Tom M'Guinn

Assuming this is a turbo, also check if the boost gauge works. If neither work, then look for a connector in the passenger footwell, on the center console side. On early turbos, it is a bright red square 4 pin connector, and on later cars its a flat black 8-pin connector. The signal to the tach and boost gauge run through that connector. If it's still connected and the tach doesn't work, then see tach testing here:

https://rennlist.com/forums/944-turb...achometer.html

Yes, no cam sensor on 944 Turbo's. DME fires the spark based on crank position, and the spark just jumps to the closest post inside the cap.

Tom M'Guinn

Posts crossed, but no the connector for the tach is in the passenger footwell, not under the hood. If the 3-pin connector you saw under the hood was a round barrel connector, it was probably the O2 sensor. Worth figuring out why someone would unplug that...

curtisr

I couldn't agree more. Just such a warning should appear as a sticky or banner at the top of this Forum. I, too, jumped through most of your same hoops only to discover that switching to Bosch fixed the no-start condition. $@#$(^(!!!

Joe6pack

I skimmed through this thread last night as I am going through pretty much the same thing. The car is a 1987 944 NA, but the symptoms are identical. It had been running fine, but I pulled it into the garage after a drive one evening about a month ago and that was it - no start. Having had a 1987 911 and this 944, I pretty much know Motronic. I have no spark, no injectors and no tach bounce. The injectors pass the ground test at the DME connector and the coil produced spark when tested separately. I ran all the usual tests on the sensors and wiring and everything checks back to the DME connector. I regapped the sensors and used my jumpered DME relay so I could hear the fuel pump. Fuel pressure checked out as well. I tested for spark before the cap using a plug and I had a spare coil to cap wire and neither showed spark. So, I suspected the DME. Here's the thing: this was a rebuilt DME purchased from ECU Doctors in November of last year with a five year warranty. So, I sent it back to them and they checked it and said it was fine. They even sent me a video of it in a running 944. So at that point, I decided to throw parts at the problem. The speed/reference sensors that were in the car were purchased a couple of years ago, but I decided to replace them anyway. They had no markings and I know they aren't Bosch. So I ordered new Facet brand sensors, a new Bosch coil and new plugs. Still no go. So, after finding this thread last night, I am sending the Facet sensors back and I have ordered new Bosch sensors. Granted, these are the BMW sensors, but they are half the price of the Porsche sensors and I am hoping they will work. If they don't work, I will return them and pony up for the Bosch 944 sensors. If that doesn't work, I will drag it into a field and set fire to it.

Dan Martinic

Hold that lighter just yet: the Bosch sensors for the BMW work perfect; it's what I installed. The only difference is the cord length, and it's negligible at best.

I think you'll be fine. Hopefully

PS: be sure to post back what happens!

Joe6pack

Absolutely. There's nothing worse than a thread that ends without resolution. In the meantime, I am going to remove the bracket that holds the sensors and clean the area of the flywheel where the reference sensor picks up. I am also going to replace those #$%^&* 6 mm allen key bolts with hex bolts.

Dan Martinic

I'm a little confused by this. What do you mean by "clean the area where the reference sensor picks up"? The sensors pick up rotating threaded pins....

Droops83

The original bolt for the speed/reference mark sensors is a hex-head bolt, but of a special type with a deep shoulder to clear the void in the bolt hole of the sensor. A past owner/mechanic probably lost these bolts and installed Allen-head bolts for this reason. A standard hex-head bolt will not work unless you use a stack of appropriately-sized washers underneath, which would be hokey . . . .


I have replaced lots and lots of speed/ref mark sensors on customer 944s and 911s over the years, and my shop has always used the OE Bosch sensors. The price of the Bosch sensors has risen dramatically over the past few years (even as the quality of a lot of Bosch stuff has gone downhill); while we still use the Bosch sensors on customer cars, I cheaped out and got the Facet sensors for my own 951 about 6 months ago for preventative replacement due to deterioration of the connectors and insulation old Bosch sensors. I quickly swapped them in after work one evening and they worked perfectly well then, and still do.

