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I agree with the order. It's now past 20+ years for most E30 models and the crank sensors are failing. Most mechanic were telling me "Those things never fail" True... 15 + yeare is the equivalent to 3 lifetimes on an engine part. Therefore, most BMW mechanics never expect crank sensors to be defective. My approach now is simply replace the 2 sensors if you have a E30. Spare yourself future headaches.

Like the other guys mentioned, try the fuel pump

relay first. This one's really easy, as you can temporarily swap in any of the other 4 prong relays to test. I believe there is an o2 sensor relay right next to the main and fuel pump relays. On my very first e30, this was one of the first issues I experienced. You can also hear/feel the relay "click" (energize) when trying to crank the car. If there's no click, the relay is bad. It's not a bad idea to check / replace the main relay as well. The main relay is the 5 prong relay in the bank of 3 relays on the drivers side strut tower, by the fuse box.

Re: 91 325ix won't start

Checking the main relay is definitely a good start, as well as checking the fuel pump. Another good thing to check is the two "speed sensors" (black and gray). These sensors are hooked right up to the flywheel and tell the coil when to spark. If the sensor is broken, you will not get the spark to the engine. Three of my four BMWs had one of these sensors go out and they just would not start. I do not have the Bentley manual any longer, but there is a section in there that specifies the Ohm reading you should have on both of these speed sensors.

Before jumping to fuel pump..

On the drivers side of the engine bay, remove the black cover over the three relays. The white relay (should be the one toward the front) is the main relay. Does the relay click when you try to start the car? If yes, does the fuel pump relay right next to it click as well? If yes, then move on to test if you're getting fuel and spark.

Blah blah blah.. If you really want to find out, use the wiring diagram to trace it.
Follow the link.
http://shark.armchair.mb.ca/~dave/BMW/

Re: 91 325ix won't start

Make sure the fuel pump is running.

Remove the rear seat, jump the fuel pump relay and listen for the pump. If it is not running, you probably need to change the pump, but you should check to make sure you have 12v at the plug.

91 325ix won't start

I have a 1991 325ix. I was on my way home the other day, and I stalled Thinking that I made some bonehead move with shifting, I tried to restart it. I wouldn't start. I hear the starter spinning, but it doesn't sound like it is trying to start. I got it home, opened the hood and tried to start it again. I hear the starter spinning and the belts are also turning, so I'm guessing that the starter is engaging the flywheel. However, the engine doesn't sound like it is turning. The timing belt was just replaced 10,000 miles ago, so I don't think it is that. Anyone have any ideas?