E28 Fuel pump relay issues

[moktar 7]

Hi All,

Im stuck and in need of some advice from anyone who might be able to give me an idea on what could be going wrong or something im missing.

1985 E28 M535I M30B34

So i have an issue where im pretty sure the ECU is not telling the relay to start the fuel pumps.

I have checked the main relay and socket...fine.

The fuel pump relay and socket..fine

Injectors are pulsing

All earths are clean and tight.

New battery

Crank angle sensor is sending a signal

The thing is if i use a jumper wire to bypass the relay both fuel pumps fire up and the car will start and run fine.

So since it runs fine im assuming the AFM side of things is working ok?

But this is where im stuck. is the only other fault thats likely to cause this going to be within the ECU?

How do I diagnose the problem from here?

Are ECU's avaliable and is it as easy as swapping them straight out?

Thanks for any help!

[moktar 7]

Ive been using this write up to get this far but it only goes as far as diagnosing a faulty relay.

http://www.hiperformancestore.com/motronic.htm

[will 169]

Have you tested the relay itself? i.e. remove relay from socket, put +12v on pin 86, ground on pin 85 -- it should latch. When latched, it should show continuity through pins 87-30.

You should also test the load ability of the relay by connecting an indicator globe (21W) in series from the battery across pins 87-30, latch the relay via 85-86 and make sure the globe burns full brightness. The reason for this is to make sure the contacts are latching properly. Just testing with a meter does not guarantee it is working under load, it might just be making enough contact to indicate on the multimeter, but will break down when a load (such as the fuel pump) is applied.

If the relay checks out as working, connect a multimeter between pin 85 of the relay socket (without reinstalling the relay) and ground, set multimeter to check continuity, then crank the motor and check if the meter shows continuity between pin 85 and ground. If you get continuity, the ECU is switching, if not, there is an issue with either the lead between the ECU and the relay, the signal from the crank speed sensor, or the ECU itself (or a combination of them).

Test and report back..

[moktar 7]

Thanks for taking the time to reply.

Yes old relay was tested under load and came up good. However I brought a new one just in case.

Just checked continuity at socket 85 and earth and that came up good. Car actually started from the fuel left in the rail.

Chased the leads back to the ECU as well as i could and everything looks fine from what I can see.

Is there a way to test if the crank speed sensor is sending a signal?

Thanks for the help.

Sent from my GT-I9295 using Tapatalk

[will 169]

Normally you will not get a spark if the speed sensor is faulty. You will need a scope to read it. You can do a basic check by measuring it's resistance at the ECU plug.

Did you test if the ECU is switching(grounding) pin85 on the fuel pump relay?

[moktar 7]

Ok so i tested between pin 85 and 86 with a test light and cranked. the light came on constantly but did not turn off. I was told that this should turn on and then off?

Is it possible to take the plug out of the ECU and test if it is sending a signal? So i can eliminate any wiring issues from the ECU to the relay.

Worst case would it be safe to use the jumper wire to fire the fuel pumps and drive the car into my auto sparky?

[will 169]

You need to test between pin 85 of the fuel pump relay socket (with relay removed) and ground, not 85 and 86 of the FPR socket. Multimeter to be set to continuity/resistance test. It should show continuity when cranking and open circuit when not.

You can confirm the connection between the ECU and the FPR by unplugging both, then do a continuity test between pin 20 of the ECU socket and pin 85 of the FPR socket.

You can further confirm the FPR switching circuit is functioning using this procedure..

1. unplug ECU from socket.

2. Leave FPR plugged into it's socket.

3. switch on ignition, check that main relay has energised by testing for +12V at both pins 87-87.

4. ground pin 20 on the ECU plug- the FPR should latch and switch on the fuel pump.

The FPR switching circuit is a safety feature. It is designed to turn the fuel pump off if the motor stops running, specifically if you have an accident like rolling the vehicle and ending up on your roof. If the fuel pump keeps running, it would likely cause fuel to pour out and cause a fire.

You could bypass the safety feature by simply wiring pin 85 of the FPR to ground. The main relay will control the switching of the FPR, meaning the pump will run with the ignition on, and switch off with ignition off.

