for info copied: '90 535i No Start
I've had this e34 since last fall; got it with zero maintenance history, but it was in decent shape and the price was right. Aside from a minor overheating issue that I resolved a month ago, it's been a great running & driving car. At least up until this weekend. I've been messing with it for way too long, so figured it's time to hit you all up for some better help.

It ran fine driving to a meeting on Saturday morning, then on the way home I noticed the tach dropped to zero; engine died. What the heck? I coast to the next intersection and park it around the corner, and try the starter several times; nothing. No tools with me, so I couldn't do anything at the time, so I called my son to come drag me home. Moments after hanging up the phone I try the starter again, and it starts like nothing ever happened. Called him back to cancel, put it in drive, and started for home again. A block later it died again, but started again after a couple of starter bumps. In the ~2 miles to home it died two more times, both times deciding to start again after numerous tries. During those stops I also pulled the main and fuel pump relays to reseat them. Didn't seem to make any difference.

I kinda suspected the fuel pump, so left it running when I got it into the driveway, then went to the trunk to listen for any strange noises. About that time the engine shut off again; the fuel pump stopped at about the same time, maybe a little later than the engine. It was like the ignition was shut off; it just quit. Weird. And since getting it home I've been unable to get it running at all.

So it was an intermittent issue that seems to have become full-time. Here's where I am with troubleshooting so far. I jumpered the fuel pump relay, and the fuel pump runs, so that's not it. Wouldn't start with it jumpered either.
Checked for spark; none.
I jumpered the main relay and tried another known-good relay, no change.
I had recently changed the spark plugs, wires, and distributor cap. Getting nothing on all six made me think maybe it's the coil or coil wire, so first I swapped in the old coil wire; no change. Checked the voltage to the coil and got ~12V at terminal 15; normal. Resistance on the coil is normal, as is the coolant temp sensor.

Any thoughts on where to start tracking things down? I have a copy of the ETM and a Bentley manual; I'll be pawing through those things this evening looking for anything that might help, but would greatly appreciate any advice you all might be able to offer. Thanks!
Make sure the pressure from the pump is enough first. My pump died on me and when we went to check it it was pushing fluid still so we looked elsewhere for an issue. After searching for a while go no avail we put a gauge on the pump, it was supposed to pump at like 50psi and it was at like 10. Just to rule it out. Also check vacuum leaks often the intake boot can get tears in the folds.
So the only piece of the triangle you are missing is spark ? Air is getting to the motor, you have fuel going to the motor, just no spark. Did you check to make sure the battery cables are on tight ? You checked the ignition coil and that was yielding positive results. The tach dropping to zero could be a false positive if you need a new capacitor. Did you put a new rotor in the dist, or reuse the old one ? I think it would make a difference, if the ic is testing well and the coil wire is okay then i would think the next thing to investigate would be the dist cap. I would also check the connections from the power terminal under the engine bay that supplies power to the starter, also check the wires that run from the starter to the alternator, if they've never been changed you may want to do that as well. +1 for the fuel pressure..
Check the crankshaft position sensor
Using digital multimeter, check resistance between terminals 1 and 2 in crankshaft position/rpm sensor connector.
Crankshaft Position/rpm Sensor Specifications: • Coil resistance (approx.) @ 20°C . . . . . . 540 ± 10% Ω , • Air gap (sensor distance from toothed wheel) . . . . . . . 1.0 ± 0.3 mm (0.04 ± 0.01 in.)
This ^^ classic symptoms of this part failing.
Just checked the crankshaft position sensor; looks to be normal. Showed 0.5 on my DVM’s 200 ohm scale, but the temperature this morning is 10 degrees C. Position looks ok. I sprayed some cleaner on it and wiped away some grime. I don’t have a feeler gauge, but was able to slip a bit of cardboard in between that measures about 0.5mm, so it looks to be just under 1mm gap. Is there a way to check that sensor’s input at the DME, to confirm that the connections are ok?
Are you saying it reads 100 ohms then? If so then it's dead. Your symptoms mirror my experience when my CPS died. Care of Shogun.
They aren't reading it correct. Shogun says it should be 540 ohms and they are using a multimeter set for 200ohm range. Ohms readings of sensors is always a bit duff anyway - can confirm a fault but not really accurate enough to confirm a part is fine. You need working volts at least for that, or oscilloscope results; and all behavior points to CPS fault.
Doh. I was thinking that it was reading 500 ohms with the DVM on that setting, but in thinking about it that's not right; 500 ohms shouldn't be able to be read on the 200 ohm scale. With the DVM on the 2000 scale it reads zero, close to a short. If the sensor has failed, is it more common for the resistance to be too high or too low? Either way, I think I'll order a replacement.
Failure resistance can be all over the place, even within spec.
shogun and ross1 get the prize on this one; it was the crankshaft position sensor. Many thanks guys!
Getting the old sensor out and the new one in sure could've been made harder, but not by much; the allen head screw holding the sensor to the block was a bear to get out, and threading the connector up under the intake runners... I used a fish wire to pull the new one through, but it's fairly large and there's barely enough room for it to pass. Good thing it's something that doesn't have to be replaced very often!