FIXED?! 20/2 electrical problems 944 2.7

[peanut]

ok that makes sense. Right then... It is clear that the ECU is not earthing terminal 85b thereby energising the second relay in the DME (fuel pump) By taking terminal 85b to earth you are effectively bypassing the ECU earth and directly energising the second relay of the DME. You achieve the same result when you put the 3x way bypass in, which takes the +12v on terminal 30 and supplies it directly to terminals 87 and 87b thus supplying the ECU ,injectors and fuel pump without either of the DME relays working. Why isn't the ECU earthing the second DME relay ? mmmm well it has to be because either the ECU isn't receiving the +12v from terminal 87 of the DME relay or it must be because the ECU isn't receiving the necessary inputs from one or more of the sensors. Do you get a very slight twitch of the tacho when cranking the engine ? Have you cleaned the terminals in the electrical connector from the crankshaft sensor ? Have you tried getting someone to wiggle the sensor wire whilst you crank the engine ?

 

[Hilux]

Thanks for the reply Paul - I checked the DMEs according to the Clark's guide and all had the stated correct resistance when the relay was energised.Does this still sound like I have an earth problem from the DME?

You shouldnt have resitance on 30 to 87 in my opinion, if you have there`s a potential issue. Also, the ECU needs to see a minimum turnover (revs) for the injector trigger.

if terminal 87 has +12v on it when the ignition is on, then the primary relay is energised and therefore the injectors and the ECU should both receive +12v

I presume that you have verified that the injectors are receiving +12v from terminal 87 of the DME relay with the ignition on yes ? The injectors (IIRC) are supplied via a permanent live energised by an ignition fed relay - this should show no resistance. This has a common path across all four injectors and the ECU earths the injectors in phase. There should be some resistance across the injectors - if this is too much there`s another issue. You should be able to trace the feed to the injector plug and test for battery voltage on the battery fed live when cranking thereby eliminating this aspect also.

 

[doctor924]

Thanks for advice - will check out these aspects of the weekend when i have more time.

 

[peanut]

Doc I checked all my schematics and it would seem that in 1989 Porsche switched from the Motronic 1.5 to the motronic 2.1 system for the S2 On your 1988 car terminal 85 on the primary relay is not earthed by the ECU but is taken directly to the earthing point MP ll which is on the chassis just in front of the relay/ fuse box somewhere ? My 1989 S2 has the later Motronic 2.1 system and terminal 85 is earthed by the ECU. Up to 1pm this morning checking through all the original porsche schematics for several 944 models so I could be sure about this. I need to get this quite clear in my head for future reference as the motronic system is quite complicated and isn't helped when you realise how many different variations there are with the Jetronic and Motronic systems including different ECU earthing arrangements for automatic cars and A/Con equipped cars. Here is my appraisal of where things are at present. DME primary relay is working as it should. DME second relay is only working when you take an independant earth to terminal 85b (bypassing the ECU) So ... something is preventing the ECU grounding terminal 85b and this can surely only be a either a fault within the ECU ,or poor earth to the ECU or more likely a faulty CPS or missing signal from the CPS (crank position sensor) to the ECU. The ECU will only ground terminal 85b on the DME relay if it registers 225rpm from the CPS . I would concentrate on the CPS. Unclip the connector at the back of the cam cover and clean the contacts. Measure the resistance as per Clarkes. Check the cable down to the bellhousing carefully for breaks, nicks cuts or any damage. If this checks out ok you'll need to check the CPS signal is getting to the ECU. Make sure ignition is off. Disconnect the ECU and check continuity between the CPS sensor connector terminals 1,& 2 and the ECU connector (not the ECU itself) It should be 8 & 27 on your ECU . As the CPS is such a critical component and quite likely 23+ years old on your car I would recommend replacing it anyway to be sure . The wires from the sensor to the connector become brittle with heat and age and you may get intermittant faulty starting and running and never know the cupret. Good luck

 

[doctor924]

Wow! []Thanks for the effort to determine the potential causes of this problem - it is MUCH appreciated. I'll focus upon this over the weekend.

