no-spark problem finally resolved
 

This is my attempt at giving back to this awesome resource. I couldn't have successfully completed this task without the knowledge and dedication of the other forum members.

Determining If No-Spark Is Reference Sensor Problem:
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This forum has many, many threads for 'no-spark', especially for the 84-89 Carrera. I must have read scores of threads on this subject going back several years. When my 1984 3.2L Carrera decided not to start one morning, three years ago, I did some brief and spotty research on various forums, and convinced myself that the no-spark problem was the darlington transistor in the DME that feeds the coil. I went so far as to replace it, only to be disappointed that didn't fix the problem. I removed the part and put back the DME to it's original condition (hopefully). The car sat, gathering dust...for 3 years!

So I finally got a week where I could work on this issue without interruption (i.e. kids and wife away for two weeks, off work one week), and I revisited some old threads. In one, user 'Lorenfb' offered up 10 steps for resolving no-spark, which coincidentally share the first 5 steps in sections (28-15/28-27) of the Factory Workshop Manual. I thought this would be a good start and hopefully I wouldn't get to step 10.

1. A missing +12 volts on DME pins 1, 18, 35, (bad DME relay) or
2. A missing ground on DME pins 5, 16, 17 (check grounds on engine, tranny), or
3. A bad speed sensor on DME pins 8 & 27 (1000 ohms/2.0VAC), or
4. A bad reference sensor on DME pins 25 & 26 (1000ohms/.100VAC), or
5. A speed sensor plugged into a ref sensor connector or opposite, or
6. A bad ignition coil, or
7. A bad ignition switch (no +12volts on coil), or
8. A missing flywheel pin, or
9. A bad +12 volts on the injectors, or
10. A bad DME ECM.
(steps courtesy of user 'Lorenfb' http://forums.pelicanparts.com/porsche-911-technical-forum/311570-3-2-sensor-question.html)

I had gotten to step 3, but was looking at the waveform directly with an oscilloscope before doing a resistance test (Workshop Manual method). Oddly, I was getting what looked like the correct waveform for reference and speed sensors, but after checking resistance, it was reading infinite ohms (open circuit inductor) on both reference and speed. I checked at the connector in the engine compartment and at the DME, but infinite ohms on both. I still can't figure out how the oscilloscope was showing somewhat correct waveforms. However, it turned out the infinite ohms reading on the sensors was the smoking gun that pointed to the cause for no-spark. I replaced the sensors and it immediately started.

So the above 10 step-by-step checks should get you very close to figuring out a no-spark problem.

In many of the threads I read, many refer to 'replace the reference and/or speed sensor'. Many also mention that this is no small task. I can now concur that it is no small task, and also I would like to offer up a step by step procedure and hopefully some tips for doing just that: Replacing the Speed and Reference Sensors on a 1984 3.2L Carrera.


Step-by-Step Highlights:
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1. Get a 3/8 socket wrench, 12" extension, U-joint, 3/8-1/4 coupler and either a 5mm hexhead or 5mm socket with a cutoff 5mm allen wrench.
http://forums.pelicanparts.com/uploa...1278688849.jpg
It will keep the cussing to a minimum. I will refer to this assembly below as the 'Thinga-ma-jig'.

2. I recommend buying the BMW sensors instead of Porche. There were no visible differences and they have proven to work. $62 each vs. $125 each (both available from Pelican).


Step-by-Step Proceedure for changing Speed and Reference Sensors:
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1. Jack up vehicle in rear. Use jack stands for safety.

2. Remove left rear tire.

3. Using an handheld light, you will see the sensors up under the wheel well behind the shock absorber. Sorry, I didn't take photos at this stage.
http://forums.pelicanparts.com/uploa...1278689105.jpg

4. Loosen the 5mm allen bolts that secure the sensors to the holding bracket. This was considerably painful with just an allen wrench since you can only move it about 1/8 turn before repositioning the wrench. Much cussing ensued. Use the 'Thinga-ma-jig' for easy removal.

