No Start - During Restoration

[2011z4]

Ok, here is another "won't start" post...

My car is in the middle of a restoration and nearing completion. I am putting the engine back together and want to do a test start to be sure everything is working and I want to do this before I put the rest of the car together. The engine cranks when I turn the key, and sounds like it wants to, but I don't hear the fuel pump, but I do hear the main relay clicking. When I test the voltage on the fuel relay, with the key in the "start" position, there is no power...less than 1V. I have 12+V at the 30 pin on the main relay. There is no spark, and I don't hear the injectors. I replaced the ignition switch as well in the process of the restoration b/c the PO changed out the ignition with a push button start (that was an issue for him).

The interior is completely out, and I temporarily hooked up the instrument cluster, OBC, and the check control module. Am I missing something simple here? Is it something odd that I don't have hooked up that relates to the start process? Everything in the engine compartment is hooked up. I replaced all the relays with new from BMW as well as all the fuses. Before I started the restoration 2 years ago, the car ran great. I left all the harnesses in the car and labeled everything!

I'm pretty good with troubleshooting, but this one has me 100% stumped. HELP?!?!

[hornhospital]

Have you tested the speed and position sensors? If the ECU can't 'see' the engine turning through those sensors, the ECU won't turn on the fuel pump, fire the injectors or fire the plugs.

You can jumper the main and fuel pump relays to test function of the pump and ECU.

[Brucey]

you should check that you have a 'start signal' at the ECU (pin 4 IIRC) when you are cranking. Without that, the ECU won't make sparks, won't close the fuel pump relay.

Can't be sure it is that, but it is one thing on the list...

hth

cheers

[2011z4]

you should check that you have a 'start signal' at the ECU (pin 4 IIRC) when you are cranking. Without that, the ECU won't make sparks, won't close the fuel pump relay.

Can't be sure it is that, but it is one thing on the list...

hth

cheers

I tested the ECU #4 pin and I have almost 12V (I think my battery needs a little charge). I also tested the speed sensor by testing the part of the harness that goes down to the block to the sensor. While cranking, there is less than 1V there. Does this mean I need a new sensor?

[BenM635i]

I am not the most technical here but do know mine won't start unless my battery is in good condition. If you are seeing less than 12v then i would charge the battery properly or stick a power pack on it before doing anything else.

[Brucey]

. I also tested the speed sensor by testing the part of the harness that goes down to the block to the sensor. While cranking, there is less than 1V there. Does this mean I need a new sensor?

You should get an appreciable AC voltage (refer to the manual for details) from the correct pins on the speed sensor during cranking. When the speed and ref sensors go bad, they normally go open circuit. When they are OK, they measure ~1000 ohms. If the resistance is OK the sensor is usually OK but the output waveform (as measured using a 'scope) is the final arbiter.

cheers

[2011z4]

So I went into the sensors a little further, and saw that the PO must have removed the actual connector at the top end of the wire and just plugged the 3 flat prongs into the harness. Needless to say, I ordered new sensors and will retest once they are installed. I just don't trust what was done previously.

I am also going to go and get the battery tested tomorrow just to make sure that it is up to par and not needing replacement.

I'll post an update soon. Thanks everyone!

[Brucey]

BTW apologies if this is obvious but the arrangements vary with build date. Did you mention your build date?

Post 6/87 builds have a single crank sensor at the front of the engine, and a toothy front pulley.

Pre 6/87 motronic builds have a front sensor that is only there for maintenance purposes, and a front pulley that isn't toothy. The two crank sensors that actually run the engine are at the back.

As well as faulty sensors, if the crank sensors are connected the wrong way round, (which is easily done) the engine won't run.

cheers

[2011z4]

Thanks Brucey. Mine is a 11/84 build date. I order 2 Bosch sensors, so I should be good. Thanks again!

[2011z4]

Ok...the replacement of the crankcase sensors didn't help. This is what I did and what I found. Any advice from here?

1. Pin 4 at the Motronic control unit (black/yellow wire) coming from the starter and I made sure there is continuity between the starter end and the ECU end of the wire.

2. I checked for power to pin 35 on the Motronic when the ignition is in "start" or "run". There is .06V when we start or run. Not getting power full power. I checked continuity from the main relay to pin 35 and I have full continuity.

3. I checked the ignition coil for signal from pin 1 at the Motronic control unit. There is continuity here, but the coil is not getting power.

Thoughts? Thanks again everyone!

[2011z4]

Ok...the replacement of the crankcase sensors didn't help. This is what I did and what I found. Any advice from here?

1. Pin 4 at the Motronic control unit (black/yellow wire) coming from the starter and I made sure there is continuity between the starter end and the ECU end of the wire.

2. I checked for power to pin 35 on the Motronic when the ignition is in "start" or "run". There is .06V when we start or run. Not getting power full power. I checked continuity from the main relay to pin 35 and I have full continuity.

3. I checked the ignition coil for signal from pin 1 at the Motronic control unit. There is continuity here, but the coil is not getting power.

Thoughts? Thanks again everyone!

Bump...any thoughts? Thank you.

[Ralph in Socal]

Did you try swapping the flywheel sensors? We had someone here have a bad brand new sensor a few years ago. It would ground out when it was bolted in place. Those are always suspect to me whenever there's a crank-no-start condition.

The ecu provides the signal to the coil so that is suspect if it's not providing necessaary power. It could be something very basic also but check your grounds. You may have power to a part but it may not function properly unless it's grounded.

Any power to the fuel pump? Good luck.

Ralph

[slofut]

On an '88, the ref sensor will cause no spark, no inj pulse, no fuel pump.

[2011z4]

Thanks.

I replaced both the speed and reference sensors, and still nothing. I'll check the grounds again, but already went over them a couple of times to check. I am getting power to the fuel pump and the fuel tank gauge is showing an accurate reading. This is why this is so maddening for me!

[Brucey]

are you sure you have the main and fuel pump relays the right way round?

If you don't have power at the coil then you have a very basic fault probably between the fuse box and the coil itself, most likely the main relay.

A simple test is to (temporarily) replace the relays with jumper wires instead.

cheers