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Suddenly stranded? Testing your fuel tank levels, fuel pump relay, and fuel pump in 10 minutes with a piece of copper wire

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70K views 91 replies 22 participants last post by  68FB  
J
#1 ·
Hi all,

So tonight my car suddenly no longer started up. It had literally been driven within 5 minutes and suddenly no longer started, yet it cranked healthy. No problems, no lights, etc.

Thus, I suspected fuel system related since our lovely beasts seem to have several known failure points - that the fuel tanks are no longer being balanced somehow and the tank your fuel pump is in empties; that the fuel pump has died; or that the fuel pump relay has gone bad suddenly.

To figure out what the problem is in 10 minutes with your bare hands and a piece of copper wire, here's my guide to what I did (Big hat tip to Rao):

1. Step one: Put key in position 2 so that everything is powered up, put your seatbelt on, and then open up your secret menu on your gauge-cluster and go to test: 05 or 06 (forget which) and look at your tank levels to confirm they are at least close to each other:

Image


2. If your levels look equal and sufficient in quantity, next up is testing your fuel pump and fuel pump relay together. For this, you'll need a ~6" piece of copper wire or similar.

3. Open up your trunk and pull open the right-hand access panel behind the right taillight. You'll see a row of box-relays as such - the TALL BLACK ONE is the fuel pump relay. See below:

Image


Now, if you have someone with you, put your hand on the fuel pump relay while they turn the key to position 2. If the fuel pump relay is working, it will notably click under your hand - you won't miss it. If the fuel pump relay is dead, a lack of a click is a clear indicator. However, most people don't have individuals with them, soo....

4. Here is the fuel pump relay pulled out (just tug and wiggle a bit and it'll pull out with some force)

Image


5. Now you want to access what the relay plugs into - that grid underneath it and the wires that feed that grid. Pull the grid straight out and it'll unlock from it's slot and give you a few inches of room to manuever it around.

Image


6. Going into this relay is ~4 wires. 2 thick wires and 2 thin. The 2 thick wires are red & violet stripes and red & white striped. AFAIK, these represent power supply to the relay and then a power wire that runs to power the fuel pump - they become connected when the relay is signal'd to connect them.

To test the fuel pump and relay, you want to stick the wire in the grid-slot that connects to the thick wires (one red-white striped, one red-violet striped). You'll need to really jam it in there to make contact. Even with no key in the car, this has power with the trunk open. When your make-shift wire makes contact on both ends and connects these power supplies, you may see a spark and, if the fuel pump is working, you will hear it begin whining/pumping as long as you maintain contact - it won't cycle on/off, because it not has a steady supply of power.

Image


If the fuel pump does not turn on now and cannot be heard, it is dead. If it does turn on and you can hear it, keep the wires in contact and either start the car yourself or have someone else start the car. If it fires up, your fuel pump relay is dead.

...

Hope this helps others. Ironically, I've been eyeing up the fuel pump/filter/relay kit for a solid month and just putting it off since the car was running fine. And then tonight, my fuel pump relay dies. Time to pony up.

Thanks to this wonderful community and Rao in particular for making it possible for a basic DIYer like me to narrow this down to a specific relay at night, with a flashlight, and pretty much no tools.
 
R
#2 ·
Glad you were able to get your problem sorted out quickly. :)
 
#3 · (Edited)
Glad you got your problem sorted. That Rob guy seems like a pretty good guy.

NOTE: I NEED TO VERIFY THIS INFORMATION (see Post #5): Just so no one is confused, your jumper should be placed between Terminals 30 and Terminal 87 assuming that this relay and circutry follows convention. Terminal 87 is a shut contact when the relay coil is energized via Terminals 85 and 86.
 

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R
#5 · (Edited)
josiahg52 said:
Glad you got your problem sorted. That Rob guy seems like a pretty good guy.

Just so no one is confused, your jumper should be placed between Terminals 30 and Terminal 87 assuming that this relay and circutry follows convention. Terminal 87 is a shut contact when the relay coil is energized via Terminals 85 and 86.
Click to expand...
I've read differently :rofl:

The BMW relays don't always follow the Bosch convention so the pins can be in just about any location.

I walked Joe through the wiring diagram so that he would be able to find the right wires to work with.
 
