davek9

Unlike the S4 the S3 is a cruder ISV and thus are more prone to add a mechanical resistance, then the Driver circuits get over heated and burn out (I think I have 3 LH's now on a shelf with bad ISV drivers), two go directly to ~1,400 RPM, one sometimes works just great, then it creeps up
In both the S4 and the S3 the ISV's are constantly moving, it's not an open or closed thing many fine adjustments, a bench tested unit that opens and closes by adding 12V to one or the other outer two pins (center is ground) won't always function under the load and or heat.

I've installed the after market (BMW) ISV's in two of my own cars and so far they are working just fine, one is in my 86.5 Twin screw, w/o issues.
I'm just saying if all other parts check out (TPS wiring back to the ECU's(not just hearing it "click") etc..) don't rule out an intermittent or failed LH.

As to your question is it the correct power(12v DC) I think it is more of the correct current.
Have you swapped out the LH yet for a know good one?

Dave K

Mrmerlin

I am going to have the computers checked as well as the MAF everything else seems to be working as it should,
and i did find a lot of fouled up things with this intake refresh.
I installed a new Lowe ISV and the idle was able to be lowered via the screw,
then removing the test jumper the idle will sometimes stay low ,
and then after driving it for a few miles it will suddenly go to 1500 or so RPM,
Thats why I am with Dave on this idea of damaged computers

abrescia

jbrob007

Stan and Dave, Joel Roberts here. This is mine and Mike's project car, which is soon to be my daily driver. We've gone through almost every system on the car and about to button her up when this problem raises it's ugly head. We've gone through all the intake connections, vacuum hoses & lines, connectors, etc, multiple times. All the hoses, lines & connectors, O-rings, sleeves, etc, are new. We checked all the throttle linkages (first thing) and turned the throttle body idle control screw all the way down so the idle SHOULD be very low. We tested the TPS, swapped out the MAF with a known working one and tried two different ISVs - all with the same result, a 1200 rpm idle. Our last vacuum test included plugging the vac hoses to/from the intake center "T" to isolate any vac leaks and see if the car would idle without the ISV in the system at all. As we suspected, the car would start but immediately die - can't breathe. No vac leaks found. Then one by one we reconnected the two vac hoses to the center "T" and restarted to eliminate all potential leaks - still, none to be found. Finally, we hooked up the ISV (without plugging in the connector) and the car would start & idle around 1,000 rpms (still too high). When we plugged in the ISV to power the idle went straight to 1200... and stayed steady. The ISV doesn't appear to be the problem and is trying to regulate the air intake... unsuccessfully. We've used two ISVs (stock and after market) with the same results. Long story short, we're slowly coming to the same conclusion (bad computer) as everything else has checked out...

So unless we've missed something obvious (doubtful), or you guys have other suggestions (we're open to any and all suggestions), then it appears the computers are the next thing to test / repair. Any of you guys know a good LH & EZ computer repair shop? Preferably in the USA?

Thanks for all your help, insight and considerable expertise!

Joel & Mike.

Mrmerlin

I am going to send these computer to Rich Andrade, he fixed my S4 computers and it was a time consuming issue to effect repair.

Their might be other vendors that will repair these .

davek9

Stan please let me know if we can get a "group" discount, I have three

Thanks!

Dave K

skpyle

Joel, I second what Stan said.
I had Rich rebuild the LH 2.2 and EZF for the Red Witch.
I am having him do the same to a spare LH 2.2 and EZF also from an 1986.5 auto, and will have him install the Porken S300 chips in those computers. Rich was not cheap, but was reasonable for the work done.

From what I understand, ALL of the S4 and later LH 2.3 computers WILL fail due to the 'tile' inside. The earlier LH 2.2 computers are a little more robust, but are now suffering from age. Meaning failing solder joints, individual components, etc... Worth spending the money to have them rebuilt now at your leisure instead of later when they are dead.

Also, Stan, to my knowledge, Rich Andrade and John Speake are the only reputable rebuilders of 928 computers. I believe there may be others, but Rich and John have the best reputation and are 'part of the crowd.'
Having said that, if I am wrong, I meant no offense to anyone.

buccicone

Seth, Dave and Stan: We're in the process of removing them now. Gonna send both to Andrade... ya know, while your in there Seth, I've changed out the PORKEN 300 chips in my 86.5 every time Ken came out with a new version all the way to the 4.0 chip set. My computer tabs were toast, but I got really good at it, and the car ran like a raped ape... for those of you reading along, that's FAST and it pulled all the way to red line in every damn gear! Very noticeable HP & TQ increase without pinging, etc. You'll need to regap your plugs and get an S4 FPR... those are getting pricey! BTW, If anyone has a spare S4 FPR I'm in the market for one so I can do the same with my 86 (once we get this idle speed resolved).

Thanks again! 928 Buckeye Landsharks RULE!!!!

Joel Roberts

skpyle

skpyle

Mrmerlin

Well I will give Rich a call and discuss this with him .

928S MN

I am VERY much looking forward to running the Red Witch with the S300 chips. It is my understanding that a well sorted S3 with the S300 chips is wicked fun. ��

Yes it most certainly is!

Mrmerlin

I spoke to Rich tonight and he is interested in checking this out ,
I am going to send him my computers the MAF and the ISV thats made by Lowe.
When I find out the diagnosis,
I will update this thread and maybe Rich will also check in here as I advised him of this thread.

buccicone

Rich9928p

Hello all,

I recently repaired a 25-pin Porsche 928 LH that had an ISV circuit failure for a customer on the east coast.

I will check out this 25-pin LH ISV circuit and advise what I find.

Rich
928 Owners Club Membership Chairman
1993 928 GTS Cover Girl
1987 928 S4
1979 928 5-speed - now in print
1979 928 track beast