Fuel pump relay connector measurement less than a volt

[Cea123]

Hello All,

My son's car is a 1993 525i that's powered by a M50TU engine. The problem he's having is that it appears there's not enough voltage being supplied to the fuel pump. We thought that maybe the fuel pump was bad so he replaced it, but after installing it the engine still doesn't start. The troubleshooting that we've done are as follows: checked the fuel pump fuse, it wasn't blown; sprayed fuel directly into the throttle body, which does causes the engine to start; removed the new fuel pump from the tank and powered it directly, it worked fine; connected a jumper to the fuel pump relay terminals 87 and 30 and measured the voltage at the fuel pump's connectors 4 (-) and 5 (+), the resulting voltage measurement is less than a full voltage (0.09).

If anyone has any ideas as to how I can resolved this problem, please let me know. Lastly, I also read some information on another thread that mentioned something about fuseable links so if anyone has pictures of the location of the fuseable links, please send them to me.

Cea123

[Javier]

I would ask if you did measure also the voltage at the B+ post? Was it also that low? (Guess you verified the DC voltage setting to the tester, Not AC setting).

I was left in the road once by the Fuel pump fuse, it was not blown but mechanically damaged so it looks fine but was opened (where the filament meets the crimped lead), a quick exchange with a similar fuse in the box will rule this out.

[Cea123]

Hi Javier,

No, I didn't measure the voltage at the B+ post. Where exactly is the B+, is it one of the fuel pump relay post and is it also suppose to measure 12 voltage? (Yes, I use DC voltage).

I'll change out the current fuse as a good measure.

[Javier]

B+ post is a small square black plastic cap with a red + hiding the Battery + post in the engine compartment (Guess you also have your battery below rear seat, so this is a handy access to the Battery +). It will reveal the Battery voltage (though I'm sure you have it right as the car cranks Ok). Measuring B+ post after reading a funny lecture will rule out a tester issue.

The Fuel pump fuse is after the Fuel pump relay contact, so jumping 30-87 does not feed the pump if there are fuse issues, no mater you can read 12V+ at pin 30 of Relay pump socket. If switching fuses does not solve the issue, you should test for voltage at the F23 (With 30-87 relay pins jumped, once verified 12V+ at relay pin 30) to verify voltage reaches the fuse, and if fuse is powered at both sides, you will have to rule out a wire issue from fuse to pin 5 in the pump/sender connector (measuring 12V + at pin 5 to ground). If Ok, be sure to confirm there is a good ground at pin 4 of the pump sender connector. Remember measuring no voltage between pin 5 and 4 does not necessarily means you are missing 12V feed, but also a bad ground at pin 4.

[Cea123]

Javier,

Here's what I found after checking the things you mentioned. After jumpering terminals 30 and 87 the voltage at terminal 30 is just above 3 volts, the voltage at F23 is 12v on one side (the side nearest the driver side fender) of the fuse terminal, the other side reads 1.77v, and the reading at the fuel pump's connector pins 4 (-) and 5 (+) were 0.04. Next I'll try to location where pin 4 ground leads to, which seems to require that I lift the carpet alongside the passenger doors. If you can tell me where the pin 4 ground terminates, I'd appreciate it greatly or a picture would be worth a thousands words.

[Javier]

The only way a fuse can read 12 volts at one side and 1.77 at the other is it is opened or somehow damaged, The fuse resistance is insignificant and the only way it could drop 10+ volts is that the current is extremely high which would lead definitely to a fuse blown. Also terminal 30 is directly connected to B+ post, so it could not have 3 volts unless wire damage between B+ post and relay socket pin 30, additionally, it feeds F23, so voltage can not climb from 3 to 12 in its way to fuse F23. Something is wrong with your readings (probably the way you are taking them).

Please consider following procedures:

Find a very good and solid ground connection in the vicinity of the Fuse box (that is also besides the B+ post, I usually use the shock strut nuts). Hook there the black lead of the meter in a way that you can steadily and repeatedly get a 12V + reading at B+ post (using red lead of course).

Jump 30 to 87 in the pump relay and remove pump fuse F23. Take voltage readings at both posts of the fuse, one should read 12V+ and the other 0V. Note the one with 12V+ is the upstream terminal of the fuse (probably the fender side?), and will always read 12V+ volts as long as 30-87 is jumped.

Replace the fuse and get the reading at the side it was 0V, if it is not now identical to the voltage at the upstream terminal of the fuse, then the fuse is gone and should be replaced.