J
#4 ·
Josiah,

I'm very new to relays and electrical work in general and thus when I pulled the relay and examined your description above and what not, I became dis-oriented - I wasn't clear which ones were which since the box looks very similar on each side.

So I followed Rao's advice and jumped it based upon wiring going into the lead from below - the large wires represent power supply to and from the relay, they are red/violet and red/white striped, and thus I jumped those directly. Worked and the fuel pump immediately started running non-stop (I wouldn't leave it like that for long!)

I'm not disagreeing with you at all, just saying why and how I proceeded a little differently due to my inexperience.
 
#7 ·
Nothing wrong with verifying the wiring! I was trying to preempt the question as to which ones to jump.

rao said:
I've read differently :rofl:

The BMW relays don't always follow the Bosch convention so the pins can be in just about any location.

I walked Joe through the wiring diagram so that he would be able to find the right wires to work with.
Click to expand...
I will find out tonight, at least for the fuel pump relay! Using a wiring diagram is the proper way anyway.
 
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J
#6 ·
Wanted to give a plug to Bimmerzone - Vince is getting me a fuel pump relay TODAY (note: first business day since this happened, but possible also cause I live 2 miles away). Being off the road for a single day to get a part is a livable situation and I'm saving in car rental costs enough to pay for the fuel pump relay itself, if I had been stranded for one more day.
 
J
#8 ·
Josiah, by your diagram I believe I jumped 85 and 87.

85 was my violet/red striped power supply wire, and 87 was my red/white striped power-to-fuel-pump wire, IIRC.

There are 9 pins available to plug into IIRC, and I jumped a center-left terminal and a center-top terminal (i.e. diagonally to each other). I did NOT jump anything from bottom to top.
 
R
#10 ·
You jumped 30 and 87. The numbers are hard to see at night with a flashlight or in the day if you are over 15 hiha. Plus as mentioned BMW goes it's own way with relay pin orientation. That is why we went with the colors and gauge.
 
E
#9 ·
Thanks for the pics and write-up. I'm seeing a part # 61-35-7-892-976 for the relay. Does that appear to be correct? Thanks!
 
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J
#11 ·
Rao, sorry I wasn't clear, I meant to indicate to Josiah that if I followed that diagram, then i jumped the wrong leads, but by following the wires, I was ok. Yes, you taught me the superior way 'O wise one. ;)

ECB - That's the part number I gave to Vince @ Bimmerzone this morning, so it better be correct!
 
B
#12 ·
Nice write-up Joe, thanks. My 90k mile fuel pump and relay are scaring me now.

I have a dumb question about the relay - if stranded, can you drive the car at all with the terminals jumped? I'm guessing this is more complicated that e.g. jumping a temperature switch to keep an aux fan running. Maybe a better question is what function does the relay serve?
 
R
#13 ·
Add it to the e39 M5 list of things to be worried about hiha

The relay allows the DME to decide when the fuel pump is on.

When you bypass the relay you take the DME out of the equation and the fuel pump will run ALL OF THE TIME,even when the key is off or the car is upside down, etc. Obviously, if you are stranded, this is a decent way to get the car to a safe place. The car will run just fine with the terminals jumped, but you obviously don't want to run it that way for long.
 
J
#16 ·
rao said:
Add it to the e39 M5 list of things to be worried about hiha

The relay allows the DME to decide when the fuel pump is on.

When you bypass the relay you take the DME out of the equation and the fuel pump will run ALL OF THE TIME,even when the key is off or the car is upside down, etc. Obviously, if you are stranded, this is a decent way to get the car to a safe place. The car will run just fine with the terminals jumped, but you obviously don't want to run it that way for long.
Click to expand...
Do you happen to know how long a fuel pump would run for in a continuous state with, say, high RPM operation on a track?

I'm trying to guess how long a fuel pump sitting in a good full tank of gas could safely operate continuously in....i.e. if this happened in the middle of a road trip, could you run it continuously and safely for the fuel pump for 6 hours assuming a good tank of fuel to keep it's heat dissapated?

