mhariush

I'll source out a new CPS though i was pretty sure this was the same sensor as on the N/A and it did run for a day with it.

While I wait for the sensor, maybe it's worth trying to bypass it like you suggest? To rule out the cable altogether?

The car was pretty empty on gas when i started it, filled almost full tank. But will keep that in mind for next time.

I checked the DME Computer, no water leak, no damage on it and I even got a second computer that was said to work and tired it, no go.

I didn't do any pressure wash under the hood or under the car. Also, the starter I had in this car, was a stater that i had running in my NA so it hadn't been sitting for a long time. The starter from the now broken car is running in my old NA.

william_b_noble

I spent some time with Marius on this car today, with a scope. This car is an S, it has one flywheel sensor only, it has a hall sensor in the distributor body as well. The flywheel sensor shows a 2.5 to 3V sinusoidal wave when the engine is cranked (with plugs removed to reduce load on the starter) - I conclude that the sensor is good.

There is a test to verify the ignition circuit from the DME main connector by putting power to pin 1 if i remember right - whichever pin carries the ignition pulse to the drive circuit - this works, a strong spark is generated.

There is power to the DME at both the continuous power and the DME relay power pins.

There is a diagram that shows a mixed reference and mark pulse stream measured at the DME connector - this waveform that is a 2-3V sinusoid with another 2-3V peak superimposed occasionally does not appear - but I don't see how it could be different at the DME connector than at the one sensor since the wires just connect - is there a magnet or other device to cause the sensor to output a larger pulse once (or 4x) per revolution? This particular waveform makes no sense. Further, the troubleshooting guide doesn't say anything about the hall effect sensor, which I am starting to suspect.

The reason I wanted to focus on the flywheel sensor is that it is possible that the sound that haippened just before the engine stopped could have been a piece that broke off a part of the flywheel. I seem to recall a magnetized allen screw being used in some cars, but I can't find anything about exactly how the S creates the waveform, or even confirm that the rather odd waveform is correct. Further, I don't think the tach bounce test is applicable on an S, can anyone confirm for sure?

peanut

firstly yes the tach bounce is the same for all 944 . The CPS is essentially a magnet and generates its own output to the ECU . No tach bounce means either a faulty CPS or the cable /connector .

It might be easier to follow if you use DME for the relay and ECU for the engine control unit otherwise using DME for both can lead to confusion.

The first thing you need to establish is ..is there 12v+ at terminal 30 on the DME relay socket with the ignition off ? if yes then I would remove the DME Relay and fit a 3x way bypass cable in its place .
(Check the proceedure at Clarks Garage Workshop manual .) Basically you bypass the DME relay by connecting DME relay socket terminals 30, 87 & 87b This will supply a continuous 12v+ to the fuel pump and power the ECU and ignition system.

See if you have fuel and a spark.
Warning
Whatever you do .Do not put a multimeter on any cable or connector that goes directly to the ECU controller. Some of the ECU chips work on voltages less than 3v+ and can easily be damaged by a multimeter
I would strongly recommend that you do not mess with applying voltage to the ECU. If you make the wrong connection the op will have to fork out $200+ for a replacement used one.
There is no test you can do by applying voltage to pin 1. Instead check that you have 12v+ at terminal 15 on the ignition switch ? this supplies the coil and the ECU with 12v+ when the ignition is switched on.

mhariush

There's no bounce, and the scope showed the speed signal but not the ref signal. But the car did run with the sensor as installed now, so the distance should be fine and the sensor itself is working. How can I check if the magnet in the flywheel is there or correctly installed? What would make the ref signal not appear?

If I jumper pins 30, 87 and 87b on the DME relay, that will bypass the ref signal and fire the engine up? There is power in terminal 30, but will recheck to see if it is there with ignition off. With these pins jumpered, do I turn ignition to see if it fires? If so, will that point to the ref sensor not picking up the right signal?

Thanks

william_b_noble

Peanut - thanks for the input - we verified the following over the month:

1. voltage to computer on the two sets of pins
2. battery voltage is low, about 10.5
3. fuel pump runs when energized from computer pins
4. spark works when energised from computer pins
5. flywheel sensor outputs 2.5 to 3V peak to peak sine wave when engine is cranked.

have not tested hall sensor. couldn't find connection diagram for 12 pin test connector that is near the computer - found it now, so will check RPM there when I get a scope onto it again

it is not the DME relay for sure, nor is it the fuel, or the flywheel sensor, or the coil. What I don't see with the scope is the longer pulse that is the mark pulse - what causes this pulse? is it a magnetized allen screw like in the regular 944?

william_b_noble

maruis - you can jumper the DME relay all you want, the engine won't start because the computer is not commanding a spark. the computer is not happy, we need to figure out why. you cannot "bypass" the reference signals, that tells teh computer when to make a spark.

peanut

William you must not supply voltage to the ECU under any circumstances ! The slightest miscalculation will result in a toasted ECU . they are too expensive to put needlessly at risk. I mean no offence but you clearly do not understand the L-jetronic fuel and ignition system and your meddling with the ECU is going to cause big problems. You are using 'rocket science' to crack a simple nut.



First of all the ECU needs to see a minimum of 225rpm from the reference sensor before it will ground the DME relay supplying fuel delivery ijector pulsing and ignition pulsing. If your engine is not turning over this fast you are completly wasting your time.

