Alan

Calling Jim Corenman, John Speake, Rich Andrade (any one else*...?) [*edit - should have added PorKen of course]

What does LH Pin 15 do on an 89+ car? What is connected to it seems to have the same function as pin 14 - AC compressor except that it comes from before the Freeze switch Vs after.

Why did they wire both? do they only use 1 of them? Did they wire both so they could decide later which one to use? If both are used - why? This came up in a discussion with Hans on rewiring for a new engine controller.

Alan

StratfordShark

According to the WSM, pin 15 is not used on 89+

ddire333

From another thread

Alan

Wrong module (earlier cars)

Alan

Alan

Yes I know - but it IS ACTUALLY WIRED - which is why it is interesting...

Alan

ammonman

I wondered the same thing as I worked through some idle issues on my '90 S4. Pure speculation here, could the purpose be to the idle elevated to ensure adequate alternator power for the cooling fans/flap system even if/when the freeze switch has interrupted power to the compressor clutch? A second input rather than simply relying on the input from the feed before the freeze switch I chalk up to CTO (Compulsive Teutonic Over-engineering.)

StratfordShark

You’re right Alan (well of course you are). Now I look at the wiring diagram I can see the ‘undocumented’ pin 15 wired to freeze switch, and pin 14 wired to LP switch which is later in AC clutch chain.

Could it be that the signal from the LP switch confirms AC clutch is actually engaged and compressor running? Could there be some internal logic that says from the freeze switch AC has been demanded, but a +12V from closed LP confirms compressor running so revs should be slightly raised to compensate? If that’s the case then I don’t know why you wouldn’t just use the pin 14 signal which would confirm sufficient refrigerant and no iceing. Those smart Porsche engineers must have had a reason!

jcorenman

Pin 14 is after the freeze switch and indicates that the compressor is actually running. I believe that is the signal that the LH uses to goose the ISV and maintain a stable idle as the compressor cycles with the freeze switch.

Pin 15 simply tells LH that the AC button is pushed in, not whether the compressor is running or not. I'm not sure why the LH would care, but it would have to be part of the idle logic which is certainly CTO. Auto-trans-in-gear is also part of that twisted logic.

So here's an experiment for someone with an '87+ A/T: Disconnect the freeze switch, idle in neutral, push in AC button. The compressor won't start because the freeze switch is open (disconnected), but does the idle react in any way? I am pretty sure that it won't. Now put the auto-tranny in gear, and repeat: Does the idle react at all when the AC button is pushed? Is it maybe some combination of AC-button-in and trans-in-gear??

For Hans, the question is what does his controller need, and what does it do with it?

Cheers, Jim

Alan

I think all Hans will need is the after the freeze switch version (pin 14). You'd still goose the idle unnecessarily if the pressure switch cuts out but so what - its a relatively benign failure mechanism. Seems odd that they'd wire both - it doesn't seem that useful. Testing would be good to see if its actually functional?

Trivia really but was interesting to find a wired input that the LH diagrams seem to just gloss over completely... The schematics for many of these LH connections are certainly extraordinarily tortuous - seems they might be just discouraging people from understanding it (that was the only explanation I could come up with).

Alan

Alan

Mike - its actually the other way around - the extra one is before the freeze switch - so its a less accurate depiction of what is going on with the compressor clutch. I'd say the pre-freeze switch version sees a few less spikes than the post - but not lower in magnitude, as long as the suppressor is good it should be fine either way. The idle may get boosted unnecessarily if the evaporator freezes up or if refrigerant is low...

BTW - never remove the suppressor from the CE panel - without it the LH, and the itty bitty HVAC relay contacts see every switching spike from the compressor clutch unquenched - and its a pretty big inductance. You really don't want to pump huge negative voltage spikes into your LH. The part of the suppressor you are using here is the reverse biased diode half (it serves 2 purposes).

Alan

PorKen

S4-up the A/C input is coded to tell the LH to disable fuel injector cutoff on deceleration(?)


I have been mentioning this pin for a while
https://rennlist.com/forums/928-foru...l#post12220439



Note 1: Pre-86.5 S3 has the same number pin and function but it is unwired (see PDF in post #3)
Note 2: This input is often used by other makes, EG Volvo, SAAB, as a coding input for automatics as it helps keep the torque converter spooled up for better throttle-on response
Note 3: Re note 2, S4-up LH does not know what transmission is installed - the transmission coding input tells the EZK to pay attention to the shift sensor

Alan

Ken - Pin 14 is AC on Also - so what's the difference? they are almost the same thing - only the freeze switch separates them (literally).

Besides I run my GTS with AC always on and I always get fuel cut-off on deceleration - you can feel it and fuel economy pegs at 90 MPG on the digi-dash (it maxes there) - so I'm not sure I buy that for a GTS anyway. I'm glad it provides such strong engine braking much better driving control.

I do realize now I should (of course) have added you to my initial list of likely suspects names... Apologies.

Alan

PorKen

To tell you the truth, now that I think about it I traced that input in the code early on and just assumed that's what it did. (It was not something I could use as an additional input so I moved on.) I seem to recall it was in the section of code for idle management.



S2/S3 definitely are programmed to disable fuel injector cutoff with that input.

Alan

Yes I'd expect it to bump the idle up a bit with AC on to avoid risk of stalling from the added compressor load - but why have 2 of them...? Maybe they only really use one?

Alan

PorKen

14 AC COMPRESSOR - raises the minimum idle speed 125 rpm (idle speed is allowed to vary between min and max rpm tables)

15 AC ON - inhibits the momentary 50 rpm idle speed drop when [R] or [D] is selected - AFAIK, this is its only function





I 'knew' this five/six years(!) ago when I was working on a S4 fuel chip but totally forgot it