This is not to say that there isn't an issue with these sensors (it seems that there are a number of people that have had no-start issues that have been instantly cured by changing sensor brand), but I wonder if it is a bad run of Facet sensors, or some other variable such as deterioration of the engine harness side of the sensor wiring (maybe the cheaper Facet sensors are more sensitive to noise/interference).

Anyway, I'm glad that Dan finally found the issue, and hope that others do as well.

Dan Martinic

Joe6pack

Sorry, I meant clean under the bracket - basically remove the bracket and clean it. And the allen head bolts I was referring to were the ones that hold the sensor bracket to the bellhousing. I can't seem to the get a 6mm allen key into that stupid hold in that tight space when adjusting the sensor gap.

Well, the Bosch (BMW) sensors came in yesterday. I dropped them in and ... nothing. So that was a waste of time and money. Still no spark, no injectors and no tach bounce. The car passes every test for the harness electrical connector test as in post #9 of this thread except for: terminal 10 and ground continuity and tachometer/upshift light (11 and 21) continuity. I believe the original poster had this issue as well which is what led me to this thread. I didn't test volume air flow, oxygen sensor or air conditioner ON signal as I didn't think them relevant. I don't really know what I am looking for in the coil test at the harness. With the key on and terminal 1 grounded, I read 12 volts. While turning the engine over, the voltage drops to 10 volts. It could be pulsing, but I don't have an oscilloscope. I have verified continuity between terminal 1 and the coil (green wire).

I have verified continuity of the sensors back to the harness and there are no shorts, no crossed wires, nothing. I also verified that the allen screw in the flywheel hasn't fallen out.

If anyone has any ideas on something to check, I would love to hear them. I don't have an oscilloscope to verify the sensor signals and I don't trust my multimeter for AC voltage readings. That said, I'm on my third set of sensors (counting the originals), I have set the gap three or four times now and the wiring is all correct. Assuming the reference sensor is picking up the allen screw and the wiring is correct back to the harness, shouldn't I at least have tach bounce? If I hadn't already sent the DME back once, I would swear it was the DME.

Dan Martinic

Sorry to hear the new ref sensors didn't change anything (for future readers, you can perform the continuity check between the sensor shield and any of its pins before ordering new ones)

You've had the DME confirmed as good. What about the KLR?

gpr8er

qualifier I'm certainly not an authority on Motronic, but I have had the same weird issues outlined in this thread. My car would suddenly die while going down the road, no start etc. Could never track it down. Changed virtually everything, harnesses. sensors, TPS. Sometimes the car would go for weeks fine then problems. I suspected maybe 30+ year old wiring in the harness at the front of the car to the coil. If you've ever dissected wiring on these old cars you'll see that the copper has oxidized under the sheathing. Could never say for sure what the problem was. Anyway I changed the KLR and the issue seemed to resolve. But....now after all that monkeying around with this and that I moved to VEMS. Now I'm trying to learn how to get this configured.

Dan Martinic

On page 29 of the DME/KLR test plan (test points 14 & 15) there are signals that should be present on an oscilloscope while cranking. I'm pretty sure you'd get a flashing LED at the same connections. I would give this a try just to confirm the DME is sending the KLR a signal... then if the coil isn't flashing an LED, could be the KLR not returning a signal back to DME (which finally leads to DME signalling the coil pulses)

EDIT: Test plan link again http://www.arnnworx.com/dme-data.htm

Joe6pack

I should clarify that this is a non-turbo, so no KLR. I only posted here as the OP's saga was so recent and similar and I really hoped it was just the sensors.
Oh well, I am in Florida for a couple of weeks, so I won't be able to do anything further until I get back. Thanks for the help.
I do have a question of you Motronic experts that might help with future troubleshooting: if one disables the ignition system on a fully functioning car (i.e. disconnects the negative or positive wire from the coil) will tach bounce still be present? Depending on the answer to this question, a lot of items could be eliminated.

Tom M'Guinn

Never tried it, but based on how the system works, I would sure expect to see tach bounce even with the coil disconnected.