No Start, help answer a few questions

[Earendil]

Just rebuilt the head, put head into car, car isn't starting. I'm on the path to trouble shooting but a few things aren't adding up for me.

Straight up questions with no background:

1. How many volts does it take on the switched side of the fuel pump relay to get the relay to open?
2. Without a CPS reading, I know the DME cuts fuel and spark. Where does it cut fuel at, the injectors or the pump? EDIT: Further research suggests that most of the internet believes it cuts it at the pump (relay).

Thanks for the help!

Background

1.
Fuel pump wasn't running.

• Pulled relay and jumped it, fuel pump runs fine.
• Checked the relay switch connection: 0v in key-off, 10.5v in key-start position.
• I'm thinking the relay is bad since the DME is obviously(?) trying to turn it on.
• Try and run car with fuel relay jumped. No Dice. So car won't start with pump jumped and running, but the pump also won't turn on with the relay plugged in? Wha? So I have a multiple failure points, either induced by the DME or stupidly bad luck.

2. I've confirmed I get system power at the capacitor when the key is in the start position.

3. Stomp test gives a 1211 reading, which is a fault in the DME its self.

I'm going to try and borrow a DME next, but it bothers me that the fuel pump doesn't turn on by its self, UNLESS the DME cuts power to the fuel using the fuel relay and not the injectors. But if that were the case why is the DME putting 10.5v on the relay with the car in the on position?

[Earendil]

Okay, given further research that the ECU cuts fuel at the pump and not the injectors, and given that the relay is getting 10.5 volts when the key is in the start position, I'm inclined two believe that my CPS signal is good. That leaves two possibilities:

The DME is bad, and is causing weird things to happen, including failing to provide enough voltage to open the fuel pump relay.
The DME is good, and there are multiple failure points, the fuel pump relay AND something that is preventing spark.

I should know more by tonight.

[Earendil]

So I go back outside to poke at it some more, because this bugs me. I knew that, among other things, I had tested the CPS sensor and the resistance was okay. I didn't test it from the DME plug though, so I did that. I didn't get a good reading. I then tried something I was sure would never work, I swapped the CPS for the Spark Wire Sensor, and sure enough the plugs are identical. I swapped them, car fired up. I'm giving BMW partial credit for this face palm. Every electrical connector on this car is different EXCEPT for the two that are right next to each other and have no markings? Thanks.

Case closed... Except I'm still not sure why I was getting a 10.5v reading on the switched wires for the fuel pump relay. That just seems weird...

[Regnar75]

your welcome.... and yea i did the same thing (except before i attempted to start it) i noticed that the plugs didnt want to rotate the way i was trying to make them go...ones "upside down" and the others not. not the best idea but it is an idea i guess.

[Earendil]

your welcome.... and yea i did the same thing (except before i attempted to start it) i noticed that the plugs didnt want to rotate the way i was trying to make them go...ones "upside down" and the others not. not the best idea but it is an idea i guess.

I think that's why I always thought they were different, because they were rotated 180 degrees from each other. What got me this time is that I put in a new CPS sensor 12 months ago that still hasn't developed a natural bend, so when the first set I grabbed fit, I didn't question it.