Alex Sol

my electronic expert and i were going over the wave forms on my crappy running 91 964 c2.

The spark plug wires and coil plugs seems fine.

we went over to the hall sensor as i am getting a fault code from piwis stating hall sensor.

we expected 5 +/- volts on the hall sensor with the key in 'on' position but car not running.

we are getting:

0.4 volts and 1.28 volts when checking voltage on the two pins, the third being ground.

it seems to be a floating ground also...

can someone with a voltmeter handy do a check on what voltages you are getting on the hall sensor plug - the one plugged into the distributor?

thanks!

Lorenfb

The Hall Sensor supply voltage from the DME ECM is 12 volts. The signal on pin 8
of the DME ECM is a 5 volt to ground pulse. Using a voltmeter which will average the
voltage signal, one will measure between 3 -5 volts with the engine running. With engine
stopped and key on and the sensor not aligned, one should measure about 5 volts.
The sensor grounds the Hall input (pin 8) when the sensor is aligned properly.

Alex Sol

thanks Loren! Good stuff. We expected the hall sensor to be about 5 volts so if we are only getting .4 to 1.3 volts we suspect there is a broken wire? or crossed wires? or is there something we may be missing?

Lorenfb

Just last week one of my Porsche shops hard the same problem, i.e. the signal wire
was shorted to the shield wire (ground). They replaced the distributor and the DME ECM
with no results. We finally used a scope and saw no signal and then used an ohmmeter
to find the short, i.e. pin 8 to ground.

Alex Sol

we are going to 'hot wire' the hall sensor

have you ever replaced the hall sensor in a dizzy? i have read that it's difficult but no impossible. any suggestion on where's the best place to get replacement. we'll test the part first.

'We finally used a scope and saw no signal and then used an ohmmeter
to find the short, i.e. pin 8 to ground.'

this is why i hate throwing parts at problems. one, it's expensive. two, it doesn't always solve the problem.

i was told to replace the ignition control module as they are a 'part that can wear out'.

is there a way to test these? voltage, resistance, scope while running?

PhatPhlatSix

In my experience with ECUs, all grounds (sensor and power) end up on the same central connection. Perhaps someone with knowledge of the 964 ECU can confirm. In your original post, you mentioned a floating ground - you should check that first, i.e. get continuity to engine casing, and then make sure you get battery voltage at the sensor-supply pin. Without power and ground, a new sensor would also not work...

Alex Sol

loren,

how did you fix the short?

Alex Sol

phatphlat

thanks for the comments on floating ground. i'm not sure what that is and my electronics friend is going to help me figure this out but doing something similar to what you are suggesting. getting into the ecm and making sure the voltages are correct and 'look' right. ie: proper wave forms using oscilloscope. we'll dig into it this saturday...

Lorenfb

Your problem is exactly the same as my 964 shop's customer's problem, i.e. a short on
pin 8 to ground. The shop cut the shield at the DME ECM and at the Hall connector.
Then he ran a separate ground from the Hall connector to the DME ECM ground pin 30.
If you're lucky you might find where the short in the shield is (maybe by the connector
shell for the DME ECM) and fix that. Once the short is removed, the signal should be
O.K.

Last year another shop had the same shorted Hall Sensor wires. After stripping back
the outer insulation a few inches from the end near the distributor connector,
the short was found and repaired without much effort. That's most likely where
a short might occur.

Don't waste any money on a Hall Sensor, they rarely ever go bad.

robt964

Pulling back the boot on many of the engine bay connectors revealed split insulation on my car. It would then only require a twist of the wire for a short to likely occur. The age, and environment these looms live in means alot of the insulation has gone brittle and started to break down. As I was adding additional wiring/sensors to mine, I took the plunge and made a new custom loom with raychem automotive grade wiring. When dismantling the old loom I was surprised how even some of the well covered sections had degraded. The years of heat cycling takes its toll.

Alex Sol

loren, good stuff. what was the symptoms of the bad wiring to the hall sensor?
mine is: it runs like total crap and is undriveable. unstable idle. stalls. misfires, will not rev over 4,500 rpms

so i might have more than one issue...

robt - you replaced the entire 50 wire loom? i replaced the harness in the engine bay with pretty fresh used set - much much better and lower miles than mine. insulation still had some bounce vs hard and stiff and brittle... but.. oddly came with no reverse light to the transmission -

another hotwire that i will be running... and part of a potential rear deflector improvement that i am planning. mine is all pink and cracked...

boxsey911

I agree with checking the wiring but I'm sure I read somewhere that not being able to rev over 4500 is due to a broken distributor belt?

I also had the hall sensor code, constant stalling and down on power (although I could still apply full revs). Simply unplugging the hall sensor at the distributor made the car run fine as it goes into a default retarded ignition mode. This sort of confirmed to me that we had to do something about the sensor. My shop then replaced the sensor with one I bought for about $20. Car has been perfect since.

Lorenfb

Alex, the above are the typical Hall problems when the pin 8 is shorted to ground.
You also may have other problems.

Alex Sol

we hotwired the hall sensor by soldering wire to pins # 8, 30 and 31 on the back of the ecu and plugged back into harness.

the wire was brought back directly (thru the window) and to the hall sensor.

still exact same 1.29 volts

does this indicate hall sensor issues or still 'other problems'

Lorenfb

The short to ground was on pin 8, so the original wire on pin 8 of the DME ECM
must be removed before a direct wire to the Hall sensor is connected.
Hopefully that was done. At the sensor connector with it disconnected,
one should measure; 5 volts - pin 8, 12 volts pin 31, & ground.