 

[peanut]

no worries Doc i'm doing it for everyone really and when I've knocked it into shape as a troubleshooting guide I'll post it on Tipec and elsewhere too. I've been in the process of constructing a Flowchart for the past 6 months ! Hopefully it will be useful for anyone in the future that has a non start poor running problem.

 

[Frenchy]

Dedication indeed Nick ! Good on ya []

 

[peanut]

ORIGINAL: Frenchy Dedication indeed Nick ! Good on ya []

you know what they say Frenchy..... [] what goes round comes round

...... I did ask Santa for a Ferrari last Christmas but I think he's been a little busy [&o]

 

[doctor924]

DME primary relay is working as it should. DME second relay is only working when you take an independant earth to terminal 85b (bypassing the ECU) So ... something is preventing the ECU grounding terminal 85b and this can surely only be a either a fault within the ECU ,or poor earth to the ECU or more likely a faulty CPS or missing signal from the CPS (crank position sensor) to the ECU. The ECU will only ground terminal 85b on the DME relay if it registers 225rpm from the CPS . I would concentrate on the CPS. Unclip the connector at the back of the cam cover and clean the contacts. Measure the resistance as per Clarkes.

Resistance is OK as per Clark and for peace of mind i bypassed the alarm as per 944online forum (not Clarkes- as later alarm modules require different bridging) - but the non start persisted

Check the cable down to the bellhousing carefully for breaks, nicks cuts or any damage. If this checks out ok you'll need to check the CPS signal is getting to the ECU. Make sure ignition is off. Disconnect the ECU and check continuity between the CPS sensor connector terminals 1,& 2 and the ECU connector (not the ECU itself) It should be 8 & 27 on your ECU .

Not sure I did this correctly - I checked continuity on the big multiplug between contacts 8 and 27 - and saw infinity (i.e. no continuity) - is this correct? I also swapped in a spare ECU and the car had the same fault. So - a continuity problem...? Thanks Ian

 

[peanut]

Ian you need to check continuity of both the wires between the CPS (crank position sensor) connector and the ECU connector to be sure the connecting wires are ok. If you have checked the sensor and the resistance checks out ok then check the continuity between the Sensor connector terminals (rear of the cam cover) and the ECU connector .If there is a break in these two wires then your ECU isn't getting the essential start/run signal from the CPS (crank position sensor) There should be 3x terminals in the CPS connector,( I believe it is terminal 1 & 2 that you need) and the 2x relevant terminals in the ECU connector should be 8 & 25 according to my schematic. Have you actually got a slight jump or any movement of the tacho needle when you crank the car ??? this is important please try to check this for me. The ECU will only energise the second DME relay when it receives a run signal so it is highly probably that this is where the problem lies.

 

[doctor924]

Hi Nick Thanks for the info again - I have checked on these issues this morning with the following results. Crank position sensor (BG label on cable and mounted furthest from the bulkhead) - 3 terminals . I have numbered these as: 1) terminal nearest bulkhead when CPS connected to loom 2) terminal in the middle when CPS connect to the loom 3) terminal furthest from bulkhead when CPS connect to loom I thne checked these connections with DME connector terminal 8 and 25 for continuity -the results were: Terminal 8: infinity for 1, 0.2 ohms for 2, infinity for 3 Terminal 25 infinity for 1 , infinity for 2, inifinity for 3 I also checked terminal 27 (as you noted this in a previous note) - but the results were the same as for terminal 25 on the DME connnector. So - it looks like I have a continuity problem - with only 1 of the CPS feeds being delivered to the ECU? Ian p.s. the tacho bounces when trying to start

 

[peanut]

