nc_growler

Hi,

I own a '91 GT.

I've been reading the following thread with interest:

https://rennlist.com/forums/928-foru...ter-tests.html

I've got a copy of 928Diag software and I've been able to run tests on the LH using this tool.

I get zero response from the TPS (off pedal, or at WOT). All other values are collected from other sensors.

I started noticing that I do not consistently get full fuel cut-off on deceleration. I believe that this is indication that idle switch of the TPS is failing.

The manual for 928 Diag says this about the idle switch function:

Throttle idle. The light signals that the gas pedal is in idle position and the throttle valve under the intake is closed. This activated the micro switch (closed) at the valve. If idle speed switch has a break, there will be no coasting shutoff. If idle speed switch has a short circuit, there will be a single cut out at high idle speed. One additional note: the idle switch will move the ignition (EZK) to the idle map, 10˚ BTDC. To im-prove throttle response, the ignition is immediately advanced when the idle switch opens.

I believe this also explains why my car's idle will dive when I roll to a stop or when the AC is running... Without the timing modification, the engine is stalling from time to time?


And on the track, the car appears to be under powered and over fueling. There is fuel residual all over the back of the car. This appears to be what you'd experience when the WOT switch is not working correctly?

Here's what the manual says about that:

Throttle full. This light signals that the gas pedal has opened the throttle valve under the intake to its wide-open position and activated the micro switch at the valve (switch closed). This switch is also called the WOTS, the Wide Open Throttle Switch. Worth mentioning is that WOT starts anywhere from about 2/3 to 3/4 throttle as far as the switch is concerned. If full load switch is faulty, always open, there will be no full load enrichment. If full load switch has a short circuit, enrichment will be too early and consequently fuel consumption too high. The ecu will also retard the ignition 10 degrees upon full throt-tle signal. There is a slight possibility that extensive full throt-tle operation, but without the switch closing, could result in engine damage on some earlier LH engines. Later engines have the knock sensors to help avoid damage, and the LH au-tomatically enriches the mixture when it sees open throttle operation.

Is there a way to test the TPS once you've removed the intake? Or do you just replace it?

Any other ideas before I tear into it?

FredR

The idle switch is normally bomb proof- it is the full throttle contact that usually and commonly fails.

John Speake wrote a nice thread on how to repair this. I found with the TPS when it is failing it sometimes works and sometimes does not. You can easily test the switch contacts by removing the LH plug and continuity testing the appropriate contacts.

To remove the switch you have to remove the inlet manifold- when repositioning it has to be positioned correctly but by then an easy job.

Regards

Fred.

nc_growler

Thanks Fred. I'll double check the contacts on the LH connection. That's a good double check of the 928Diag tool's report before I go to the trouble of pulling the intake off.

Can the TPS be bench checked out of the car?

harveyf

I don't have direct experience with a car as new as yours but in general all throttle position switches will click once coming off of idle and click again near WOT. Its' pretty easy to hear with the engine off and your head next to the manifold.

I'm looking at the wiring diagram for a 91 car and it connects to the ECU at pins 2 and 3, with a 3rd wire going to a chassis ground. On my 86.5 car there is also an accesible connector under the manifold near the front of the engine where you could do in intermediate check. I usually check with a ohm meter back at the ECU. I also have a loose switch on my workbench that I can do show and tell with. FYI, on my car that "accesible connector" was shot and was being held together with black electrical tape. I'm replacing it as part of a top end refresh.

If you want to drop by on Saturday, I'll be around my shop and we can give it a quick check. Cell is 919-604-2188

Rob Edwards

TPS can definitely be bench tested out of the car, see my confused (then increasingly clear) comments in this thread:

https://rennlist.com/forums/928-foru...questions.html

But before you start pulling the intake off, continuity can be tested on the LH (pins 2 and 3, ground point IX on pin 5) and EZK connectors (pins 8 and 26, GP IX is pin 18)

LH connector:



EZK connector:

nc_growler

Thanks Rob!

And great to know that you are right down the road Harvey Ferris. I will definitely give you ring if I don't figure it out this week.

PorKen

If you have Photoshop still running...

#15 is A/C on.

(#26 is Shift Light - not used on 928s but can be programmed to switch 12V- here)

Knock sensor 1 = front (of car)
Knock sensor 2 = rear

nc_growler

Just checked LH points 2 and 3 against 5.

