No power to the fuel pump on car

THOMAS GREY

MEMBER
1994 BMW 325I
3.2L
6 CYL
RWD
AUTOMATIC
190,000 MILES

Few days ago the car didn't start when cranking it over and over. Eventually it did end up starting until yesterday it has done the same thing and not yet started. I have checked the fuse and seems good. I found the fuel pump under the back seat and disconnected the wire harnesses to it and used a cheap test light to try and find power but could not. I made sure the key had been on for only a few seconds before turning off key and turning on again but still nothing. I then switched a relay from another relay under the hood that was for high speed cooling fan, but still didn't crank. Maybe that relay was bad too? Maybe its not a relay at all maybe im really an idiot and not connecting the test light correctly? Any guidance here?

1 Reply

WRENCHTECH

MECHANIC
20,761 POSTS

All "crank, no start" conditions are approached in the same way. Every engine requires certain functions to be able to run. Some of these functions rely on specific components to work and some components are part of more than one function so it is important to see the whole picture to be able to conclude anything about what may have failed. Also, these functions can ONLY be tested during the failure. Any other time and they will simply test good because the problem isn't present at the moment.
If you approach this in any other way, you are merely guessing and that only serves to replace unnecessary parts and wastes money.

Every engine requires spark, fuel and compression to run. That's what we have to look for.

These are the basics that need to be tested and will give us the info required to isolate a cause.

1) Test for spark at the plug end of the wire using a spark tester. If none found, check for power supply on the + terminal of the coil with the key on.

2) Test for injector pulse using a small bulb called a noid light. If none found, check for power supply at one side of the injector with the key on.

3) Use a fuel pressure gauge to test for correct fuel pressure, also noticing if the pressure holds when key is shut off.

Once you have determined which of these functions has dropped out,
you will know which system is having the problem.