Hi Ian the fact that the tacho needle bounces would indicate that the ECU is receiving the CPS sensor information. I feel we are very close to the solution here. Clearly it is not convenient to have a DME bypass cable permently but if the worse comes to the worse you could run the wire into the cabin and have a switch on it to switch off the fuel pump until you solve this problem. The ECU has many sensors which it uses to modify its own programmed fuel and ignition map to get optimum performance. if any of these sensors become faulty the ECU is programmed to deliver a pre-programmed 'limp home' performance. many 944's are running with faulty sensors and are probably unaware of it except for difficult starting and poor fuel consumption and power delivery. I will try to find out the exact ECU /CPS connector terminals today so we can be certain we have the right ones but it sounds to me that the sensor is working ok and the signal is reaching the ECU. I would suggest the next step is to find the ECU earthing point MP ll and strip and clean it thoroughly . Alternatively find the large brown cable coming out of the ECU Connector and run that to a good earthing point on the chassis and see if there is any improvement. good luck

 

[doctor924]

Cheers for the update Nick. Aside from the evidence presented, my only other observation is that the car went from starting fine to not starting instantly - i..e no gradual deterioration - and after i'd managed to blow a few fuses trying to resolve a wiper problem discussed elsewhere on the forum.Pretty sure something has failed somewhere!! I;'ll have a look at all the earth in around the fusebox and clean them anyway. Cheers Ian

 

[peanut]

very good point Ian...[] I had forgotten about the link with the wipers. looking through the schematics there are a surprising number of direct wiring links to unrelated electrical components within the 944 loom. Naturally they used convenient power and earth supply points wherever they could. There are lots of soldered joints hidden in various parts of the wiring looms . I'll take a look later tonight to see if I can see any possible links. I have to say the ECU is extremely suseptable to electrical spikes and surges which is why you must always disconnect the battery when welding and avoid arcing when jump starting. As a matter of interest have you tried running the car with the wiper/washer electrical connectors completely disconnected yet ? just a thought !.....

 

[doctor924]

Hi Nick Well - congrats to that man! It seems my earths were the cause of the problem. I cleaned four near the fusebox and the car fired straight up as good as new. I still have a few wiper problems to resolve - and any link you do find between them and the starter system - would be warmly welcomed. One thing I did note when running the bypass on the relay was that timing seemed to have been retarded on the engine. When the car started as normal the engine seemed fine and timed great. Many thanks again Ian

 

[pauljmcnulty]

Hi Nick Well - congrats to that man!

Indeed. [] Nick, if you want to condense this in to a fault-finding faq it can go into the technical articles and be stickied for life!

 

[peanut]

thats great news Ian . There were limited possibilities to be honest but without seeing the car I was reliant on your excellent troubleshooting skills. Well done.! I would really appreciate a picture of the two earthing points in front of the fuse/relay box . They are MP ll and MP lll and are not on the S2 that I have. cheers Nick ps the reason that your engine was running retarded is because the bypass you fitted bypassed all the sensors so your ECU simply ran its pre-programmed map from its EPROM Its basically a 'limp home mode' Now your ECU is registering the AFS and engine temperature sensor and 02 sensor etc and making subtle adjustments all the time.

 

[doctor924]

Hi Nick Will grab some pics when I find the lead for my camera! Mooching around the fusebox area and there 's quite a bit of wiring 'unused' there - i.e. connectors that aren't connected... Given the car's long lay up - I never got the alarm fully working (the led door pulls never flash) nor the sunroof....[8|] All good fun! Cheers Ian

 

[peanut]

hey doc hope hope the car is dtill running fine. if you get a chance to photo the earthing points that you cleaned I'd really appreciate it, it will hopefully help me get someone elses running. They are differant earth points on your car to an S2. cheers

 

[doctor924]

Hi Nick Yes - its fine thanks and happily cleaning the earths also brought the sunroof back to life! I lost my camera cable- but have ordered a new one and when it arrives will post up some pics thanks again ian