[moktar 7]

Yes sorry, I did the continuity test between 85 and ground and that showed continuity there while cranking.

Ill have a play around with the tests you have stated above and hope to find where the problem is originating from as I would prefer to have the car running 100% properly.

But if I can bypass by grounding pin 85 I will do that to get it up and running and into a auto sparky.

Thanks for sharing your knowledge!

Let you know if I find the cause!

[will 169]

Ok, if it shows continuity while cranking, then your issue is not with your ECU, rather with your pump circuit.

Do the following tests..

1. Switch on the ignition and test that BOTH pins 87-87 of the main relay are outputting +12V. Do a load test with a 21w globe to verify that both outputs are providing the necessary amps.

2. If the above test shows power/amps from both 87-87 outputs, you then need to verify that you have power/amps on pins 86 and 30 on the FPR socket. pin 86 is switched power from the main relay and pin 30 is a permanent live directly from the battery.

If both above tests check OK, combined with you already verifying that pin 85 on the FPR socket shows continuity to ground while cranking, your fuel pumps should fire up.

If test 1 above shows an issue with the 12V outputs on pins 87-87, remove the main relay and bridge pins 87-87-30 on the socket, then crank the engine with the FPR in place and see if the pumps fire up.

If the above test fails to fire up the pumps, you have an issue with the supply circuit to the pumps, and/or the pump/s are faulty. Just to clarify, the pump in the tank is purely a lift pump designed to prime the main pump which is above the rear axle. It is that external pump that provides the fuel at pressure/volume to run the engine, so it is essential this pump is running and providing the correct fuel pressure and sufficient volume of fuel.

Let me know what results you get with the above tests.

[moktar 7]

Ok so pins 87-87 of main relay show no output.

Bridged 30-87-87 with the FPR in place and fuel pumps worked and car started.

Checked all connections/wiring leading to and out of both relays and all looks pretty good.

Sent from my GT-I9295 using Tapatalk

[will 169]

getting there..

Test that pin 86 on the main relay gets a +12V supply when the ignition is switched on (should be a green wire.) Also check that pin 85 has a permanent ground.

If both of these tests check OK, it is likely your main relay is fubar.

If you don't get a switched +12V on pin 86, you will need to find out why. First thing would be to check that the ignition switch has a 12V output on the green wire with the ignition on. If you have an OBC with an immobiliser function, that could be the problem as it cuts the green wire supply to the main relay.

[moktar 7]

Hey will,

I spent all weekend going starting from scratch and going over every test again and replacing any dodgy looking connections and cleaning up all wiring leading to the ECU. I took off all earths i could find and cleaned the contact area and also cleaned up the relay sockets.

I unplugged the ECU and plugged it back in.

Again i could find nothing obvious but there were a few grimy connections and earth contacts.

I did this test again:

1. Switch on the ignition and test that BOTH pins 87-87 of the main relay are outputting +12V. Do a load test with a 21w globe to verify that both outputs are providing the necessary amps.

2. If the above test shows power/amps from both 87-87 outputs, you then need to verify that you have power/amps on pins 86 and 30 on the FPR socket. pin 86 is switched power from the main relay and pin 30 is a permanent live directly from the battery.

Which showed that there was 12v and load tested with 21w globe. Both showed good results.

Moved to FRP Checked continuity at pin 85 while cranking and this came up good again..

Put everything back in its correct place, Sat down crossed fingers and turned the key..

Boom fired up straight away, Fuel pumps cranking and working fine!

The only thing I can put it down to is a dodgy connection or bad earth somewhere. It goes to show that even if connections look clean and tidy there may be a broken connection or one not making enough contact. Or maybe the ECU just needed a wiggle around..

Thank you so much for your help and giving me the inspiration to carry on through this ordeal... as i was getting to the point of frustration.

Once running, Clutch hit the floor... gave that a bleed.

Once reversing out of garage, brake pedal hit the floor.. gave them a bleed.

Road test came up good!

Showroom thread to come soon.

Thanks Again!