If you get 12V+ at both sides of the fuse, proceed to the trunk and:

Find a good solid ground in the trunk area (an exposed metal of the body will do) and hook there your black lead. Take the voltage reading at terminal 5 of the pump/sender connector, should read 12V +, other wise there may be a broken wire from fuse F23 to terminal 5 at the connector (very unlikely).

Set your tester to Ohms reading and check the resistance between terminal 4 of the pump/sender connector and the previously identified ground, it should read near 0 ohms. Otherwise, there may be a broken wire from terminal 4 to ground.

Your readings, up to now, do not take the fault out of the fuse box. Discarding the 3V reading at terminal 30, which seems wrongly taken, the rest of your readings leads me to an open fuse.

I'll be checking this thread more frequently so to be able to provide you quicker responses.

[Cea123]

Javier, I'll redo the measures and let you know the results. In my last reply I intended to ask you whether or not the DME relay could somehow be faulty.

[Javier]

Once you bypass the Pump relay (30-87 jumper) you remove all control automation on the fuel pump, so if it does not run after bypassing, the culprit is not the DME relay or Pump relay, or the DME itself. The only remaining components are the fuse F23 and the pump itself, besides wiring. It is not usual to find wire issues, contact issues may be, but not wire issues. Overheated contacts in fuse, relay, connector sockets and ground connections may lead to high resistance current paths dropping the available voltage to values below operative requirements, but you would notice through visual inspection such damages. I have not seen this damages in the fuel pump circuit, but I do have seen fooling fuses (visual inspection of the filament presence is not enough to check a fuse Ok).

Regarding your interest in locating the grounding point for the pump (Terminal 4), I'd suggest you to download from Shogun's web site http://twrite.org/shogunnew/topmenu.html => Reference Data => wiring diagrams => wiring diagram others => E34 directory of files => e34_93.pdf (Bookmark Shogun's web site, it is plenty of resources).

In the downloaded pdf look after page 7100.0-02, figure 2 and identify location of X495 underside the rear seat Right hand side.

Page 1210.13-02 shows the Fuel pump circuit, note that B tag leads to B+ post.

[Javier]

By the way, you can find in Walmart this funny tester for fuses. And can be used without removing the fuse if space permits.
8285

[Cea123]

Thanks, Javier! I'll certainly invest in one of these!

[Cea123]

Hello Javier,

I'm still pursuing an answer to the problem. Here's what I've done since our last correspondence 1) Replaced the current F23 fuse with another one, but disconnected it while performing the following steps, 2) Jumped fuel pump relay sockets 30 and 87, 3) Measured voltage output of socket 30, found a reading of less than 3 volts, and I did ensure that I had a good ground for my multi-meter, 4) Measured voltage at F23, which was zero and I didn't expect anything since socket 30 didn't measure well, and oh by I did switch on the ignition while taking the measurements, 5) Since I wasn't receiving a favorable voltage reading at socket 30, I assumed that there may be a fault somewhere in the B+ cable leading from the battery to socket 30, so I disconnected it from the battery and re-measured the voltage, there was no change still less than 3 volts, 6) Next, I checked the ground for pin 4, it was clean and tight. 7) I checked the fusible link near the battery, it wasn't broken, 8) Out of curiosity I decided to disconnect the fusible link cable from the battery and reconnected the B+ cable, followed by measuring voltage at socket 30 and 12v were present, but changed back to less than 3 volts after I reconnected the fusible link cable to the battery. 9) During the time I had the fusible cable disconnected from the battery, I also measure the voltage at F23 but there was nothing there and I figured that since the fusible link cable was disconnected from the battery, it must have some bearing on the no volts measure at F23, even though there was 12v at socket 30.

Is there some other fusible link that I should check?

If you have any other suggestions, please don't hesitate to send them to me.


Cea123

[Cea123]

If anyone else has some information you'd like to share about correcting the problem with my car, please send it to me!

[genphreak]

If anyone else has some information you'd like to share about correcting the problem with my car, please send it to me!OK this intrigues me:

I checked the fusible link near the battery, it wasn't broken, 8) Out of curiosity I decided to disconnect the fusible link cable from the battery and reconnected the B+ cable, followed by measuring voltage at socket 30 and 12v were present, but changed back to less than 3 volts after I reconnected the fusible link cable to the battery.

Fusible links are located inside shrink wrap in the 2 largest wires (the main power 1cm one and the 4-5mm one), approximately 8" from the battery post (if battery is under the back seat). If not there, one may be relocated to a large plastic holder behind the battery under the back seat- or very obviously fixed to the front RH shock tower.

