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Thread: Fuel Pump Electrical Diagnosis Help

  1. #1
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    Fuel Pump Electrical Diagnosis Help

    So far: 1991 850

    No start
    0 fuel pressure bank 1-6
    fuses good
    Jumpered pin 30 & 87 in relay socket, good fuel pressure, engine start.
    Installed new relay - 0 fuel pressure - no start
    Check voltage in relay socket across pin 85 & 86 = 10.82 volts. Shouldn't it be 12v? Doesn't seem right, but maybe I'm wrong.

    Where does triggering voltage come from for fuel pump relays so I can back track, or what else am I missing?

    Thank you
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  2. #2
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    One of your crank sensors is bad. You can measure the ohm resistance between pins 1 and 2 I believe should be around 500.

  3. #3
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    Dragon850, thanks for getting back so fast.

    Since I posted, I back tracked to source of trigger power for fuel pump relay. Apparently it's fed from K6310 which is DME relay cylinders 1-6 and powers injectors and coil supply on fuel pump relay as well as the evap emission valve.

    Checked across that relay (85-86) and got 11.82v. Can all this be caused by crank position sensor? I hope not, because I just got the engine back together and now may have to pull shroud and fan again, but hopefully not the radiator, unless there is a trick way to get to it.

    Can I test the cps at the connector under the oil filler? Is it the top or bottom cps?

    (Deep sigh of frustration)

    Thanks again

  4. #4
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    K6310 is the DME supply, it will operate with no input from the crank sensor and switches ON when the ignition is in position II. You say the car ran correctly when the fuel pump relay is bridged - but, you didn't say if it started on all 12 - if it was running on all 12 then both crank sensors are working and you can ignore them - if only running on 6 then the 1-6 crank sensor could be a candidate.

    The fuel pump relay (K8311) is controlled by the DME (contact 85) which is pulled to around 0V during a quick blip when the ignition is switched to position II - and then permanently when a signal from the 1-6 crankshaft sensor is seen.

    http://www.meeknet.co.uk/e31/e31_95.pdf#page=132
    Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i


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  5. #5
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    Timm;

    When the car started, it seemed to run on all 12, idled smoothly. I didn't drive the car, so don't know if it was down on power. I was just ecstatic that it started. The second I jumpered the fuel pump relay for 1-6, the fuel pump kicked in and I had full pressure as I had left the pressure gauge attached. Key NOT on. When 1-6 fuel pressure was 0, the engine would not start. Are they supposed to be able to start on 1 bank? I know they can run on 1 bank, but I didn't know it was possible to start them that way. Besides, with full fuel pressure in bank 7-12, wouldn't it have started on that bank if that's possible?

    When I backtracked to the DME relay, I checked it and it was not clicking (12v across pin 85-86), so I think it's a goner. It's a bit different than the fuel pump relay in that it is a normally open dual 87 contact relay, so it can operate 2 circuits. Nothing is open this weekend (BMW dealer) due to Columbus Day here in the colonies, and there are no electronics stores around, so it will have to wait til Tuesday. I hate waiting for parts...

    I'm praying to the electronics gods, that it is not a Crank Position sensor! Looks like a real PITA. Specially since I just got it back together after having a bunch of powder coating done.

    Thank you
    Drives:
    2015 Ram 3500 Dually
    2014 Dodge Dart
    2002 Mercedes Benz SL500
    2000 BMW 528iA
    1998 BMW 740iLA
    1998 BMW 750iLA
    1997 BMW 528iA
    1997 BMW 740iLA
    1994 BMW 540iA
    1994 BMW 740iA
    1991 BMW 850iA
    1990 BMW 750iA (Not an "L")
    1985 BMW 635CSiA
    1967 Buick Riviera

  6. #6
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    Yes they will start on 6. I think relay is fired, let us know what happens when you replace.

  7. #7
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    Replaced both relays, no change. Guess I'm going after CPS after all. Drats!

    Gentlemen, thanks for the help and will follow with updates.

    Regards

  8. #8
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    Just checked resistance on both. Lower right = 513, lower left = 516. I'm kinda lost at this point.

