Dougs951S

Today I learned just how important the DME temp sensor is; even to a hot engine. In fact, unplugging the sensor on a running, fully warmed up car will instantly stall the engine like someone turned the key off. The engine will not start (at least not a warmed up engine) with it unplugged, and it is sluggish restarting for the first time after replugging in the sensor. I had no idea the DME temp sensor was this important, but clearly it is. Can someone explain to me how exactly the DME uses this info, and why it's so important that it stalls the engine/keeps it from restarting? I thought the sensor was only needed on a cold engine, and even then only for starting by activating the warm up enrichment to keep a cold engine from idling rough due to running lean (from poor fuel atomization in a cold engine).

odonnell

They're NTC sensors, so as temp increases, resistance decreases. When you unplug it, the DME reads infinite resistance (open circuit). On a warm engine, it expects a low resistance. So it's going to massively enrich the mixture as much as the enrichment table allows and stall.

Here's the datasheet for our DME temp sensors, it's the same part as the E30 and it's what I'm using with my MS setup. http://www.farnell.com/datasheets/307042.pdf

Check your phone nukka

tempest411

It's not just a 944 thing, try unplugging the temp sensor on almost EFI car and see what happens.

marc abrams

That's not entirely true, every car to a class 8 truck I've ever worked on with OBD will run with just about any sensor unplug. In fact a little trick here, if you have a engine with a hard or no start problem and you don't have a scanner you can unplug each sensor one at a time and when you've unplug the bad sensor the engine will start right up.

I think you are just confused with Jodi Arias who was ADD and when she was "unplugged" her brain started to misfire.

Jfrahm

Some ECUs are smarter and will see an open circuit, throw a code, and use a nominal temp value for mapping. I think others will skew rich and kill the engine. I had a Mustang (1984) that did that, but I forget if the temp sensor was going open or just very high resistance, it was the sensor itself and not the harness.

Tiger03447

Some of the "performance" chips that are out there(maybe not Porsche) will skew the temp reading of the temp sensor enroute to the DME . They will always give the temp sensor reading as cold, throwing more gas to the mixture. The owner will feel that his engine is doing better since his seat of the pants computer tells him that it is.. The mileage will suffer accordingly..

tempest411

That could be...My Ford truck and Audi play dead with the temp sensor unplugged, and I've heard much the same over the years from other people with varying makes/models.

marc abrams

That's interesting. My son's 2007 Ford Ranger with a 3.0 l engine will run with both the coolant temperature and MAF sensors unplug. My Daytona the coolant temperature sensor must unplugged in order to set the ignition timing. A Mack truck will even run with the speed sensor unplugged relying solely on camshaft position sensor. A engineer from Mack trucks told me when they developed their V-Mack system ("the computerized bulldog", the company's slogan) it was designed so the engine would still run so not to leave the truck stranded if any of the sensors failed or was disconnect. I'm pretty sure car manufacturers though of this also, except for Porsche.

V2Rocket

Doug, the coolant temp sensor is used all the time by the DME as part of the fuel trim strategy.
It is "on" 100% of the time. As Michael said, without sensor plugged in the DME sees infinite resistance (air between the plug contacts)...and doesn't know what to do with fueling.

Normally, cold temps make for lower electrical resistance, but I believe the way the 944 sensor works (like lots of other car's coolant sensors) is "negative temperature coefficient", higher resistance cold and less when hot. So might be that, upon seeing infinite resistance, the DME fuel trim dumps extra fuel because it thinks it is suddenly very cold.

Dougs951S

Yes, I know all of that. I guess what I meant to say was, I didn't think the fuel trims had enough "play" so to speak to actually cause a hard cutoff at one extreme of the other on a warmed up engine running in a mild climate. Rough running and bucking sure, but just didn't expect it to stall the engine, or do it so fast that it felt like someone just cut the key off.

I guess I shouldn't be surprised, the car still has to run right even when it's -20 below freezing in germany or some Nordic country, so the fuel trims have to have quite a bit of wiggle room I guess.

But yes, my final assessment of my stalling issue was intermittent moments of no contact on the DME temp sensor, which would pull the fuel trims rich enough to cause a stall; after which I would attempt to restart and flood the engine because the ECU thinks I'm in Antartica. I was able to replicate all of this in my driveway with the car idling by wiggling the DME temp sensor harness plug.

V2Rocket

I don't remember off-hand but I recall from my Rogue Tune and now VEMS that the trim % is quite large based on ECT...I think it could be as high as 25% at extreme cold, IIRC.

I'll try to remember to check that later.

88Silver924S

Is there any reason that a failed sensor doesn't just trigger a "limp home mode"? I think that is the question. Everyone hates being stranded at night in the cold freezing rain on a shoulder-less road.

V2Rocket

Because the 944 DME was developed in the late 1970s before they had thought of such a thing...

Although in the case of the ECT sensor, a failed sensor will just throw off your AFR, not kill the car. May run like **** though.

JacRyann

It's probably a logic or math thing. For example:

Fuel = (AFM * RPM * O2 * altitude * FQS) / (AirTemp * CoolantTemp)

What happens when when CoolantTemp = 0 Zero ???

StoogeMoe

You guys make me laugh. Comparing 07 engine control electronics with the first Motronic developed by Bosch. Do you know how many generations of micro controllers separate the two? The Motronic used the first generation micro controller. Microprocessors had just been invented! It barely had enough memory and processor power to even run the engine, let alone be able to account for every failure mode possible. I'm sure they thought about these things, but they just weren't possible at the time. We are so spoiled by what we have available now. That phone in your hand is more powerful than the mainframes back in the early 80's.

Has someone else tried unplugging their temperature sensor and see if the engine dies? I don't believe that can happen. I think he has some other problems going on. Possibly a ground problem. I would try it on mine, but I have the early sensor which is the one wire model. It may not behave the same. And my 86 is sleeping for the winter.