1991 318i: NO POWER to main relay!!! HELP!!

[tfritz]

I have had some running issues with my 1991 318i for a while now but it finally stopped running all together a couple months ago. The car cranks but will not start. I just replaced the cps and the fuel pump but now realize that the pump is not getting power. I started checking the circuit and realized the main relay is not getting power either.....where do I start with this problem????

Thanks, Tyler.

[golde30]

Ecu is a good place to start. Or swap out the main relay first, then try ecu.

[tfritz]

I have tried two different ecu's and tried swapping out the main relay

[golde30]

Double check all your ground connections.

[tfritz]

What all ground locations are there?

[romeomike]

Google search for this electrical troubleshooting manual (ETM):
e30_91_modified_090512.pdf

http://www.m42club.com/forum/index.php?topic=8474.msg71055#msg71055

[bmwman91]

My bet is on the fusible link. It happened to me and others.

First, is your battery in the trunk or under the hood?

Second, you should know where the big 12V terminal block is on the firewall. Just above it you will see another smaller terminal block that is also at 12V (or should be). It is in fact a SEPARATE 12V circuit. You see the gigantic black wire coming from the battery's + terminal, and the smaller one that runs parallel to it? Those feed the terminal blocks.

Get a multimeter out and check for 12V between any ground spot and the smaller upper terminal block. If you see less than the battery voltage, then your problem is the fusible link. It is basically a 50A (I think it is 50A anyway) fuse that is crimped onto the smaller black 12V wire. It commonly corrodes and breaks, causing intermittent and total failure to run. The ENTIRE fuel injection circuit (pump, injectors, O2 sensor, ECU, etc) is powered from that and when it breaks...well you get the idea.

If your battery is in the trunk, then the fusible link is about 18" away from the battery under the plastic wire cover and then under some shrink tubing. I went to a local junk yard and found a clean one, chopped it out and crimped it in. Yellow butt-splices aren't big enough to handle the wire so you will need the big uninsulated ones where you stack the wires and then crush them together. Buy some heavy duty crimpers and some QUALITY electrical tape or some quality heat shrink tubing and a heat gun. For an optimal crimp on a mission-critical electrical system, you need to put a LOT of force on there to ensure a good electro-mechanical bond.

Oh, and make sure you disconnect the battery while doing this.

[tfritz]

Okay soooo, I replaced both crank and cam sensors which solved nothing. I then cut apart the covering on the fusable link in the trunk and it appears fine and I have 11v on the car side of the fuse......

Things I know:
no fuel, pump not turning on (new pump)
no spark
no power to main relay

What now....

[bmwman91]

So there is no 12V at terminal 30 or 86 of the main relay?

[tfritz]

negative!

Dash and everything has power and the car cranks but wont start which is very puzzling

[tfritz]

The post with the red arrow is the only place on the distribution block where I am getting voltage (11v). Should I be getting power at any other place on the block??

[tfritz]

Figured out the problem guys!! the small wire running from the battery, with the fused link, was broken just as it entered the cabin of the car.

[bmwman91]

Glad you found it. That smaller wire is the dedicated 12v supply for the whole engine management system.

[tfritz]

Yeah, you couldnt see where the wire was broke. I just decided to tug on it and out came a broken wire..

Thanks for the help!

[StereoInstaller1]

LOfreakingL...good catch!