No continuity to ground at any position of throttle for either.

Either the connector has come off the TPS or the TPS is failing. Ordering parts.

Rob Edwards

Hi Ken-

Got it, will update the pics appropriately.

PorKen

Rob, cool. For extra credit...

LH:
Display (check) = Check Engine Light (same as EZ - DX plug 11)


EZ:
DX Plug 11 31 = CEL
DX Plug 04 14 = knock (output - shown as input in WSM)

Cntrl Unit 1 15 = 1,4,6,7
Cntrl Unit 2 32 = 2,3,5,8

Coding Input 10 = catalyst
Coding Input 27 = Australia
Coding Input 28 = automatic

nc_growler

Pulled intake. Connection was solid. Pulled TPS and bench checked per Rob's thread. I can hear the idle position click, but the switch is completely non-responsive on the ohm meter.

Will be installing new TPS next week after part comes in (they've suddenly become extremely hard to find!)

UpFixenDerPorsche

Gosh - good info.

Is there a vacuum function involved here, or should zero fuelling be switched in as soon as the throttle is closed?

On a long gentle downhill near here, coasting at 35mph: in 5th, always a small instantaneous consumption reading. In 4th, zero consumption.

But at 40mph in 5th - zero until the speed falls back to 35 or so, then fuelling re starts.

.

FredR

The common failure is the wide open throttle contact and the bit that fails is not the switch itself but a discontinuity in the soldered connection. Without the full throttle switch the WOT ignition map will not activate nor will the WOT fuelling map that throws incremental fuel into the mix so the top end will have a performance deficit.

If the idle switch does fail you probably lose stable idle especially if the a/c is running. I believe the ISV takes some kind of intermediate position to permit some kind of crude idle but not sure of the circumstances for that to function as intended. When my ISV connector came undone [I run without the circlip to facilitate inlet manifold removal] I lost idle but was able to cold start OK. Coming off cruise the engine would die.

Rgds

Fred

nc_growler

I definitely experienced the challenges with the AC running and engine dying. And Idle switch was definitely dead. In my case, the WOT was dead too.

nc_growler

Update on my continuing saga.

First some back story: I contacted one of our regular 928 parts suppliers looking for a replacement TPS. They indicated that they were running out of the item and that their new price was nearly 2x retail plus shipping.

I decided to shop around.

Found the Bosch part on ebay from a reputable supplier. Placed an order for retail price and free shipping. 12 hours later got a cancellation notice saying the item was not in stock.

Found another ebay supplier. Repeated the experience of placing order and having it cancelled.

Called another of our regular 928 suppliers and spoke on the phone confirming the part and the order. Got a cancellation email 12 hours later. ARGH!

Ready to throw in the towel, I called AutoZone and asked them if they had the part. They said, no, but they had a DuraLast/Weller part for $77. And that they could get it delivered to my local store for free.

Figured, what the heck and ordered it.

It showed up 4 days later. I walked in and paid retail. Took it home and put it on the counter. My son-in-law walked in and opened the package. With a surprised look on his face he asked if this was the new part. I said yes, and he held up a bright new shiny Bosch part. Son of a gun. I didn't have to compromise afterall (on price or product).

One happy customer! I wonder if Duralast bought out Bosch or if they just had some Bosch inventory on hand?

Anyway, on with the story. After replacing the TPS, put everything back together and the car ran great. Drove to work the next day. Took it to cars and coffee the next day (Saturday). On Sunday, I drove it to a friend's house and back.

Sunday afternoon I decided to take it out and really shake it down to see how it performs at the top end. On my second hard push, the car stuttered at high RPM... Then it rolled to a stop on the side of the road. My first real "stranded' experience.

Had the car towed 6 miles back home.

Called a few friends. Determined the only thing that could really cause something like that to happen was likely to be the CPS. And being that I had just been "in there" it seemed likely. Also turns out one of my local buddies Larry (NC928S4) has a spare new CPS on his shelf in the garage.

Feeling bold, I dived back into the car this evening. Only took about 45 minutes to get down under the MAF and drain the fuel out before looking under the throttle pulley and seeing bare connector wires. Reaching under the pulley, the connector crumbled to my touch.

Guess the CPS is ok. Ordering the rebuild connector parts now.