Dougs951S

Yesterday I did a 320 mile round trip in my 944. the first 180 miles went fine, but then all of the sudden the car started missing. At first it was just a momentary miss and then all was well. It did that a few times over the next 40 or so miles but then it started happening more often until finally on the last 100 or so miles I was on the highway doing 75 mph and the engine would just go dead periodically. It felt like the car hit a wall and the pedal went dead (no response) but the tach was still up. The car would be "dead" for sometimes up to 5 seconds; pumping the pedal and waiting would restore the power. I barely got the car home. I experimented with pedal position and going different speeds (to change the airflow/load) to try and see if it was related to a dead spot in the TPS or AFM tracks, and it appears to be completely independent. It was even bucking and running like **** as I went down my street in 3rd gear at 1800 rpm. Also, when the car would cut out for several seconds and then suddenly cut back in, I could smell fuel which tells me the issue is ignition related. I'm amazed I got the car home, I drove it 100 miles running like that, it would cut out and then run fine for 5 miles or so, then cut again. Repeat for 100 miles. When its running fine, it sounds and runs perfect. Sounds great idling in my driveway. Started up perfect this morning.

I'm thinking it's related to the speed and reference sensors since the car is also occasionally hard to start, but I'm not sure since when the car cut, I still had tach signal which means the speed sensor was still working. I was under the impression the reference sensor was only used for starting. Would an issue with the reference sensor cause what I am describing?

My other thought was it could potentially be a faulty DME relay, and the coil inside (it has one for spark, and one for fuel) for spark is intermittently bad.


On a side note, I REALLY need to megasquirt my NA also, so sick of **** like this with motronic. Crap like this is why I MS'ed my 951 in the first place.

V2Rocket

dme relay doesn't do anything related to spark, fyi...unless you set your car up differently.

i had a similar-sounding issue and it turned out to be the black wire from the ignition switch, through the central electric panel to the ignition coil +.

the wire had gotten super hot at some point and cooked its pin/socket in the central electric panel and would occasionally fault making the car die randomly on the highway etc.

i ran a whole new wire from ign switch to coil and all is well...

my test for this was disconnecting the original black wire from the coil, and running a test wire from battery + to coil + and making the car run.

Dougs951S

thanks Spencer, I was actually second guessing myself as I wrote that. I was reading through some old threads and someone mentioned a similar issue and made mention of the dual coils in the DME relay so I was posting that here so someone like you could tell me I was wrong

RE: The reference sensor, am I correct in my deduction that the S&R sensors are not at fault as the tach signal was good, and ref sensor is only used for starting? Sounds like what you described is plausible, especially since I was monkeying around under the central fuse panel about a month ago!

I may try your test Spencer, just don't leave it like that as the coil would be hot with key off! (potential melt down, or would it just kill the battery?)

V2Rocket

both speed/ref are used all the time by DME for tdc/revs and spark angle control.

dme relay does have 2 coils but they are sort of a "loopdeloop" in function...ign power goes through the relay coil#1 which powers the DME which triggers coil#2 and runs the fuel pump..

88Silver924S

First, I don't know what your problem was/is, hope you find the problem and let us know.

It could still be a DME relay with one of the contacts "fluttering" on and off occasionally due to temperature, sun spots or whatever. When my OE relay went out it was intermittent for a while, the car would sputter and die, but would restart and run for awhile. During the necropsy of the relay one of the coils had a copper corrosion green stain. I think that coil was weak and not always keeping the contacts tightly closed. A new relay fixed the problem.

I would like to recommend keeping a quality brand, tested, spare DME relay handy so if you find yourself on the road like that you can swap in the spare and eliminate it as a problem. After several thousand miles on my replacement relay I bought a second relay and put that new one in keeping the thousand mile "tested" one as my backup. Good luck.

jhowell371

Check for a loose O2 sensor too.

CVR_Rally

Check connector to the AFM. I had a dead moment and the connector had backed out because I didn't have the wire clip lock. I have remedied this so it doesn't happen again.

Dougs951S

That definitely would not cause the issue. This is obviously a hard fault, something electrical that is cutting the ignition.

10-4 sir, I try to never leave a thread open ended for posterity sake. And for the record, I too keep a spare dme relay in both my cars.

I am unfortunately also familiar with what it feels like when the AFM or TPS dies, typically at highway speeds/revs the engine will buck hard if you give it any kind of load but it is still very much running. This was not like that, the pedal was totally dead and unresponsive, not bucking. No power at all because the ignition was cut.

For what it's worth, a dead TPS and then a dead AFM (both confirmed by direct electrical testing on the bench) not even a week later are what prompted me to ditch motronic forever and go to stand alone EFI on my 951.

odonnell

It could be the reference sensor. You need both to run the car. Motronic doesn't have a missing tooth on the ring gear so it needs something to sync the crank position. This happened to me on the NA when I first got it. Remember switching those out at Veedub? Same symptoms.

It could also be the coil itself, I have 2 or 3 if you want a backup. You're getting fuel so your dme and dme relay are probably fine.

Dougs951S

Thanks dude, I'll let you know pending an investigation but I think my coil is ok, it came out of my 951.

Yes I remember that day at Veedub, we looked through every fricken 944 and bmw in that yard before we found those sensors, all while it was 100+ out in the middle of the summer. good times


Does anyone know if a failure in the reference sensor (but not the speed sensor) while the car is running will totally cut the ignition?

mel_t_vin

Not correct...both speed sensor and reference sensor are required to start the car.

V2Rocket

yes.

Dougs951S

That wasn't what I meant, I should have been more clear. What I was asking was if the reference sensor was used only for starting the car, but not used once the engine is running. I am aware the ecu needs both to know both speed and TDC location to start, just like any other engine. I just wasn't sure if; once started, the ecu still needed the TDC signal. In theory, a smart enough ecu with a high enough sample rate could keep track of rpm in real time and once started, it would always know were TDC was just based off engine speed and time. I guess it was silly to assume motronic was anywhere near that capable when even most stand alone ECU do not have that capability.


Spencer, is the total spark cut a failsafe to prevent the DME from massively retarding/advancing spark if it loses track of TDC?

V2Rocket

The Motronic doesn't have any "fail safe" modes, other than running rich without an O2 sensor

It needs both sensors' signals 100% of the time to know RPM and crank angle for spark. The SPEED sensor counts flywheel teeth to get crank angle...360 degrees/130 teeth (NA) makes a 2.77º resolution...the DME counts the teeth to know when to fire spark/time the injectors.

The Reference sensor just tells when the engine is at TDC to start the reference tooth count over again (1,2,3...128,129,130,1,2,3..etc)

Dougs951S

Thanks for the clarification Spencer