ISV using a battery so only 12v and grd and it opened fully.

If you have access to a variable power supply you could test the ISV and various voltages.

dr bob

Don't have the answer to your question but...

The ISV, plugged in and sitting, engine not running. should **probably** go to full-open position. It's trying to add bypass air to raise the idle RPM's from none up to the normal idle speed.

One of the all-knowing guru's will jump in here with any luck.

Alan

I believe it is indeed a pulsed ground from the LH, the power side is the LH power supply from the LH relay. However as Dr. Bob says pulsed can include anywhere from always off to always on too...

Alan

worf928

The S4+ ISV is barely open (~10%) in its unpowered rest state. At a normal idle in a normal S4+ it seems to vary between 40% and 60% open.

the flyin' scotsman

So above idle its WFO?

worf928

No. It isn't WFO. It's mainly there to adjust idle air for load with A/C on/off (and for slush box guys when its not in P or N and the cars stopped.) Precisely what it does off-idle (heh, or on-idle) very few folks know for certain (and I ain't one a dem dat knows) but it isn't WFO AFAIK.

I've been watching mine with the Shark Tuner (or, as one would pronounce it in Bean Town: The Shack Tunah) for a while and it's a weird beastie.

-- D

worf928

Actually, Malcolm, the ISV seems to stay in the mostly-half-open state, most of the time. Idle, cruise, or WOT. It really is weird. Of course, I may be driving it insane on the Twin Screwed '91. However, it seemed not much saner when I gave it a sanity check on my stock '89 GT the other day. But that was not an extended session.

the flyin' scotsman

?NoInteresting.................I wonder if the racers worry about the ISV or just eliminate the 'beastie'.

If it does operate at a small range I wonder what it could be replaced with or indeed eliminated.

the flyin' scotsman

Driving it insane...............too much,

You must be Aladin as prophesied by Bowie.

Tails

Interesting Subject.

My initial research when trying to determine why my car was cutting out and would start again after a rest I undertook a full diagnostic fault finding protocol as per Porsche Supplementary Manuals. It turned out to be a LH control unit failure

John of JDSPorsche repaired LH control unit and fault rectified - it was the extra "tile" that failed- time dependent failure.

With regards to the Idle speed , the 928 S4 LH-diagnosis/Troubleshooting manual states on page D 24 - 28:

The 928 S4 is equiped with an adaptive idle-speed cylinder filling control. This makes it un-necessary to adjust the idle speed of any version. On cars with adaptive idle-speed cylinder filling control, system adaptation must only be carried out after disconnecting the LH control unit from the permanent positive supply and after each inspection.

Note: The idle speed will increase when A/C and automatic gear box is engaged in gear. IIRC by 50 RPM.

Idle speed control value with engine at regular operating temperature:
GT 775 +/- 25 RPM
S4 675 +/- 25 RPM

On page D 28 - 5

Test Point 1

Idle speed contact - ohm
Ground short
Break (continuity of wiring)

For testing Idle Speed control valve manually via a DVM, if you don't have a Hammer, Spanner or Theo's PC based diagnostic equipment the following description is on page D 28 - 5 of the EKZ-Diagnosis/Troubleshooting Manual:

Disconnect the plugs from the EZK and LH control units.

Using test lead, connect ohmmeter between terminals 8 and 18 of EZK plug (8 - Throttle valve switch Idle Speed and 18- Ground point MP IX)

DVM Display:
Throttle valve closed < 10 ohm ( my reading was 6 ohms)
Throttle valve open angle > 1 degree = infinity ohms (OK)

If idle speed contact values are not reached during this test, it must be repeated directly at the throttle valve switch. (This is to determine loss of continuity or short in the wiring).

Pull plug off Throttle Valve switch. Connect ohmmeter between terminals 2 & 3 of the throttle valve switch , using test leads.

Display:
Throttle valve closed < 10 ohm
Throttle valve open angle > 1 degree = infinity ohms

Check the setting is correct and adjust if necessary.

Possible cause:
Incorrectly adjusted throttle valve switch or accelerator cable.

To see whether it is a varying voltage or pulsed supplied from the EZK control unit to the Idle Speed Control Valve it would appear that a oscilloscope would need to be bridged into the power circuit between the plug and the throttle valve switch as appropriate.

Tails 1990 928 S4 Auto

aggravation

When I say I had a spare one plugged in and visible to me it was with the car idling. It popped wide open and stayed there regardless of how high the idle was, 850 rpm's, and regardless of the temp which was quite warm.
Unplugged it rotates back to closed under spring pressure and the back edge of the rotating "gate" leaves a small slot open for air to pass through.
Upon start up of a cold engine, with ISV unplugged, it gives me a nice steady 600 rpm's...once it warms up it gives me 800 rpm's.

I'm thinking my O2 sensor is screwing up since the idle increases to 800 once the engine is warm, that would be when the closed loop takes over I think. I'm pretty confident I've eliminated any kind of air leak as a cause for it and I've tried a loaner MAF and LH ECU with no difference in those idle symptoms so I'm zeroing in on the O2 sensor.

Does anyone know where the ISV should be when first starting a cold engine and what tells the LH that it is experiencing a cold start up? Intake temp sensor or water temp sensor (which would be temp II right)?

aggravation

Crap!
Tails, are you telling me the EZK and not the LH is the brain that controls the ISV?
That's the one part I haven't tried to replace

worf928

No. He's providing directions on how to test the idle switch. It can (and should) also be tested on the LH along with the WOT switch. But, you are concerned with the ISV.

The wiring diagrams would seem to indicate that the ISV receives constant power via the LH relay output and that the "pulsing" is a switched ground.

The ISV is indeed controlled from the LH pin 33:


On your 88 check for continuity from w13 at the fuse panel to the ISV connector. That's the power side. Then from the other ISV connector check for continuity to pin 33 on the LH connector. If no continuity then check for continuity to a ground. If the former then the bet is your LH is weirding out on you. Hope for that. If the latter you have a short in the engine harness. Given that you've swapped LHs with no change.... you are probably looking at a harness repair at minimum. If you are lucky you may just have a broken solder joint on the LH ECU plug.

worf928

I am, sooo not that old; I had to google it.

When that album came out I was still listening to Disney Soundtracks...

Still though, it does mention motors and clutches, at least those sensational and of sad remains.