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Thread: Fuel pump fuse keeps blowing

  1. #1
    Join Date
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    Fuel pump fuse keeps blowing

    Got a ‘83 that I bought about two years ago and hasn’t run for about 10 years. Slowly going through it and just bought two new fuel pumps and filters. Got fuel and car starts but after like 3 seconds of being on, the fuse pops. I’ve tried the original fuel pump, still does it. Doesn’t do it with just the in tank plugged in or without it, so I know it’s not that one.

    Is there a common pinch point for the wiring for the main pump? I’ve looked all over and can’t find any wiring that’s rubbed through. Can a bad relay cause it to pop?

    Thanks

  2. #2
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    Quote Originally Posted by Jim530 View Post
    Doesn’t do it with just the in tank plugged in or without it, so I know it’s not that one.

    Is there a common pinch point for the wiring for the main pump? I’ve looked all over and can’t find any wiring that’s rubbed through. Can a bad relay cause it to pop?
    I'm not sure what you meant by the "doesn't do it" statement. Since the problem has happened with both pumps, it's reasonable to look at other places. Yes the relay can be the problem.

    Steps I would take include:

    1) Making sure the fuse is the proper one. (Not a higher or lower rating. Don't just replace it with what it had before. Check the chart.)
    2) If it's easy to do, swap the fuel pump relay with another relay. Usually cars have several relays of the same type for different functions. If the problem goes away, the relay is probably bad.
    3) Pull out the relay and test the circuits, using a wiring diagram and/or the diagram on the side of the relay. It probably has 4 terminals. 2 of them come from the switch that turns on the relay. 2 of them feed power from the fuse to the pump. Set your multimeter to ohms and check the circuits. Also check the circuits to ground, to see if there's a short to ground. Since we think the pumps might be ok, unplug the wire at the pump, and check the circuits to ground.
    3a) Check the circuit between the fuel pump and the fuse. There should be one of those that has a good connection (no resistance, no ohms). That would be the hot lead for current to the pump. The other matching terminal in the relay socket will be to the fuel pump. With the pump unplugged, it should have no connection (infinite ohms) to ground. If it has a connection to ground, then yes, look for kinks or a short in the wiring to ground.

  3. #3
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    (One of the leads in the relay socket probably does have a good connection to ground, for turning on the relay. So that one is ok. But just that one should have good connection to ground. The one to the pump should have some connection to ground, with some resistance, but only when the pump is connected.)

  4. #4
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    The fuel pump relay powers more than just the fuel pumps, so check all the wiring to those items as well. Also unplug those items for testing. If by chance the relay is causing trouble, you can remove the relay and put a jumper in the relay's socket for testing too (between 15 and 87).

    Fuel pump relay pic for reference (also lists those other items the relay powers)..
    click to enlarge
    e21-fp-relay-bottom.jpg

    1981-1983 e21 electric manual for reference...
    save a copy!
    Tbd

  5. #5
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    Cool

    DSCI0009t.jpg I'd make/use the one on the right to jumper the fuel pump socket, it has white-8 amp ceramic fuse ,Fuel Pump Fuse uses red-15 amp ceramic fuse in the fuse or junction box. I have used the one on the right for years, more than enough for the entire circuit when jumpering for testing purposes.

    The main and in tank fuel pump are connected together by a Green-Black stripe wire and both use brown ground wire, both are powered by terminal 87 on the fuel pump relay when its contacts are closed. terminal 87 has 3 power wires, one goes to the fuel pumps, one goes to the WUR/Air Aux Valve and one to the Lambda/Main Relay, its a work horse relay and due to the loads can go on the blink. I addressed this problem with an update some time ago and have run it for some time flawlessly.

    Randy
    Last edited by 320iAman; 04-15-2019 at 12:32 PM.

  6. #6
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    So the day after I posted this, I didn’t touch the car and suddenly it’s not blowing the fuse pump. I assume there must be a short somewhere but the car is on jack stands so not like anything is shifting. I got it to start many times without any issue. Not sure what happened but we will see if it continues to work.

    I’ll look into that wiring diagram, thanks for posting because I couldn’t find it for the life of me.

    I’ll also look into that upgrade for the relay if I understand correctly

  7. #7
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    Cool

    The top is how its wired from the factory, the bottom is the Hella update,,Hella and its contractors made the original wiring harnesses for these cars and the update. Fits perfectly and nothing less.
    IMG-5_page1_image1.jpg

    I'm using Bosch -109 version, both relays-hella and Bosch-Tyco add diode protection , so that flyback voltage is sent thru the on module coil and dissipated there by Looping the High Spike Break Contact Ve, that way it does run into the Fuel Pump Relay and it has to dissipate it--Called Hella Update-came from BMW Employee that Migrated to Hella Originally, you'd need to get a additional relay block(bmw dealer item) and wire up ground and a fused power wire. The diode relays brings in a modern touch to the wiring harness, a Fuel Pump Relay protected electrical circuit and puts the Fuel Pump Relay to running the Fuel pumps and providing minimal signal Ve/Amperage too the hella relay so the Hella Relay runs the Warm Up Regulator and the Air Auxillary valve,, the update lessens the load on the FP Relay. In terms of load reduction signal Voltage is 200 milli volts from around 12 Volts and corresponding amperage reduction for that leg of the Fuel Pump Relay DCve Circuit.

    The added hella relay DSCI0003.jpg,,DSCI0006.jpg

    Installed Look
    DSCI0007.jpg

    Randy
    Last edited by 320iAman; 04-18-2019 at 11:53 AM.

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