Scuba Steve

Today I got a multimeter with a computer program that lets it act as an oscilliscope. Voltage readings on the reference sensor were under .8V + and -, but what kind of refresh rate does the oscilloscope need in order to capture this signal? Is it a fast signal that only the real thing could see? I cranked the engine for 2 or 3 seconds and got maybe three or four complete oscillations out of it on my screen. I have a sampling rate of every second.

Gerald Allgire

It might be more about where the signal triggers the meter. Even if you had a real o'scope you may have trouble getting it to trigger at the desired time. Does the reference sensor pick up just once per revolution? Trying to view the pulse while running might be easier. Just some ideas.

Scuba Steve

I'm not sure if it does once/revolution. I think the peaks corresponded to about the rate the engine was turning but that's just my guess. According to the manual the fastest sampling rate is once/second. The engine was turning slowly because it's cold out and the fact that I've been turning the engine over a lot while trying to bring this thing to life.

gtroth

One sample per second is way, way too slow. Totally hopeless.

Gerald Allgire

That's not a very fast sampling rate.... well unless you're using it to view 60cycle ac. I've seen some of those meters and wondered just how well they worked. Just wondering.... how many cycles does the screen display when sampling 60cycle ac?

Scuba Steve

There goes that idea. At least Radio Shack has a good return policy, but now I'm back at square 1 again. Sensor clearance vs. DME... from the measurement I took with a dowel the gap is about 2.3mm, but I have never been able to move the bracket intentionally (can't even get a tool in there). It's not the sensors; they're new and the wiring checks out, leaving only the DME. But I've resoldered that part twice so now I'm back to trying to make the bracket move.

Very frustrating.
As many as you want it to. The computer program is decent, you can have the x axis show however many seconds, or have the signal continually fill the screen (it becomes more and more compressed). The screen on the unit is a regular multimeter window.

Gerald Allgire

Well except for the fact that it's not helping you, the meter sounds pretty cool.

Scuba Steve

It is a nice meter, the thing does capacitance too and the meter itself refreshes really quickly. But I already have a decent multimeter and really don't need a second one. If a new DME would make my car work again I'd trade it for one of those!

Sam Lin

Just use your sound card with a sound program that displays the sound waveform, hook the sensor up to the mic input.

Sam

Scuba Steve

Good idea - I'll start the search now.

I realized I had been making a stupid mistake for awhile now. The gap I checked was for the reference sensor. My guess is in a few minutes I'll be searching threads where people resoldered the DME for where I've been doing it wrong. I've already remelted and added solder to every connection on the board.

austin944

I have a book called "How to tune and modify Bosch FI", and it says to test either sensor in a Motronic system, crank the engine and check the AC voltage of the sensor is between 0.5 and 1.0V.

Scuba Steve

Looking good there, thanks for the info. The computer oscilloscope is cool too. I guess I get to spend more quality time with the soldering iron after I come back from visiting family over the holidays.

If that doesn't work, where can a DME be found that's not $500? An '86 or newer would work with a 924S?

Waterguy

The 1987 924S used the same DME as the 85.5 to 87 944 (P.N. 944.618.121.04). The 1988 and later DMEs are different (944.618.121.05). There seem to be a number of Rennlisters with parts cars these days, so that might be a good place to start.