5. In the wheel well, remove ref/speed sensor wire hold down with 10mm socket. Set it aside.

6. From the engine compartment, remove the plastic heated air elbow for accessiblility.
http://forums.pelicanparts.com/uploa...1278689040.jpg

7. Remove brass key on sensor holder in engine compartment.
http://forums.pelicanparts.com/uploa...1278689182.jpg


8. Disconnect bottom two (speed/ref) connections. Middle is 'speed', bottom is 'reference', so note which is which. You may even want to use tape and ink. Save the metal wire clips for re-install.

9. Remove ref/speed sensor wire hold down with 10mm socket in engine compartment on the manifold. Set it aside.

10. From the wheel well, remove sensor grommet. Poke a very skinny screwdriver through one of the holes for ref or speed sensor wire. Pry out the grommet partually until you can grab it with your fingers and pull out completely.
http://forums.pelicanparts.com/uploa...1278689259.jpg

11. From engine compartment, tape a flexible wire to the head of one of the sensors. Pull that sensor from the wheel well through the grommet hole. You will use this later to pull the new sensors back into engine compartment. Tie wire to something near the wheel well so it's out of the way. Pull the other sensor into the wheel well.

12. Slip sensors out of the sensor bracket. If possible, do not remove bracket to save time in gapping the speed sensor. I was not that lucky. The speed sensor wouldn't budge so I had to disconnect the bracket (two bolts not hex head as shown in photo) and tap it out with a hammer careful not to ding up the bracket.

13. I used BMW sensors 12-14-1-708-619-M14 ($62 each from Pelican) instead of Porsche sensors (911-606-215-01-M14 ($125 each from Pelican). The were identical to my old ones except the cord length was about 1/2 inch longer.

14. The gap on the speed sensor is 0.8mm.
http://forums.pelicanparts.com/uploa...1278689373.jpg.
After all my pain of getting to this point, I was a bit anal about getting this gap precise to prevent any disappointment down the road. I found a pressed board washer in my shop that was slightly thicker than 0.8mm. A metal washer would have been ideal, but I couldn't locate one. I rubbed the washer on sandpaper with my finger on both sides, until it was dead nuts 0.8mm (0.0315in), verified by my table saw micrometer.
http://forums.pelicanparts.com/uploa...1278689473.jpg
I epoxied the washer to the end of the old sensor. Superglue would have been faster. I had to wait 10min.

15. I sqirted some silicone spray into the bottom sensor hole and put the old sensor with the washer attached into the bracket in the speed sensor position (bottom). This needs to be easily extracted later.
http://forums.pelicanparts.com/uploa...1278689601.jpg
I installed the ref sensor in the top position while I had the bracket out because only the speed sensor is used to set the gap. I lifted the bracket into place, and inserted the lower 'pivot' bolt (with loctite) into the bracket. This was difficult because I could only feel where it was supposed to bolt to the engine. Once it was in, I tightened the bolt so that the bracket could still pivot slightly on the bolt. Push the washered speed sensor all the way down until the washer touches the teeth, then insert (blindly) the top bolt (with loctite) and tighten it. Then tighten the lower pivot bolt tightly. I couldn't get a torque wrench in that space, but they were snug. The clockwise rotation of the wrench helps keep the washered speed sensor against the teeth.

16. Using the 'Thinga-ma-jig', loosen the allen bolt on the lower washered sensor and slide it out. Install the new replacement sensor in its place.

17. Label with tape and ink the connector ends of the sensors as 'ref' and 'speed'. Or peel the 'BG' and 'DG' from the old sensors and use them. Using the wire that goes from the wheel well to engine compartment, loop the wire around the 2 sensor connectors and also tape the wire to the connectors in series. If you bunch them together they won't fit through the grommet hole. Pull the sensor wires into the engine compartment.
http://forums.pelicanparts.com/uploa...1278689669.jpg

18. In the engine compartment, mate the speed/ref sensor connectors with the corresponding speed/ref sockets through the brass holder. Push the brass key back into the holder. Re-attach the metal wire clips on the sockets. Make sure reference connector goes to reference socket and speed connector to speed socket.
http://forums.pelicanparts.com/uploa...1278689865.jpg

19. At this point, you may want to try and start the engine to see if you should rejoice or cry.

20. Assuming the rejoicing has come under control, from the wheel well, insert the sensor wires into the grommet slots/holes, lubricate the grommet with some silicone spray an just push it back into position. Goes in a lot easier than it came out.

