Where are you in Cumming?

Interesting comment on the tachometer. My conversion ran fine using the stock 1973 tachometer.
Maybe if the original tach already had problems, but you can try buying a good used Carrera tachometer.
1973 Porsche 911 - YouTube

Sully,
NICE car. it looks like you could eat off of that motor.


Tspringer,
Too bad the ECM is fried . Just remember that before you throw away the "Motronic crap", it still has value - even the non-functional ECM. Since your tach didn't work, did you get a chance to check pins 11 and 21 of your ECM connector to make sure you don't have a problem from a PO?

Mike

I went through and started testing everything again - a back to basics full "reset" on the testing process.

I think the issue is related to the reference sensor. In my notes from testing before, I have a check mark next to this one indicating it tested to spec. But when I tested today, both at the DME plug (pins 25 and 26) and at the engine harness plug (bottom of the 3 plug in connectors) I got nothing. No continuity and 0 Ohms - an open circuit. Spec appears to be .6 - 1.6 k Ohms.

If this is correct I have a bad reference sensor. Perhaps a bad internal wire connection such that sometimes it makes a connection and other times not?

The gap seems correct, but I will reset it using the paper method after I remove the sensor itself.

Anyone have suggestions for a more definitive test (if possible) to tell if this reference sensor is no good?

Tspringer,
It sounds like you're making progress - that's good news.

I haven't tried this, but if I wanted to test a reference sensor, would do something like this -

http://forums.pelicanparts.com/uploa...1379223601.jpg

I believe if you put a piece of metal in front of the sensor, the voltage at the meter will change. Maybe someone can verify this.

Mike

I did a LOAD of reading on the speed and reference sensors when I was going through my no start problem. It sounds like the only real test is using an oscilloscope. I checked my NEW sensors with a Ohm meter and would have said they were bad based on my reading of the subject. IF you decide to go with new sensors, Pelican sells a sensor for BMW's that is half the cost and works fine. I'll try to find the part number if you go that route.

My problem was part the fact that my wires were crossed so the speed was going to the reference and the reference to the speed sensor. The other problem was the cursed DME relay that I would swear to this day was good, would not let the engine start. Make sure your DME relay is good. I can't stress that enough after spending a few days searching for everything else and it ends up being the relay. I have read where the DME relay can be a hit or miss thing meaning one day it works and the next it doesn't.

I hooked up the circuit from my previous post, using a good used sensor. The sensor responded to metal moving by the sensor with a momentary increase in voltage measured on a voltmeter.

When I was troubleshooting my Motronic issues, I just bought 2 new sensors, because they were 25 years old, and provide the timing for the engine. If you come up with an oscilloscope, and get to the signals (PITA), Here's what it should look like -

http://forums.pelicanparts.com/uploa...1379292867.jpg

Mike

Interesting stuff Mike...

I did the test for the reference sensor with everything hooked up first - tested pins 25 - 26 at the DME plug - got nothing. Same test for pins 27 and 8 for the speed sensor and I get just over 1K ohms, exactly as the testing chart Sully posted said I should.

I removed the reference sensor over the weekend (boy that was fun) and tested it. On the bench I got the same results - no continuity and 0 ohms across any connection.

So I decided to cut the wire and test it. There are 3 wires in there.... yellow, black and a bare metal wire wrapped around both with the entire grouping wrapped in aluminum foil and plastic. I was not expecting that.

I tried testing at the wires and got nothing... but honestly not sure how to test the 3 wires.

Mike - in the diagram you posted, which 2 of the 3 wires were you using?

My best guess is that the reference sensor is bad, but other than the Ohms test based on the troubleshooting chart that Sully posted I have no means of really knowing.

Whats the BMW part number?

I used 2 of these on an 88 back in January, they were a direct replacement for the originals -

Pelican Parts - Product Information: 12-14-1-708-619-M14

Of the 3 wires on the sensor, I used the 2 that had ~1k ohm across them. I used 5 volts because that's what powers the sensors in the ECM.

Mike

Thanks Mike. I ordered a new sensor online yesterday after searching google on the part number listed before. It was $63 including shipping.

My bench testing of the old sensor shows 0 ohms across the two wires (ignoring the shielding wire). I tried waving a hunk of metal in front of it and sticking it to my giant steel vise - nothing magnetic made any difference to the ohms reading. Given that the speed sensor when tested does show the correct ohms reading, my assumption is that this sensor is bad.

Steve Wong called me back about my DME - it is totally toast, cannot be repaired. He said that the tach circuit had been plugged into an older style tach (the PO did this I know...) and that had fried the DME. He has shipped me a totally new, updated unit. When it arrives and the new sensor arrives, I am hopeful to have this all fixed.

It is still all very strange. The reference sensor tests bad - yet the car did start before - sometimes - with 2 different DME's.

Terry,
I am still curious about the Steve Wong thing that using the old Tach burned out the DME. I used the original '73 tach, and 2 others that converted also used the original tachometers, '73 and '72, without any problems burning out the DME. The one using the '72 tachometer says his reads a little high, but no other issues.
I hope the new sensor gets you back on the road.

Steve Wong told me that hooking an early model tach up to the black/purple tach wire from the DME OR to the solid black shift light wire from the DME could either way fry the DME. He believes that is what happened to mine. He says the black shift light wire should not be used to try and get an rpm signal or bad things can result. He indicated to me that this was a common problem.

I have no idea about this. Neither wire has been hooked up to my tach since I owned the car, but I do know the prior owner tried to make the original '73 tach work and could not get it to do so.

Other that what Steve said to me... I have no clue. I have the "new" DME back from Steve and only need the new sensor to put it together and hopefully have it fixed.

Fixed!

I installed the new DME from Steve Wong and the new reference sensor. Car fired right up and runs great. It runs much smoother than before.

I have driven the car around twice, started and stopped it over a dozen times and it now appears to have no issues.

My best guess is that the old reference sensor was bad but that it still worked - barely - sometimes. Also, the old DME was a mess and not functioning properly and somehow in the process of trying to diagnose problems it finally gave up the ghost. I believe there were thus multiple problems at the same time that caused the diagnostic process to be confusing.

Anyhow, she is running great now!

My '72 tach works fine with my '84 DME. I didn't use the shift light wire.

Roger

I can see how miss hooking the Black shift light may cause a problem with the DME, but hooking up the Bk/P does not seem to.
Anyway, it's good to see you have the car running. Now you get to enjoy it.