BrianG

While down in the bowels of my '87 S4, I discovered that both of the knock-sensors' wiring harness plugs had badly deteriorated insulation which, in all likelihood was shorting out.

Can you fellows explain the operation of the knock-sensors in terms of the lead functions, and the consequences of either "short" or "open" faults.

Given terminal wiring harness failure, what wiring configuration would provide "un-retarded" EZK function if the knock sensors were not connected?

John Speake

Hello John/Bill,
The idle control system is quite complex. It also interacts with the O2 loop.

The O2 loop offsets the basic fuel mapping with a correction factor to keep the CO at stoich with the loop nicely settled in the middle of its adjustment range. This is the thing that takes several minutes of driving to adapt.

The idle control loop bleeds an amount of air into the inlet at idle to try and keep the ISV within its adjustment range. Every time it readjusts that, the O2 loop may also adjust the fuelling offset value to suit.

Various things can conspire to mess up the idle stability.

O2 sensor bad
MAF badly aged so idle loop cannot stablise with mixture held to stoich by the O2 loop.
Closed Throttle switch bad.
Airleaks into inlet.
Stuck ISV
Faulty LH ECU
etc...

It would appear that John's car will idle OK when the O2 default is zeroed after a battery reset, but not when the O2 loop has adapted after driving the car..

johnf95111

Hey John,

The O2 sensor has about 15k miles on it. The idle stabilizer looked good on the bench and seems to operate smoothly when in service. All the vacuum lines in the engine compartment have been replaced and check good, and all vacuum actuaters for the HVAC on the interior hold vacuum.

The idle is rock stable, just a bit too high.

A couple months ago I passed our CA smog check with no problems. The guy doing the testing thought I had a new engine.

Bill has graciously offered to let me give his MAF a try. Hopefully that will do the trick or I'll be back under the hood (bonnet) scratching my head.

Thanks,
John Fagerlund
'89GT 101k miles

John Speake

Hi John
Well, you have eliminated many of the potential causes. I suggest that if you can get to Bill's Spanner again, to try the idle adaptaion routine.

At 101k miles you MAF may well be significantly aged, depending on typical journey length.

John Speake

>>>>>>>>>>>>
Hi Brian,
Any corruption of the expected knock sensor signal by noise from exposed shielding, open or short circuits will result in the igntion retard at mid to high load/rpm.

There is no way you can fool the system to not retard, if a knock sensor is not connected to each input.

When I was construcing my EZK test jig, emulating the correct knock signal was a major problem.

BrianG

So my wife is likely right........ at the end of the day, it's probably true that this car is just retarded!!

John Speake

>>>>>>>>>>>>>
Wive's are always right :-)

There are 3 leads to each knock sensor, there is a shield, a ground and the "live" signal. The sensors are a piezo type sensor, and generate a signal when "squeezed". Or in the case of the 928 when a shock wave compresses the piezo element. So the tightening torque is important.

This signal generated is of the order of a 100mV. They are high impedance devices, but I don't believe oil on them should impair their efficiency. Water is a different matter.

As Jim said, most people will replace them while under the plenum. I have never been able to persuade my engine rebuilding friend to just tape up the dodgy insulation, and try them again......

johnf95111

The miles come from both long and short drives. I'm assuming if more short drives are logged, the MAF burn-off will be done more frequenty, shortening its lifespan.

Great info,

John Fagerlund
'89GT

John Speake

Exactly correct !

johnf95111

This morning I grabbed my coffee and foggy head, and went into the garage to check my knock sensor wiring. I found that the ground pin on the female connector was recessed and shorted to the center or signal pin.

It looks like the PO replaced the connector and did a bad job doing it. People that don't know how to solder really shouldn't do it. I found globs of solder and frayed wires.

I cut off the boot and cleaned it up to make it at least functional. Back on the road terrorizing the wildlife, my SOTP accelerometer told me I had my lost power back. I'll replace the connector properly to make the fix permanent.

I don't have an SC, but this thing still stands up and scoots when the loud pedal is punched.

Thanks to John S. and all for entertaining my questions during my learning process.

John Fagerlund
'89GT

Bill Ball

Excellent! I suspected the connector but didn't do more than wiggle it after the Spanner error code. This may have been the ground fault that prevented the Spanner from doing the actuator tests. So, I can drop by one evening this weeks with the Spanner, if you have time. And we can swap in my MAF to see if the idle issue resides there. Tomorrow night (Monday) 5:30 would be good for me.

John Speake

OK John, that's ggod ! If you need any 3 pin plugs or sockets I can supply.

BrianG

I can't believe the generally poor condition of the terminal ends of most of my engine wiring harness. Both of the knock-sensor plugs were obviously shorting due to deteriorated insulation at the wire/plug junction. Same with the MAF plug and by-pass air plug.

I hope someone decides to get to reproducing the engine harnesses for the S4's soon....

johnf95111

A new harness could solve quite a few problems of the head-scratcher type. Would installing one require an engine pull?

I guess I can answer my own question; with these cars, if there are two choices, the most diffcult/costly will prevail.

Cheers,

-j

pantera928

I see several references to "codes" from knock sensors in these posts. How does one receive codes from a pre OBD-2 Porsche 928?