Also, this gives me a great idea for a snake oil car mod. I'm going to sell fuel pump relays that keep the pump on continuously, then regulators that allow much higher pressures, and sell it as

"Tired of your car's power being tuned down by the factory because the fuel pump doesn't run all the time? Turn up the BOOST - raise your fuel pressure to it's natural potential! Your cylinders will now see much better fuel injection due to much better fuel pressure levels, resulting in better performance at all RPMs!

Only $199. Not responsible for life of fuel pump or dead batteries."

...

If only I could use my powers for good...
 
J
#14 ·
I was really surprised - the relay died very suddenly on me. I guess that'sthe way they go, but stilll....never thought it would happen to me :)

The relay serves to turn the pump on and off AFAIK - my pump ran non-stop when I jumped the relay. Yes, I could drive it with a certain jumper set-up - no way it could be my copper wire though. You would need a jumper cable that would firmly be inserted/held in the relay slots - my copper wire I had to firmly hold in there for it to maintain contact.

I BELIEVE I can now take my dead relay, dismantle it, take the eletrical contact points and wire and make it into a relay jumper that would stay inserted if I wanted. So that might be an option if you have an old relay sitting around.

FYI, while I firmly held it in there my wife started the car and it fired right up. I think this was important - I had a no start condition and it appeared very clearly my relay was bad. But I then confirmed this to be the case by bypassing the relay and actually starting the car.
 
R
#15 ·
You could easily wire in a socket for a "normal" relay and carry a whole pile of spares for about $20 ;)
 
#18 ·
I started this at 8 AM and it has taken all day to hit submit, busy day. Some has been mentioned but some has not. Sorry for the repeat.
I have a question or two from some of the other posts.Where does this two speeds come from? And where does this thing where the pump is turned off during operations come from? Best I know pump fires on engine start and has one speed that runs constantly till the key gets turned off. I don't see it in the wiring diagram and with what little is used of the EKP it is not there and it is certainly not in the fuel pump.
Rao is right they used the EKP module for this application. Not sure why, as they did not use all the pins. They just used the pins of a standard relay no need for the expense of the EKP module, but maybe has something to do with a something extra on pin 1 going back to the DME. Anyway you are still sort of correct and it is worth noting the correct positions of the power thru which is the location you noted of 87 and 30. Those two location are very standard and should almost be memorized by diyers. It is especially important in this application because there are two red/white wires in this plug.

One thing that Joe pointed out in #3 is only sometimes correct. The relay is not always activated when the key is in position 2. It is only activated if the car is cold. If the car is warm the relay is not activated because it is assumed the the system will still have pressure. It activates for a short time cold because no matter how hard they try it is hard to hold fuel pressure for long periods of time. If you hold it in crank for some amount of time the fuel pump runs again, don't recall how long you have to be cranking.

Another thing worth noting is always put some gas in the car as the pump tank maybe empty or very low. It will prevent you doing any further damage if nothing else. The reason the tank could be empty is simple, if the fuel pump is having problems from a fault in itself or by fault of the relay then the amount of fuel returning to the jet pump will be low. If the jet pump does not get the correct flow of fuel it won't be moving fuel so if you are low on fuel already, you could be empty on the pump tank even though the gauge says you have some. It will be in the other side. You can use the OBC gauge but since the senders are electric and you are having an electrical problem.........
 
#19 · (Edited)
This is a helluva cheap part to keep in the trunk just in case! Considering we have those little (empty) compartments in the trunk just under the carpeting I say toss it in there. I have oil in there, a tire plugging kit, flashlight, etc. For $30-$40 I just might order one myself. Someone post up the best buy location. Thanks! Dare I say Group Buy? :)
 
J
#20 ·
Yeah, if I knew 2 months ago what I know now I would've picked one up as a spare. As have been pointed out elsewhere, there's really only a few parts on this car that routinely genuinely leave folks stranded without advance warning: Fuel Pump & Fuel Pump relay being two of them. At least the Fuel pump relay can be changed in 2 minutes with your bare hands anywhere.
 
B
#21 · (Edited)
Good write-up! Thanks.

Okay, the question that I have, is does the relay have anything to do with the fuel pump speed? in other words, when my car is dying of late, i CAN verify that the fuel pump IS getting power. So, if the relay is bad, is it a nogo on the voltage to the pump?

So in simpler terms, bad relay = no fuel pump spinning, correct?

