Jumper across fuel relay doesn't run fuel pump

[redhotchilipepper]

I have an 89 325i, ran well until it just quit completely out of the blue and failed to start. I slapped the gas tank, and got it started to get it home. Must be the fuel pump on it's way out, I thought. Bought the nice TRE 255lph pump to replace the original pump and waited 2 days for its arrival.

Replaced the pump with the new TRE, and the car wouldn't start. I decided to check my pump job so I put a 12V battery directly onto the pump leads. Pump ran great. I then proceeded to follow the Bentley manual for testing the fuel relay. Jumping pins 30 and 87 of the fuel pump relay gives me battery voltage at the connector for the pump (ensuring good wiring to the pump), BUT THE PUMP DOES NOT RUN. Double checked the connectors and cleaned them but no amount of wiggling or pushing ever made the pump come on.

How is this possible? Pump runs off a battery directly but not when hooked up to the connector which has confirmed battery voltage (12.6V). Thanks for your help!

[E30SPDFRK]

You're doing something wrong, if it has power it will run. Check the wiring a third time to make sure it's the correct pins you're checking.

Edit: Or it could be loose pins in the plug, does it look damaged at all?

[EatsHondas]

You may have wired the pump to the cradle backwards. Pull the pump out of the tank and double check everything.

[redhotchilipepper]

Double checked the connectors removed from casing and pins they look also solid. Pump runs good from a battery hooked up in the same polarity as the connector so I know my pump is good.
Just discovered something new - connector is providing 12.6V until I plug it onto the pump and crank the car; unplug it and it reads 2.5V. Wait several minutes, and it is restored to 12.6V! I have done this 3 times in succession now, what the heck!?

[EatsHondas]

How old is your battery? An auto parts store can load test it for you to make sure all the cells are good. 12.6V doesn't necessarily mean a healthy battery.

If I were you I'd replace the fuel pump relay for peace of mind, can't hurt, they're cheap and easy to find.

[redhotchilipepper]

Replaced the battery as it was fairly old. No change to symptoms.

Problem is still here - jumping pins 30 and 87 on the fuel relay socket do not run the pumps. I check the voltage at pin 30 and it's at 12.6V, then when I place my fused jumper between 30 & 87 nothing happens except the voltage at pin 30 instantly drops to ~2V. After a little bit (20sec or so), battery voltage (12.6V) returns to pin 30. What could be going on?

Even though I believe the main relay is unrelated from my fuel pump jumpering issue, I removed the main relay and put in a jumper act as the main relay switched on. The ICV buzzed away, until I attempted to jump the fuel pump which resulted in the voltage drop and the ICV going silent.

What could be switching the voltage off when I jump the fuel relay? The OBC?

[redhotchilipepper]

bump*

[EatsHondas]

The high current side of the fuel pump relay is hot at all times (which is why you can jump the pump with no key in the ignition). This means you have bad wiring (could be a power or ground problem) leading to the pump, a semi-functional pump or some other intermittent problem, or an incorrect installation.