95993Cabrio

I have a1995 993 Tip and it has an OBD II socket but I guess it is an OBD I car.
The car is fine but the engine hesitates to rev above 4k or 5k rpm to the redline when driving.
I had tested the knock sensors and they have no continuity. I had also checked the connectors from the harness and it has no supply voltage when the car is running.
I had tried to disconnect both knock sensors and drive the car, it had no difference.
How can I further test if those knock sensors are working?
Why there is no supply voltage from the ECU to the sensors?
Thanks for helping.

pp000830

Hi 95993Cabrio,
Could you simply be hitting the rev limiter that cuts the engine when you bounce against redline?
The engine can get there before you see it on the dash gauge.
Andy

Mad993

Exactly the same problem for me. Hope someone can figure it out.

Mad993

Also to add I have a durametric scan tool but shows no code.

IainM

My understanding is the knock sensor is basically a crystal mic, a piece of quartz that generates a voltage when mechanically stimulated. As such it will appear as an open circuit and requires no external power.
I haven't tried this but would expect a voltmeter in low volt scale would show 0V steady with engine off but would show some level of noise with the engine running. Check on both AC and DC for activity.
The problem with my knock sensor triggered an "implausible signal" status and misfire codes. The problem was just bad connection as the one on the front gets wet when you wash the car and heat soaked when you drive it.
Not sure this is your problem as the knock sensors only factor into the closed loop ignition timing. You might be down on power but it wouldn't stop you revving.

Mad993

agreed the problem is probably not the knock sensor but what could cause this running condition

pp000830

To see the piezoelectric output I would think one would need to attach an oscilloscope to catch the signal spikes due to their sudden and short duration and very low output. I would think a meter would not see its activity. Hook up the scope and I bet if on taps on the engine they will see a small signal emanate from the sensor(s).
On the other hand, misfires that cause knocks usually are at lower RPM. If not the rev limiter, I would look at other fuel supply or ignition issues such as a missing high voltage trigger at higher speeds where greater demands are made on the coils or a fuel supply restriction or pressure issue. Also, consider running the OBDC codes as knocking is a form of misfire and the codes you get may point you in the right direction.
Andy

SwayBar

On a 928 S4, if a knock sensor is dead, the ECU knows it, and automatically pulls 6 degrees of timing with performance suffering as a result.

However, bad knock sensor(s) will not cause the engine from reaching redline.

It sounds like it could be a fueling issue - changing the fuel filter is where I'd start.

Mad993

does the obd1 95’ show misfires my durametric does not.

geolab

The Porsche system tester hooked to a porsche OBD1 993, stores (last 10) and shows Knock sensor trigger on each Bank of cylinders. I assure you 100%
But porsche system tester should be hooked to the car, and a test drive with special conditions that appear on Porsche system tester whilst doing the test, are required by the software, to determine if knock is discovered
and if so, stores it in memory.

Mad993

I understand the factory tool “hammer” does but I only have a durametric.

pp000830

I have Durametric but I have an OBDC-II car. I would think one could hook it up to an OBDC-I car and if the knock sensor data is one of the available running data items Durametric can see you will be able to see it. If not it seems that a misfire CEL is so basic to the emission system, I would think it should be in one of the items if the engine was having issues with misfires. If no misfire is being captured it suggests rev limiter or something else I am not familiar with.
Here are the Motronic codes [M04} which I believe are the early 993 OBDC-1.
[M04]
;Motronic trouble codes
11=Power supply, DME, high
14=Engine temperature sensor 2
15=Throttle Potentiometer
16=Throttle Potentiometer
18=RPM Signal
19=Speed signal / Speedometer
21=Hot-wire mass air flow sensor
22=Oxygen sensor (Sensor signal)
23=Oxygen regulation / stop (intake air leak?)
24=Oxygen sensor short to+ or 0 Volts
25=Intake air temperature sensor
26=Ignition timing change
27=IAC opening/closing winding
28=IAC opening/closing winding
31=Knock sensor 1
32=Knock sensor 2
33= Control unit faulty, Knock computer
34=Hall signal
36=Idle CO potentiometer
41=Control unit faulty
42=Fuel pump relay (DME relay)
43=Tank ventilation valve
44=Auxiliary air pump relay
45=Warning lamp Check Engine
51=Injection valve 1
52=Injection valve 2
53=Injection valve 3
54=Injection valve 4
55=Injection valve 5
56=Injection valve 6
67=Ignition final stage
68=Ignition final stage
69=Ignition final stage
Again, make sure you are not simply running against the Rev limiter.

geolab

I suppose maybe hammer does this, I am lazy to check the documents.
But on a PorscheSystemTester 2 (PST2), I have cleared knock sensor tests a lot of times on 993's stupidly.
Durametric ? No idea, but we cannot have everything in life that works

95993Cabrio

Thanks for the feedback and appreciated.
I had so far replaced the MAF, fuel filter, spark plugs, ignition coil and wires, distributor caps....and there is no vacuum leak but the engine still doesn’t want to go above 5k.
The flap in the air intake is working, it activates every time when the key is in position 2.
Also, I still don’t understand why there are no voltage supplies to the knock sensor from the ECU when the car is running.

Mad993

I’m leaning towards a weak fuel pump, bad distributor, flywheel sensor or the cylinder head temp sensor.
what do you guys think?