Put another good battery on or jumper from another car. Put the positive jump lead on the car's battery but put the earth jump lead directly to the car's engine . Use a stopwatch and count the cranking rpm. you don't need a sillyscope.

peanut

actually this is not factually correct. The ECU chip is pre-programmed with a basic starting and running map. This is often called a 'limp home' program.
When some of your sensors fail the ECU is quite capable of starting and running the engine . A lot of cars are running with faulty sensors and are probably unaware of it other than hard starting and rough idleing/ running characteristics.

The DME relay is a double relay and supplies current to lots of vital items not just the fuel pump
When you bypass the DME relay you are supplying 12v+ to the fuel pump, and the ECU which in turn pulses the injectors and ignition system and supplies 5v to some of the the sensors

Installing a 3 way bypass is also a diagnostic test and depending on the result can be very informative.

I suggest you get a copy of the Bosch fuel injection and engine management system which will help you understand how the system functions and how to systematically troubleshoot an L-jetronic system

william_b_noble

we have a terminology problem - I have been using DME to mean the computer, you are I think calling it ECU. The DME relay is working. I have tested it, marius has swapped it. The DME relay supplies the switched power to the computer, a seperate wire supplies continuous battery power to the computer - bothof these powers are there. The car does not start, there is not spark. The DME, or if you prefer ECU (thought that is not what the Porsche manual calls it) controls the spark and the fuel pump, and it is not choosing to ask for fuel or to make sparks though both the pump and the coil are known good and operate from the computer.

My suspicion is that the flywheel has a magnetic allen screw, much like the NA 944 cars have and that the screw got damaged with the stuck starter, otherwise I cannot explain how the extra higher amplitude pulse shown in the manuals is generated. If there was someone nearby with a computer to plug into the rectangular 12 pin socket, we coudl read out what the DME is unhappy about - baring that, I can (now that I found the pinouts) put a scope on the output and look at it and see if the computer is correctly seeing the flywheel generated pulses.

There are no ignition pulses coming out of the computer, I've checked. I do have a schematic for the DME but I'd prefer to not spend a long time chasing around inside the DME with a scope. It is something pretty simple, probably related to the cause of the starter problem, but since I never got to look at the damaged starter I have no way of knowing this - there appear to be no missing teeth on the flywheel though.

peanut

William.... a little knowledge is a dangerous thing . its clear that no one can tell you anything about the Bosch fuel injection and engine management system

Get yourself the Bosch manual as I suggested. It is written by the guy who invented the system Charles O Probst.

It sounds to me like you have already fried the ECU by meddling with the inputs . . Porsche may well call it the DME but Charles O Probst calls it the ECU (engine control unit) which is good enough for me

mhariush

Hey Peanut,

I have two DME (ECU, Computers) and the car starts with neither of them. Have opened them up, found no cracks in the solder nor any fried or burned parts.

If the car isn't registering the tac bounce, but the sensor itself is working and the scope reads the speed signal but not the ref signal (the high and low top on the scope), how would this be a fuel injection problem?

peanut

You have had so much work done on the car and so many parts removed replaced and fiddled with now that your original problem could have been made worse still by now. All this information has come out in bits here and bits there so it is now very hard for me to build up a sequential sequence of events.

As I said before,..... if you have no tach bounce ..that is your problem full stop.
Until your ECU sees an ac input from your CPS (crank position sensor ) it will not ground your DME relay for the Fuel pump and it will not pulse either your injectors or your ignition system.

The tach bounce you are looking for on cranking the engine is very very small barely an 1/8" you have to look very hard to see it.

You have replaced the original CPS with a used one from another car . Was it exactly the same model and year ?

Did you actually check the gap from the sensor to the flywheel ? it is critical.
If you look on Clarkes garage site there is a detailed guide.

Stop fiddling with the ECU it will not help. Unless you got water in it from the car wash it is unlikely to have been at fault . it may well be now though

Try this .
Remove the DME relay

1. Check you have 12v+ on terminal 30 of the DME relay socket with the ignition off. (you may need to use a male spade connector in the socket )
2. Switch ignition on (not cranking ) check you have 12v+ at terminal 15 on the coil and also 12v+ at terminal 86 on the DME relay socket .

If this checks out ok then ....
3. Check replace fuse 34 (fuel pump )
4. Remove DME relay. Switch ignition off and fit a 2x way bypass to the DME relay socket between terminals 30 and 87b ( Either a piece of electrical cable with a male spade connector each end or just use a paperclip Remember terminal 30 should be live 12v+ so be careful )
Your pump should now be running continuously and you will hear it as it is noisy.

let me know exactly how each check went and we'll take it from there.

william_b_noble

a long time ago, and again on Saturday we confirmed:

10.5V on terminal 30 of the DME relay - this is what the battery in the car reads, it has a dead cell

fuel pump runs with jumper in DME relay, it also runs when commanded by the computer by grounding the pin -

I did find that the 7.5A fuse to the DME had blown, and replaced it.

william_b_noble

peanut, what exactly do you mean by "William you must not supply voltage to the ECU under any circumstances !" - certainly the DME computer must have electrical power to operate, and connecting the battery supplies voltage to the ECU - what exactly are you trying to tell me?

Note that all the tests that I report as measured at the ECU as you call it, were measured with the computer not connected, so any voltages supplied were not supplied to the computer itself. All measurements were made with a 20 Mhz HP scope on auto trigger, which will synch up nicely.

peanut

william I was responding to Mhariush who was the original poster and as I understand it the owner of the car.

I am not sure that I can work with you.? I am helping 4x posters with their non start issues across 3x forums . I need someone who will listen and conduct exactly the tests I suggest in the order that I suggest and respond to every question I put without going off and 'doing their own thing'

have a chat with the op and decide who is going to do the testing and let me know if you are prepared to work with me .