21. In the wheel well, group the sensor wires through the hold down and bolt the hold down to the body. Do the same for the hold down that attaches to the mainifold in the engine compartment.

22. Re-mount the rear tire, get the car off the jack stands, start it up and give a Cheshire grin. Hopefully you won't have to do this for another 20 or so years.

Good for you
 

I dread the day but now I got a good F/F guide.

Thanks

After all these efforts, you need a beer or 2. Help yourself :D:D:D



http://forums.pelicanparts.com/uploa...1278691376.jpg

Thanks for the great write-up. Should be very helpful to many.

Good write-up.
Glad i don't have to deal with them on my 79

yes,..thanks!!!!!!!

Doyle

Great job, will save this for future reference SmileWavy

Great write up! Perfect timing as well, heading out right now to get started on my car!

Thanks! :)

I used this as my guide today and my car fired right up!!! Thanks a million for putting this together.

Also, when does the Cheshire Cat reaction go away?

Man, I had to do this crap a couple of months ago. Really could've used this excellent write-up, especially about the Thinga-ma-jig.

Oh well. Success is sweet, even if a little blood is spilled.

Just finish mine. A lot easier with the engine out but the downs side is that I won't be driving for a while...

What is the torque for the bracket bolts? Could not find it anywhere.

Thanks

JJ

Sorry, haven't looked at this thread in a while. The Workshop Manual doesn't specify a torque for the mounting bracket. I suppose they assume the sensors will slide out without having to remove the mounting bracket. Yeah...right... They do specify a torque for the 5mm hex screws that mount the sensors to the bracket, which is 8Nm (6ft-lb).

I suppose it goes away when you die, because I'm still smiling....

That is what I end up doing.

Put her back together last week-end. Crank a few time with the DME relay out to get the oil pressure up and it start on the second try.

Great write, thank you for all your work!

Hey, I am at the point of installing both the speed and reference sensors (BMW) type. Can I silicone lube up the sensors before installing? I will be sure not to mix up the sensor connections.

I have had a no spark issue and hopefully narrowed it the speed sensor. The VM checks pointed toward this as the problem.I pulled the DME out to check with a VM as mentioned in the threads. I was never able to get a reading and was afraid to jam the VM contacts into the pin areas to hard.

The car is a 84 3.2 Cab I have had since 86.

Much thanks to this thread for all the tips. Since I am not master mechanic and the first time through takes me for ever.

Excellent write up!

I don't really understand the whole "gap" thing you did. I've done this on a few of my Carreras over the years, and I've never adjusted the gap. Just took out the old and replace with new. If you remove the sensors from these 2 points, the gap does not change (the factory gap adjustment is retained) and does not need to be adjusted.

http://forums.pelicanparts.com/uploa...1278689105.jpg

I've found the job to be fairly simple, with the exception of those rubber grommets! Those are a pain to get back in place.

oops, I reread your post and see why you removed the entire bracket.

I think if you would have kept working on jiggling the speed sensor, you would have been able to pull it out without removing the entire bracket and having to regap.

I just pulled my sensors out with pb blast and a lot of wiggling as you indicated. I just pulled the sensors through the fire wall. I am at the point of trying to insert the sensors.

As I asked before is it OK to silicone spray the sensors to ease the insertion process before bolting up?

Bob
84 Cab

Bob

No problem with spraying the sensor to help the sliding in.

I measure how the Porsche Bosch sensors were sticking out of the bracket and did the same for the BMW Bosch sensors. There were no difference.

So, if you did not remove the bracket, it should be a straight forward R&R.