  9. #9
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    To Summarize:

    Fuel pump 7-12 full pressure 50psi
    Fuel pump 1-6 0 psi
    Jumpered fuel pump relay 30-87 1-6 full pressure 50 psi
    Replaced fuel pump relay (BMW) 1-6 0 psi
    Replaced DME relay (no click) (BMW) 1-6 0 psi
    VTVM on CPS's. 513 ohms pin 1-2 on lower left (drivers) connector
    516 ohms pin 1-2 on upper right (passenger)

    I think I have everything. What am I missing? New (Febi) distributor caps. New Bosch rotors. New NGK BKR6EK plugs. Wires were like new. CPS's & spark plug identifiers hooked up correctly. (Ask me how I know).

    I'm still lost, specially why it won't start on 7-12, but will start with jumpered relay and fuel pressure on 1-6.

    Thinking out loud: With fuel on 7-12 and none on 1-6 = no start, then fuel on both = start, relays on 7-12 should be good, right? Maybe I'll check just in case.

  10. #10
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    Interesting find. Pic of DME 7-12 Relay (Green) and DME relay 1-6 ( White). Pin arrangement of 30 and 86 are reversed. White on purchased relay based on part number on original DME 1-6 relay. Could it have been wrong? Can't find part number for DME relay in ETK. Car did run and drive prior to relay and fuel pressure issues. Tried reversing DME relays, still 0 fuel pressure 1-6.

    Groan...

  11. #11
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    DME Relays are White K212 & K213 pt# 61361729004
    O2 sensor relay is Green (you may or may not have this) K214 pt#
    61368373700

    At first glance, these may look the same, but electrically they differ... the White relays have diodes in them.
    Again, according to BMW, DME Relays are not Green.

    The 2 DME Relays are fed a LOW signal at contact 85 on the relay from pin 27 on each DME... there is an indication that the two DME's are also tied together at pin 27, so theoretically either DME could hold both DME Relays energized...
    The 2 Fuel Pump Relays are supplied a LOW signal at contact 85 from pin 1 on each respective DME, these are independent of each other. The High side of the coils are supplied by the aforementioned DME Relays, so each DME Relay need to be energized for the corresponding Fuel Pump Relay to activate when the LOW is supplied by the corresponding DME, pin 1.

    So... at this stage of the game, it may be prudent to test the output of Pin 1 on DME 1-6 to see if it does indeed supply the ground required to activate the Fuel Pump Relay at contact 85. If so, then if you already checked/tested that you do indeed have power at contact 86 of that Fuel Pump relay and the relay still does not activate... it is a bad or incorrect relay. If you get no LOW signal from DME 1-6, then you are going to have to figure out why it isn't there... back to your sensors again... Keep in mind that the engine has to actually be turning for pin 1 to go LOW, either running or powered by the starter... it blips for a second when the key is first turned on but not long enough to test... so you have to crank the car to actually know.

    Might be interesting to purposefully ground pin 1 of DME1-6 and see if the car starts like it did when you jumpered the relay... No, you won't hurt your DME

    0krelay.jpg
    Last edited by cartoonz; 10-10-2017 at 11:58 PM.
    '91 Dinan 860 Stage III (soon to be Dinan TT 6L)
    '91 850i 6sp (mint)
    '91 850i 6sp (getting there)
    '90 Dinan 750iL TT stage III
    '94 850 CSi (being restored to factory perfect)
    '94 840ci (4 short - obvious parts car), '91 850i parts car
    '73 3.0 csi, '08 535i, '95 318i
    ...and a few other non BMW cars

  12. #12
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    Have you pulled the codes?? Could be a cylinder position sensor , error 16.

  13. #13
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    Starting to think this is a "pour gasoline on a fire" scenario...

    Thought i would back up a bit...
    Replaced both DME relays with the correct ones (with diodes) and decided to verify fuel pressures. Autozone loaner in addition to mine, so running two simultaneously. Still have 0 pressure on 1-6, but only have 20 psi on 7-12 which is probably why it won't run on that bank. Though I tested it before (single gauge and got 50 psi (I think). It does try to start, but not enough pressure. This would explain why it started when 1-6 fuel pump relay was jumpered, that's all it had to run on

    In light of this new development, and since both fuel pressure gauges are hooked pre-regulators, would it be prudent to replace the fuel filters first? I don't know how old these are and was going to replace them anyway when I could get the car on the lift. When 1-6 is jumpered, I get 50 psi.

