Bill,
Do you or anyone know the part number for the K1 & K2 thermo sensors?
Trying to track them down.
Roger

 

Kudo's to Bill(I owe you lunch next month in SJC), and ernie and the others that led me to the source of the limp home mode. It is now jumpered so that the limp home is disabled and I am ble to drive it fine. The resistance on pins 6 to 9 of that ignition control connector shows 7.9 ohms. The circuit shows the resistor elements in series and I think that's probably a good number. So, I guess the limp home mode is working as designed, but now I need to find the source of the stumble/cutout.

Now, back to the source of the problem. I still have a serious stumble at high manifold pressure settings. When at idle and cruising on the fwy, no problem, but when I step down hard the car stumbles.

I think Wally is on the right track with coil wires and maybe even carbon buildup in the cap. I'm going in there first. This feels like an electical stumbe, because it is so abrupt. I think if it were fuel delivery it would hesitate, or burble, give blue smopke for rich or pop for lean. I don't have any of that just a quick, intermittant stumble under high load.

Thanks again, guys. This has really helps. The moxex plug numbers are darn hard to read, but I can use the print to find the pinout.

BTW - DFW weather: Temp 1C dewpoint 2C. Rain/ice mix, fog at surface, visibility 1/2 mile. Wind 190@16, gust 22. Yuch.

 

There is claimed to be a link between "limp home mode" and TT failure so you should minimise your driving till you get it fixed; might only be a link/problem on the S4s....

but even with a GT I would minimise the driving

Marton

 

Roger:

Hmmm... you're right, it seems missing from the PET. Both side are the same part.

928 Specialists shows it as 928.606.155.02
https://www.928gt.com/ps-1088-89-ign...sor-89-95.aspx

 

Thanks Bill.

 

Thanks Bill,

 

More about the thermo elements:

 

Yeah, it's in AT cars only. It happens at idle. The TT is not spinning in in that case in a MT car. Browsing today I happened across it.

 

Doc,
The limp home mode was designed to prevent cat fires. How have you determined that it's not happening for a good reason? That delta takes time to develop and register, so a stumble isn't going to cause it.

 

The Ignition Monitor Sysytem when it shuts down 1/2 the injection really is NOT "limp home mode" it the "keep the car from catching on fire mode" . Many 85-88 928s burned when 1/2 the ignition failed and the unburned fuel being pumped with air into the exhaust overheated the cats and the undercoating catches on fire then craws up on top the engine melts the hood etc... This might be better be called my car just started running like crap but the homing instinct takes over and I just had to try to make it home. Limp home mode is a stored default program in the LH which allows the car to run on 8 cylinders without the correct input from the mass air sensor. Now that you have jumped the relay when it starts " limping" your cat becomes an afterburner be careful !

 

Okay, car is running on all eight cylinders when the relay is jumpered. I think the limp home(for lack of a better term) mode is working right. I've cleaned the caps, and rotors, replaced the coil wires and we're going to test it now. I still can't find any spec for the resistance of the thermo sensors. But there can be no way to measure a delta given the circuit. The thermo connectors are wired in series, so you either get dead short(unlikely) complete open(possible), or resistance changing with temp, which is what I have. The question is, what is the range of operation of the thermo circuit? I have 7.9 ohms when warm. Now have 2.7 ohms after 1 hour cold soak. I think the resistance will go down near 1 ohm when very cold. This is common for an RTD. At least it's changing with temp. But there is no way to get a differential measurement with the series circuit.

More later, stay tuned....

Doc & Roger.

 

This is Doc posting on Roger's system. I see the difference in voltge Ernie is talking about. It has to do with the millivolt difference felt at the pins, and must be near zero to be detected right. Any deviation greater than 2.5mv in either plus or minus direction will make the compoter read the temp as out of spec and shut down one bank. This is a test I'll have to make later and report.

 

I had the same problem about a year ago- TB failure came to mind first then all sorts of crazy things were going through my head, but it ended up being an ignition control module (BOSCH Part # 0227100124). I swapped them one side to the other and found that the 4 cyls that were not working before now were on line. $97 later, I was on the road. I haven’t read the entire post yet and sorry if it has already been mentioned. I bought a couple of spares (used) if you want to test them I can stick them in the mail for you.

 

How about calling it Ignition Circuit Monitor ? That is what it is... or ICM that has a nice ring to it

 

I like Internal Cyllinder Burn Monitor.
ICBM
It's more accurate and has an even nicer "ring" to it.