In my case, I removed the bracket so I had to do the the alignment witch was very easy with the engine out and split from the transmission.

Here is a word of wisdom that was past down to me. After replacing any of the 2 sensors, do rotate the engine by hand so that the if there is any mechanical interference for any reason (Murphy law rules) you will find out at a very non destructive speed...

Put everything backtogether yesterday, after sitting for a month, started up on the first crank. Happy man!

Thanks for all the help, great instructions and made things easier.


Bob
84 Cab

How did you figure out the BMW part was the same? I see others under BMW - this one is $46.50 : 12-14-1-710-668-M85, says for Motronic also. The one you used is now up to $70.75. My 1988 needs two new ones because the insulation crumbled and fell off the wires during removal for my engine rebuild. Will the one you used or this other one work for my 88?
Thank you so much for the great write up!
John

There are a lot of threads on this part being interchangeable with the Porsche sensor. Only difference is a half inch longer and much cheaper. Pelican says they sell a lot to Porsche guys, only downside is that you get a BMW decal.

The part number is 12-14-1-708-619.

I changed both sensors, only the speed sensor checked out bad. Going the hassel of change, doing them both made sense.

Bob

btdt

Thanks to Tom Stiller for the amazing step by step to faulty sensor detection. I am confused. I drive an 84 carrera 3.2 or I did. It started on first crank for the last 15 years. The other day just stopped. I have no spark. I replaced the DME and have done all the rudimentary checks to no avail. My problem is the DME pin nos.referenced in Toms description differ to mine. The DME socket pins are 1,3,5 top and 2,4,6 bottom. The corresponding pins on the DME read 87b,85b,30 top and 87,85,86 bottom hence my confusion when trying to check the ohm reading.
PLEASE HELP

Really excellent write up, Tom. On a side note, the reason the oscope showed a waveform is that these sensors employ a strong fixed magnet with an inductance loop. The fixed magnet provides reference and amplification of the signal. Often when the sensor goes bad, the fixed magnet field is still influenced by the teeth on the reference wheel passing through the magnetic field. This induces a small amount of current flow from the sensor. Not enough to signal the DME, but plenty to see on the oscope.

Now back to your regularly scheduled praise for such a fine write up!

Speed sensor was the problem. The sensors in Australia retail at $330 each. 2 sensors from Pelican parts delivered to Oz $160. Put them in last night and fired up first crank. Thanks Guys WOO HOO. The scarab is on the road again.

Just some comments after having just done this job today. First, I wouldn't buy the FAE sensor that is available under the BMW p/n. Only one of mine worked (the one in the ref position). The speed sensor ohms ok, but the car wouldn't run. Runs fine on the old ref sensor placed in the speed position, so I have a Bosch BMW sensor on its way.

Second, ain't nfw that my speed sensor would come out, even after overnite with PB and persuasion with a hammer after removing the bracket. Had to destroy it to get it out.

A tip-I cut the head off a longish bolt to create a stud, and put that in the upper bracket mount hole in the case to hang the mount from, then put the lower pivot bolt in, removed the stud, loosely threaded in the upper bolt and set gap. It made hanging the bracket a piece of cake. Not sure I could have done it without the stud, at least not without throwing things.

Car started first try..well, after I got the connections right at the upper end.

Not a fun job, but doable. Didn't remove the shock, did remove the heater hose going to the flapper.

Hi Tom- Just a huge note of thanks for the excellent write up! Luckily, I was able to remove both sensors from their mounting bracket with copious amounts of WD40 Penetrant Release (their version of PB Blaster). The old wires were completely devoid of insulation! (see photo) I marked the old leads front and rear, and used trimmer line to snake the new wires. I kept losing the mounting bolt on the top sensor (it would fall into the abyss, once even ending up in the muffler pan) I used a length of fishing line to keep it in place for the initial tightening. Again, your write up made this a fun and rewarding project! http://forums.pelicanparts.com/uploa...1445087078.jpg

A big 'hats off' to Tom for taking the time to make such an excellent step by step procedure; photo's and all!