    Just to verify, the bottom hose from the tank, (drivers side) goes to 1-6 on the passengers side and the top hose goes to 7-12 on the drivers side. I would hate it if this is caused from me screwing up the feed lines, getting them backwards.
    Or to put it another way, the metal line with the dog-leg goes to 7-12 (drivers)and the straighter of the two goes to 1-6 (passenger).

    When it rains it pours.

  14. #14
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    After the last post, I thought I would reverse the fuel lines and did. It started and ran, seems like both sides, but I'm showing 0 psi on one side and about 11 psi on the other. With those pressures, I don't think it should run at all.

    I'm really confused now.

  15. #15
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    Quote Originally Posted by masbury View Post
    Starting to think this is a "pour gasoline on a fire" scenario...

    Thought i would back up a bit...
    Replaced both DME relays with the correct ones (with diodes) and decided to verify fuel pressures. Autozone loaner in addition to mine, so running two simultaneously. Still have 0 pressure on 1-6, but only have 20 psi on 7-12 which is probably why it won't run on that bank. Though I tested it before (single gauge and got 50 psi (I think). It does try to start, but not enough pressure. This would explain why it started when 1-6 fuel pump relay was jumpered, that's all it had to run on

    In light of this new development, and since both fuel pressure gauges are hooked pre-regulators, would it be prudent to replace the fuel filters first? I don't know how old these are and was going to replace them anyway when I could get the car on the lift. When 1-6 is jumpered, I get 50 psi.

    Just to verify, the bottom hose from the tank, (drivers side) goes to 1-6 on the passengers side and the top hose goes to 7-12 on the drivers side. I would hate it if this is caused from me screwing up the feed lines, getting them backwards.
    Or to put it another way, the metal line with the dog-leg goes to 7-12 (drivers)and the straighter of the two goes to 1-6 (passenger).

    When it rains it pours.
    OK, first, both lines 1-6 and 1-7 run on the drivers side of the car and come up through the fender then connect to both fuel rails at the back of the engine. The passenger side line is a fuel pressure return line back to the tank. If you are getting low psi your fuel pumps are bad or you have a massive pressure leak in the assembly, FPRs should regulate pressure to 39psi at idle. I am assuming you are measuring the pressure inline?

  16. #16
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    dragon850;

    Yes, measuring pressure at the two inlet hoses that go into the fuel rails on the top of the engine. Gauges are inline. That pair of formed pipes are the ones I was asking about as far as which goes to which bank. Good description or a pic would be immensely helpful. This car was pretty boogered up when I got it, so I can't trust anything existing.

    Also ran codes after the fuel line swap and idle - no dme or eml codes. I don't know if it will set codes when idling or if it must be driven. I don't want to drive it with the MAF issue as it is too lean. EML light is not illuminated on the dash (it does work). I do have a CEL, but I have 2 bad MAF's (in transit).

    Thank you for all your help
    Drives:
    2015 Ram 3500 Dually
    2014 Dodge Dart
    2002 Mercedes Benz SL500
    2000 BMW 528iA
    1998 BMW 740iLA
    1998 BMW 750iLA
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    1994 BMW 540iA
    1994 BMW 740iA
    1991 BMW 850iA
    1990 BMW 750iA (Not an "L")
    1985 BMW 635CSiA
    1967 Buick Riviera

  17. #17
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    Your pumps are probably pissing fuel in the tank, take the pump assembly out and take a look. You are not building any pressure very obvious what's going on.
    if you have 2 lines coming up through the fender and connect to 2 fuel rails at the back of the motor you are just fine, that's not the problem.

  18. #18
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    Since I know it won't run without fuel pressure, I removed the in-line fuel pressure gauges and attached them one at a time. I dead ended each on the main feed lines (drivers side). I am now back to square one.

    Cylinders 1-6 have 0 pressure. 7-12, when turning the key on, jumped to 72psi, then settled at 60. So I have a dead bank.

    I then tested the 7-12 DME relay socket 85-86 with a VTVM so I would know what to look for on 1-6. Keyoff reading .63v. Keyon 11.8v.

    Removed 1-6 DME relay, VTVM across 85-86, keyoff voltage .62v. Keyon 11.78v.

