M30 Intermittent Crank No Start-solved

[theBMWbeast]

Okay, I finally got this on camera. It doesn't happen always, or even usually, but once in a blue moon the car will crank but not start, and the tach will bounce all over the place.

https://imgur.com/19NB1DB

I was thinking maybe CPS, because I know 944 guys have a similar problem, but that's just a guess. Car starts if I keep retrying the key until the tach stops bouncing. It hasn't crippled me yet, but I certainly don't want to wait until it does.

[fo3]

I've never seen a tacho bounce around like that with a faulty crank position sensor. M30s have a distubutor and coil right? I haven't looked too far into where BMWs calculate RPMs but usually for coil and dizzy systems it's at the coil for most marques, so I'd start there as coils can partially fail where they sometimes don't start (especially when cold) but once warmed up a bit they run fine.
If it was crank position sensor cars either don't start, or if they do they run very rough, can't idle and stall a lot.

[theBMWbeast]

I haven't looked too far into where BMWs calculate RPMs but usually for coil and dizzy systems it's at the coil for most marques, so I'd start there as coils can partially fail where they sometimes don't start (especially when cold) but once warmed up a bit they run fine.

Symptoms to a tee, I’ll order a new ignition coil, was meaning to do it anyways when I got the new plug wire set.

[fo3]

If it's really intermittant though check wiring to coil, and wiring to dizzy and any earths or condensers/capacitors first.

[theBMWbeast]

If it's really intermittant though check wiring to coil, and wiring to dizzy and any earths or condensers/capacitors first.

All new parts in ignition system, cap, rotor, wires, and ground was cleaned and tightened with engine swap (still tight). I ordered the coil and will report back if symptoms persist. What capacitors are you referencing to?

[fo3]

What capacitors are you referencing to?

I just looked a m30 up and it doesn't look like it has any as the DME controls everything. I'm used to more autonomous distributors that have internal triggering sensors or points under the rotor.

[fo3]

I found this post earlier with similar symptoms:
https://www.bimmerforums.com/forum/s...6#post29807656

[JGood325]

The tach signal is generated from the crank position sensor, not the ignition coil. For the tach to be jumping like that, there is an issue with the CPS, the DME, the cluster, or any of the wiring between. My money would be on the CPS, since the car won't start either. Otherwise, maybe a grounding issue with the DME or engine harness.

[ross1]

The tach signal is generated from the crank position sensor, not the ignition coil. For the tach to be jumping like that, there is an issue with the CPS, the DME, the cluster, or any of the wiring between. My money would be on the CPS, since the car won't start either. Otherwise, maybe a grounding issue with the DME or engine harness.

This^

[theBMWbeast]

Replaced both the CPS and ignition coil yesterday, and then later that same day the car had the same problem. So based on what I'm reading here, looks like whats left could be a grounding issue. I'll check my relays as mentioned in that thread, my other question is how exactly is the DME grounded?

[Thomas525]

There is something wrong with the tach or the wiring leading to it. It won't be fun testing it, but personally I would spring for a spare tach, wiring almost never the issues for the cluster.

This is separate from whatever is causing the hard start. Typical hard start culprits are bad coolant temp sensors, and failing cps.

[fo3]

Replaced both the CPS and ignition coil yesterday, and then later that same day the car had the same problem. So based on what I'm reading here, looks like whats left could be a grounding issue. I'll check my relays as mentioned in that thread, my other question is how exactly is the DME grounded?

A quick look at the wiring diagrams says to me by a lot of brown wires, Seriously, I've noticed brown (or brown + x striped) wires going to x1500, x1908, x1913, x1541 and x1542 connections (one of the latter two go to a neg at the diagnostic port so maybe start there - pin 19 - disconnect battery and resistance check that pin to the neg terminal).

Like the thread I linked to earlier, some grounds run through relays, I noticed some partial grounds through the 'unloader' relays in the fuse box, so make sure you have the proper relays there (5-6 pin/diodes and stuff relays, not the regular 4 pin standard automotive relays)

[Thomas525]

The dme is grounded to the car body through its holder. And unplug your blue tipped coolant temp sensor near the front of the engine, see if the hard starting continues. It won't- so change it out for a new one after a few days.

[theBMWbeast]

Okay, I think I have a relevant update. Battery light came on dimly at idle a few days ago, and as of yesterday the alternator completely died. I ordered a remaned Bosch unit from pelican, and after install will see if my symptoms persist. I have a hunch that the potential low voltage was doing screwy things to my electronics, but still remains to be seen. Also got the temp sensor because it was 20 bucks so why not.

[theBMWbeast]

I’ll bump my dead thread, because I hate when someone finds the solution for a problem, and then simply vanishes off the face of the earth.

Replaced voltage regulator, and to date, no more starting problems! So my advice, check your alternator/battery, because low voltage does some wonky things. I have the remaned alternator on hand, gonna squeeze all the miles I can out of the one in car to get my moneys worth. Now to chase down my #5 spark plug fouling, idle misfire, and 1222 code. Always something....