I did it 'my way' the first time around and it was a pig of a job. I did it 'Tom's way', this time around, and it was just about what I would call a breeze.

I wonder which way 'Frank' would have used :)

Tom: Thanks so much for your post. Just wanted you to know it is still being used and appreciated. Just changed out my sensors and the 85 Carrera started right up. This my first Porsche, and although a very low mileage one, I am sure I will be making more parts replacements. Thanks to the knowledgeable folks in the forum, I am learning a lot. Sure is harder to work on than my old Austin Healey though.
Thanks again.

Great write up and pictures. Replaced both sensors along with the CHT sensor over the weekend. Insulation was worn off the sensor wires on my '88 so I thought I'd do the replacement before I had a problem.
I'd like to add that it's not necessary to use anything to fish the new sensor wires thru the tin into the engine compartment. The connector end of the old wires are pulled thru the tin into the wheel well (can't be done the other way as the sensors won't fit thru the hole). Once the old wires are out, simply push the new sensor wires thru to the engine compartment. Those wires are plenty stiff so they will emerge within easy reach just about next to the connector holder. No danger whatsoever of the wires getting lost in the abyss...

It is not a simple task to replace the sensors for a first timer. I was struggling with even getting mine out. I am fortunate to have a friend that is an experienced Porsche technician. I called him and asked if he had any tricks or suggestions. He told me to step away from it, and he would swing by.

His biggest trick was to use 1/4 inch drive ratchet and extensions. The same basic "rig" in the original post, but all made from 1/4 inch drive parts. That makes them smaller, and gives more room to maneuver.

My buddy lay down next to the rear wheel well and within a couple of minutes had the old sensors out and I handed him the new sensors. He had them back in in a couple of minutes. It was like he was a magician. I had stared and cussed at the sensors longer than he took to replace them.

I know my friend is a pro so I paid him for a full hour of work because he saved me from a day of struggle, and he made a house call!

My only real contribution to this thread is to use 1/4 inch drive tools for that task. The torque needed is low and 1/4 inch drive is up to it no problem.

Quote from Glen: "My only real contribution to this thread is to use 1/4 inch drive tools for that task. The torque needed is low and 1/4 inch drive is up to it no problem."

+1 Big Contribution -> This 'is' the secret = 'easie-peasie'

Threads like this make this the forum it is. Way to go Tom!

Great write up. Thank you, just replaced mine. Bottom sensor much easier than the top. I couldn't get the 5mm socket with extender up there but it's not that bad even with an Allen and turning it small bits at a time.

Are the #'s on the sensors in the pictures on first post from "Step 3" and "Step 7" correlated?thanks.

Hi..Are you referring to?..

7. Remove brass key on sensor holder in engine compartment

Correct -do the plugs go (top to bottom) CHT, Speed, Crank. Or CHT, crank, speed. I thought I would be able to find that in manual, but having some trouble. Hope that makes sense

CHT is easy - it’s white, ha ha.

EDIT - DUH.....Should have just looked at Step 8.

Hi...I see that you now have the info you were after... I can only suggest that you do two things:

1. Make sure not to remove the sensors bracket on the fly wheel - Just the sensors...If you do, then it becomes a lot more fiddly as they have a very fine depth fitment.
Other auto manufacturers have a 'stop' in their castings - You just pushed the sensor in until it 'stopped ...Porsche had them manually gauged as they assembled the engine = :(

2. Make sure to mark which one they plug into on the 'brass bracket' -> #7.

Other than that, other than turning your arm/body into a pretzel, it's really not that hard of a job...I would also suggest that you change the head heat sensor while you are at it. :)

All good - thanks - got them out last week and have marked the both the cable ends with red or white paint (depending on Speed or Crank) and ready to go back in tonight. Did the CHT sensor are well - "while I was there". The actual hardest part of this was getting the old grommet out where the 3 cables go through the engine tin. That and the hand cramps from the tight working area.