    So, if I understood cartoonz, both DME relays are going low (ground) on 85. (I think)

    Moved over to fuel pump relays.
    Again VTVM ACROSS 85-86 on 7-12, keyoff .27v, keyon 12.4v

    VTVM across 85-86 on 1-6, keyoff voltage .23v, keyon 1.58 v. (Houston, I think we have a problem.)

    Any other possibilities?

  19. #19
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    As you noticed, the fuel pressure jumped and then drooped. That is the action of the fuel pump relays which are only momentarily energised when the key is switched to position II. This makes it difficult to measure across the coil pins on the relays as you only get a low pulse on 85 for a couple of seconds.

    However, with the ignition in position II you should have 12V on pin 86 (which is supplied by the DME relays) - you can measure this with your meter - just connect the negative lead of the meter to the ground point on the suspension strut to get a good ground.

    Ignoring all of the above - shorting pins 30 and 87 of the fuel pump relays should get the pumps working even with the ignition off - and you should see your fuel pressure rise - if you don't manage to get that far then we can advise further!
    Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i


    My Current 840ci Sport Individual
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  20. #20
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    After doing the tests cartoonz suggested, here are results:
    Pin 1 of DME1 is only allowing/doing/giving 1.58VDC.

    Do I have a bad DME, or is there something inside I can repair, or, am I (again) missing something? It is not the original DME. It's marked with DC80795, which is an 11/91 M70 E32.

    Groan, again. But thank you again to everyone!

  21. #21
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    Ground pin one of DME 1-6 - in circuit, keep everything hooked up
    Turn on key to "run"
    Now measure 85-86 voltage at FP relay socket

    Got voltage? 12v +/- ok, trouble is crank/cam sensors
    Still no voltage? Broken wire, bad DME relay

    I'd also be pulling the pup assembly out to inspect it too...
    '91 Dinan 860 Stage III (soon to be Dinan TT 6L)
    '91 850i 6sp (mint)
    '91 850i 6sp (getting there)
    '90 Dinan 750iL TT stage III
    '94 850 CSi (being restored to factory perfect)
    '94 840ci (4 short - obvious parts car), '91 850i parts car
    '73 3.0 csi, '08 535i, '95 318i
    ...and a few other non BMW cars

  22. #22
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    cartoonz;

    I get everything, except the "in circuit" part.

    Where are the cam sensors and how can I test them?

    I've tested resistance on crank sensors - 500 ohm +/- 5% also the plug identifiers - in spec also.
    Drives:
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    1998 BMW 740iLA
    1998 BMW 750iLA
    1997 BMW 528iA
    1997 BMW 740iLA
    1994 BMW 540iA
    1994 BMW 740iA
    1991 BMW 850iA
    1990 BMW 750iA (Not an "L")
    1985 BMW 635CSiA
    1967 Buick Riviera

  23. #23
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    the "cam sensors" are the donuts on the plug wires... #6 & #12 keep in mind, if either of those ignition wires are bad/shorted - the donuts could "test" good but not work. Same scenario for a fouled plug... that can cause the sensor not to read the spark pulse either.
    "in circuit" means you ground pin #1 of DME 1-6 and keep everything plugged in... even just turning the key to "run" without starting the car should be driving the fuel pump in this condition. So set it up with the 1-6 Fuel relay out... measure voltage across relay contacts 86&85.. good? plug relay in... does it energise? Good? Does pump run? Yes? - sensors
    Last edited by cartoonz; 10-12-2017 at 11:12 PM.
    '91 Dinan 860 Stage III (soon to be Dinan TT 6L)
    '91 850i 6sp (mint)
    '91 850i 6sp (getting there)
    '90 Dinan 750iL TT stage III
    '94 850 CSi (being restored to factory perfect)
    '94 840ci (4 short - obvious parts car), '91 850i parts car
    '73 3.0 csi, '08 535i, '95 318i
    ...and a few other non BMW cars

  24. #24
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    Intermittent bad fuel pump?
    26 year old car built to run on unleaded fuel

  25. #25
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    Grounded pin 1 of DME, ulled 1-6 FP relay, tested 85-86 got 1.57VDC. Verified 12VDC on pin 86 but not across 85-86.
    What am I doing wrong? Brand new BMW DME relays. Swapped twice.

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