I am down to my fuel pump overheating due to clogged filter, or intermittent pump failure, and this helps to solidify the issue

Thanks
 
#22 ·
bibledriver said:
Good write-up! Thanks.

Okay, the question that I have,#1 is does the relay have anything to do with the fuel pump speed? in other words,#2 when my car is dying of late, i CAN verify that the fuel pump IS getting power. So, if the relay is bad, is it a nogo on the voltage to the pump?

#3So in simpler terms, bad relay = no fuel pump spinning, correct?

#4I am down to my fuel pump overheating due to clogged filter, or intermittent pump failure, and this helps to solidify the issue

Thanks
Click to expand...
#1 Don't think so. That said the relay could have a fault that makes it loose contact in such away that still lets some volts thru but not enough.
If you are actually measuring the voltage across the pump you would likely see a voltage drop.

#2 How?

#3 Yes

#4 How do you know have you seen it on a fuel pressure gauge?

Simple test; use a paper clip to jump the 2 places Joe showed, the 2 fat wires or location 87 and 30 as shown. After a min or two touch the paper clip and see if it is hot , warm or cool. The warmer it is the harder the pump is working. Could mean blockage but likely indicates an unhealthy pump.
Check for codes if your fuel pump is the cause, you should have 81 or 95 code.

Thanks Joe good write up. Now no one even needs a spare relay or to ever worry about it again. They should all know now how to jump the relay and be able to drive home and fix it there.
 
#24 ·
Can't see how. I am yet to see the inside of the ekp, maybe someone that replaces theirs and cut it apart. There would have to be someway to control the power in the EKP between 30 and 87. Pretty Sure, it is just a switch inside, no real resistance across the terminals . Could work like a cordless tool but that would take a special winding in the motor.. How would guys be using aftermarket pumps? The other thing is the DME has no way to know what to do, because it does not measure pressure or flow.
Some cut one of these things apart.
 
#25 ·
It is an electromagnetic relay.

Voltage is applied to Terminals 85 and 86 energizing the relay coil. The voltage induces current flow in the coil and creates a magnetic field around the coil. This magnetic field attracts the magnetized contact inside the "module." This magnetic attraction brings the two contacts connected to Terminals 30 and 87. The DME can control fuel pump operation by controlling the voltage applied to Terminals 85 and 86 which is a low current load of about 1/8 Amperes.
 
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#27 ·
Exactly the only thing special here is pin one. Which tests O 0hms on first contact then climbs to 800 ohms and has a reverse resistance of 1243 ohms. It only has a connection to 2 showing the DME that power has reached there.
If there was something between 6 and 2 (30 87) that could control motor speed, it would not go click. It is the job of the regulator too control the flow of fuel.
 
B
#28 · (Edited)
Regulator controls pressure beyond itself, something else controls speed, and therefore output, of the fuel filter. I agree that the circuit gets closed by the relay, and what goes through the circuit somehow must be altered. If the regulator were electric, that would be different. one would have to measure the voltage at various revs, which the DME can control, but that seems to be the only way, off hand that makes sense to me.

So bad relay is all or nothing, then.? right? As it is simply the switching portion that fails, and the rest is simply passing through as it comes from another source/controller
 
#29 ·
bibledriver said:
So bad relay is all or nothing, then.? right? As it is simply the switching portion that fails, and the rest is simply passing through as it comes from another source/controller
Click to expand...
Yes bad relay means pump gets no volts, yours makes noise when you are out of gas so relay not likely your issue. Powered directly from the battery. This concept of a variable speed fuel pump.... get some proof before you believe it. In all my time I have never seen a variable speed fuel pump not a two speed one either, nor have my mechanics. They work on everything with motors from cars to large industrial and construction equipment back down to honda pumps.
 
R
#30 ·
There are plenty of cars that vary the voltage to the fuel pump, but that isn't done by a relay.
 
R
#32 ·
If you can hear the pump change speeds you have a real problem.
 
B
#33 · (Edited)
Well, seat is out, and cover panel to the pump is off (the one with three screws), so yes, I can hear it.

If your statement is still true with that in mind, then refer to the thread where I am trying to figure out this real problem(cause I have one)! :blabla:

If I should still not be able to hear it, then you are solidifying my thought of the problem!
 
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