Scuba Steve

Are the reference and speed sensors the same part?

Last time I drove the car I noticed the tach was a little twitchy, and this afternoon it had a hard time starting (tach didn't move). I guess the reference sensor is going out even though the resistance checked out ok, the connectors weren't corroded, and the sensor itself was pretty clean. But after I messed with it for awhile the car started up again and the tach was still bouncy so it looks like it's time for a new sensor.

PorscheDoc

That or the wires (3) in the wiring harness side are starting to crack. Usually with a little fiddling, the wires make a good enough contact again to start.

Scuba Steve

Mine are cracked badly and at first I thought that was what the problem was. The far left and right wires should have at least a mega Ohm of resistance and mine show that, or no connection. I wouldn't be able to tell if the wire on the far right was making any connection without being able to get at the other end of it where it goes into the sensor. The car would run rough after messing with it for awhile, and after I had my hand jammed down there to put the thing back in it's back to about where it was last time I drove it around. The wires were plugged in while I did that and that probably pushed things back good enough to make a connection again wherever it's gone bad.

What I'd really like to be able to do is rewire the thing but I'm not sure if it's servicable. Too bad since I'd bet the sensor is just fine and new sensors are so expensive. I'd like to pick up another one that works for cheap at a local yard to have as a backup while I have a look at how the one I have is wired. As/is I don't trust the car enough to pull it out of the driveway since if that thing goes out on me, I'm stuck.

CT944

ref and speed sensors are the same part

Scuba Steve

Probably a dumb question: I see references to a TDC sensor - is this a third one, or a different name for one of the others? Next to the wires going to the oxygen sensor I have another wire bundle with a 3-prong connector on it (different from the reference sensors). It has a rubber cap over it and nothing plugged in.

Scuba Steve

Update: I got another sensor today and the problem is diminished, but not gone. The tach will bounce around some and the car stalled out a couple of times. Are there any other connections that could be causing problems? The plug the sensor connects to looks fine. Maybe the connection at the computer...? I might trade it in for another sensor, maybe I wound up with a bad part.

Danno

"I see references to a TDC sensor - is this a third one, or a different name for one of the others? Next to the wires going to the oxygen sensor I have another wire bundle with a 3-prong connector on it (different from the reference sensors). It has a rubber cap over it and nothing plugged in."

That third sensor is a diagnostic one, no use for it in on the car. Try swapping your new sensor with the other one and see if the problem is still there.

Scuba Steve

I'll try that tomorrow. What exactly does the second sensor do anyway? It has to feed something to the computer, but how would you know that one was going bad when the flywheel one was ok?

Thanks - with this car when I see something unplugged, I never know...

Danno

Both the sensors are the same. One is a TDC sensor (the front one), and one is for speed. It measures 130-teeth on the flywheel. So swap sensors and swap the connectors on the harness. If the problem goes away, then that other sensor is bad too. Usually, it's best to hook up a 'scope to the sensors and measure the waveforms. That's the first test in the DME/KLR test manual. Takes 5 minutes and saves a tonne of time in troubleshooting.

Matt H

FWIW, those sensors are used on all sorts of other cars and can be had fairly cheap if you look around. When Evil944T and I were searching for a "mystery" part we came across the speed and ref sensors cross referenced to VW and Mercedes among others.
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Scuba Steve

I wish I had an oscilliscope... But what does that second sensor actually do? I haven't found any mention of what happens when it goes out.

Standard part - cool. I'll have to search it for part numbers. Anybody know them offhand?

Danno

The forward ref-sensor is the most critical because it gives the TDC signal to the DME computer.

The rearward speed-sensor counts duration so the DME can inject the proper amount of fuel.

Of the two, the reference sensor is the more critical because if TDC is missed, the igntion won't fire and the engine may not run at all. If the speed-sensor is marginal, the injector pulse won't be the correct duration, but the car can still run.

Scuba Steve

Great explanation, thanks! I plan on swapping them out this afternoon and seeing what I get. Knowing my luck, both will be bad! The second one looks to be a pain to get at though, being farther back in the engine. If I find out what this standard part number comes out to be I'll post it here. As/is, I'll need at least one. Kind of like that DME relay - I guess it's good to have one on hand.

heybiff

but it would be great to know what VW or Merc part this cross-references to.

-Heybiff

Magown

BMW
12 14 1 708 618
12 14 1 710 668

I've never seen these sensors on a Benz, they do have one that looks similar on the newer cars but the connectors are diff.