Diagnostics Doctor
BMW 3-SERIES
Fuel pump failure
Q I have a problem with my 1992 BMW 325i with a M50 engine and Bosch Motronic 3.1 fuel injection. I tried to start the engine after work – it would turn over OK but not start. I couldn’t hear the fuel pump running. Fuse checked OK but tried another one, still no pump running. Next I took out the fuel pump relay, contacts looked clean. As I had no multimeter with me, I had to call out a breakdown service. The mechanic banged on the petrol tank whilst I cranked the engine and it started immediately. He said turn it off then try to start again – which it did. He thought I would be able to get the 18 miles home but alas after about six or seven miles the engine cut-out. I tried banging on the tank but to no avail.
Car was transported back home. Scan done with Peake Research scanner. Fault code 1A Control unit supply found. Fuel pump tested and found to have a high resistance and so was replaced. It’s at least 18-years-old. New fuel pump drawing 5.9 Amps.
I followed the testing procedure as instructed by my Bentley workshop manual, ie. testing the relays and found no power at terminal 87 of the fuel pump relay. It instructs to test for ground? At terminal 85 of the fuel pump relay, I tested for voltage and 12V was available. Am I supposed to be testing for resistance or continuity at terminal 85? How do I test for ground?
On the DME relay terminal 85 there was 11.7V with the ignition off but only 0.84V when the ignition was on. I tried a jumper wire between terminal 30 and 87 on the fuel
