FLUFF

I have slowly pinned down a no start problem on my 944/83 to be either ECU or Speed/Reference Sensors. (No signal from ECU to coil - and everything else checks out OK)

Measuring at ECU connector with ECU removed, when cranking, I get 60 mv or so from sensors. They both measure around 1K ohm resistance. Is the voltage measured high enough for the system to operate? What RMS voltage should I read at normal cranking speed? (I only have a voltmeter - no scope)

Also I took off the connectors and forget which is which. The sensors are annotated on the bellhousing as "D" and "B", with "D" being at the rear. Which is the speed sensor and which is the reference sensor? I would rather not remove them to look down the holes as the cables are stiff and I know from costly experience that they break easily.

I also noted green corrosion inside the lead plug to the distributor at the coil end - the lead was effectively open circuit (you could not see this from the outside). I've replaced the lead, but suspect that damage may have resulted to the ECU transistor circuit as a result.

Incidentally I also inspected the DME/Fuel pump relay - it hadnt failed but was about to - one of the coil wires was green and corroded.

I also repaired a 9way connector under the bonnet which was pretty corroded.

Other sensors, cables, connectors, injectors and coil checked out OK.

There is no obvious damage to tracks or components in the ECU.

Also - has anyone found a cheap source for Bosch components in the UK?

Help appreciated.

Fluff

Bri Bro

The min voltage PEAK is 2.5 Volts for the RPM sensor and 2.0 volts for the Ref mark sensor. A DVM will measure average voltage so it can't be used to measure peak voltages. The resistance of the sensor is between 0.6 to 1.6 K ohms. The resistance measurement tells you that the sensor is functional but it doesn't tell you if the gap between the sensor and the flywheel is correct. This gap (0.8 +/- 0.3mm) has to be correct for the voltage to be correct. If you haven't moved anything you could assume that the sensor gap is the same. If you want to be sure, you need a scope. If you go to your local TV repair shop, you might be able to talk them into coming to the car and doing the measurement.

FLUFF

Thanks. I only have a Haynes Manual which doesnt have any of this stuff in it. I ended up having to get my hands dirty and inspect and draw it out first hand. A copy of a Bosch Motronic M1.1 detail schematic would be useful. Does anyone have a copy they can scan for me? Also does the Motronic M1.1 have any blink error codes?

On which side of the flywheel is the reference pin situated to actuate the position reference sensor (ie front or rear side with respect to the car)? - from that I can tell which is the reference sensor and which is the speed sensor.

Bri Bro

There are two letter stamps on the Al bracket that the sensors go into. D=speed sensor and B=reference mark sensor. The cables are also marked with a wrap tape. DG=Speed Sensor Bg=Reference mark sensor. Do you need the connector diagram or are you looking for the internal diagram?

FLUFF

Brill - the letter answer is just what I was looking for. Unfortunately my cables are unmarked - any labels would probably be unreadable by now anyway - car is 1983, ex Kuwait example, bought in France in 1992 - its suffered sand-blasting and heat-stroke as part of its desert history!!!

I wouldn't mind a full connector schematic AND the electrical schematics of the ECU (I'm an electrical engineer so will have a stab at fixing ECU myself if necessary) I note that there were no obvious fuses or fusible links in the unit.

FLUFF

Tried to remove position and speed sensors tonight - wont budge - had to use an ordinary allen key - very little room and socket wrench wont fit. Any suggestions?

ERAU-944

dude! to get those sensors out you ahve to move the bell housing... that sucks! good luck solving your problem!

Bri Bro

My sensors were also stuck in place, I soaked them in WD40 and got a vice grip on them; after some work the first one came out. The second one just started to come apart, it rotated but wouldn't come up. I tried to get to the Al mounting bolts but I could only reach one. The mistake I made was removing the one bolt and continuing to wiggle the sensor. It came off all right, with a piece of the Al mounting bracket still attached. I had to put the sensor in the vice to drive it out. I was able to put the piece back with a little glue on one side and the bolt on the other. It was sound and I fixed it correctly when I did the clutch. I only have a schematic of the Turbo DME, might want to contact TT. He seems to be reverse engineering the 8751 code. Also, most of the problems in the DME are repaired by re-soldering the connector pins.

Please note in the picture that the sensors are held in by a bolt (item 6) at the top of each sensor. The sensors can be removed by taking out these bolts and pulling the sensors straight up (if they are not stuck in place). You do not have to remove the entire bracket. If they are stuck, keep soaking them in WD40 and wiggle them a little at a time.

<a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic&f=18&t=001002" target="_blank">DME thread</a>

<a href="http://boards.rennlist.com/upload/DMEPlug.jpg" target="_blank">DME Plug</a>
<a href="http://boards.rennlist.com/upload/DMEElectronics.jpg" target="_blank">DME electronics</a>
<a href="http://boards.rennlist.com/upload/DMEDescription.jpg" target="_blank">DME Description</a>
<a href="http://boards.rennlist.com/upload/DME831.jpg" target="_blank">83 DME 1</a>
<a href="http://boards.rennlist.com/upload/DME832.jpg" target="_blank">83 DME 2</a>

Mike 89 NA

Fluff,

DG should be on the top connection on your engine, and the rear most sensor on the bell housing. Obviously then BG would be the lower connection on the engine and the foremost connection on the bell housing. Reversing them won't hurt anything, just an engine that won't start!

Good luck,

Mike

FLUFF

Thanks for the diagrams, etc. Are they from a Bosche or a Porsche manual?

(I got thoroughly confused by my Haynes manual - it has the internal workings of the DME relay upside down - and leaves a lot to be desired elsewhere, and doesnt even mention the speed/position sensors)

Bri Bro

From the Porsche manual. Here is another site for 944 DME boards.

<a href="http://members.tripod.com/944dme/" target="_blank">DME</a>

FLUFF

Is there anyway of testing the DEM off the car?

FLUFF

Also what are the possible fault mechanisms of the sensors - I can only think of reduce magnetism with heat/age and possible rust. Can rust/gunge altering the airgap have an effect?

FLUFF

Oh yeah - and how can I look at all the files in the rennlist download section? I've only been here a couple of days (you're a helpful/knowledgable bunch though)

Bri Bro

I am sure Porsche can but I haven't seen anyone else saying they have a tester. Several supplies do sell rebuilt units so someone is testing them. If the material is non-ferrous such as oil, dirt etc. it should NOT effect the sensor and there is little to wear out.

As far as testing, treat it as a black box. It has to have certain inputs to generate the outputs. THe repair manual goes through each steps in checking the DME I/O. If you have a scope, I can send you the details. Too many pages for the board. Did you get them out yet?