Do yourself a favour and replace all the inline fuses* by screwing/crimping/soldering in new ones properly. Even if they are yet to prove faulty, they cost very little and are likely to fail if not replaced after 20 years. The metal simply deteriorates in a way that is only obvious under a microscope. The forums are full of issues with these- and it causes ongoing an challenge whenever anything electrical causes a problem...

EDIT: * If your car is equipped with a 4" plastic holder for any of these fuses, you may not need to be so keen to replace the fuse, I've not seen these fail as often as the inline ones- the inline ones seem to suffer more from corrosion inside the mating surfaces and from micro-cracks in the fuse material. Simply check them carefully for voltage drop and visible signs of oxidation, esp. around the fasteners, as well as cracks. I usually remove the fuse and bench test it under a light and magnifying glass/microscope when unsure.

[Cea123]

genphreak,

Fortunately, after studing the electrical wiring diagram for the car, I realized that I was placing the jumper into the Main Relay sockets (30 nad 87) rather than the Fuel Pump Relay sockets, all due to the fact someone unplugged and switched the places of the Main Relay sockets with the Fuel Pump Relay sockets. I was following the pictures of the locations of the relays from the shop manual, not relalizing that the sockets had been switched around until studing the electrically wiring diagram. So, here's what I found after placing the jumper into the correct sockets (30 and 87): 1.) When I tested the voltage at 30 it read 12v, 2.) The voltage at the B+ measures 12v, 3.) The voltage at F23 read 12v. But, after I replace the fuel pump relay into its connectors in the E-Box the 12v at F23 reads zero volts. Is it possible that the fuel pump relay is bad, even though it's new or maybe I shorted it out somehow? The other anomaly is the voltage at the 30 and B+ are not always consistent, sometimes the voltage is a low as 1.7 and 3.65, respectively, but I assuming that my ground for the multimeter is not quite good enough so I'm going try strapping it down to a good metal source and re-measure the voltages. Lastly, I learned how to test the relay so I will test it to try and determine whether or not its any good.

[Cea123]

genphreak,

I forgot to add that the fuel pump appears to be working fine and I'm saying that because, when I jumper 30 and 87 the fuel does come up to the rail.

[Javier]

Please keep in mind that the Pump relay will only close contact from 30-87 when the car is cranking or when the engine is running, so if you just replace the relay and checked for voltage at F23, it is normal to have 0V unless you crank the engine.

If jumping 30 to 87 in the pump relay starts the pump, the circuitry is Ok, I mean, the fuse, wiring, connectors and the pump. Now if you still does not have a working pump when cranking the engine with the relay installed, it is necessary to check the relay ( just remove the relay and feed 12 V from 86(+) to 85(-) pins, feel the click and verify 0 ohm reading between 30 and 87 pin), If relay test Ok, you may have other issues with the DME control, and a different approach to debug should be started.

[Javier]

This also should be checked carefully, I'll comment using the later info about this being the DME relay and not the pump relay:

I'm still pursuing an answer to the problem. Here's what I've done since our last correspondence 1) Replaced the current F23 fuse with another one, but disconnected it while performing the following steps, 2) Jumped fuel pump relay (now we know it was DME relay) sockets 30 and 87, 3) Measured voltage output of socket 30, found a reading of less than 3 volts (30 is feed straight from B+, just like pump relay, 3 V is bad), and I did ensure that I had a good ground for my multi-meter, 4) Measured voltage at F23, which was zero and I didn't expect anything since socket 30 didn't measure well, and oh by I did switch on the ignition while taking the measurements, (0 V is Ok in this scenario, ignition key in on (not start), does nothing here) 5) Since I wasn't receiving a favorable voltage reading at socket 30, I assumed that there may be a fault somewhere in the B+ cable leading from the battery to socket 30, so I disconnected it from the battery and re-measured the voltage (?? don't see the objective), there was no change still less than 3 volts (Getting more than 0V without battery is weird), 6) Next, I checked the ground for pin 4, it was clean and tight. 7) I checked the fusible link near the battery, it wasn't broken (Fusible link does not feed the B+ post), 8) Out of curiosity I decided to disconnect the fusible link cable from the battery and reconnected the B+ cable, followed by measuring voltage at socket 30 and 12v were present, but changed back to less than 3 volts after I reconnected the fusible link cable to the battery (This is weird, and should be verified). 9) During the time I had the fusible cable disconnected from the battery, I also measure the voltage at F23 but there was nothing there and I figured that since the fusible link cable was disconnected from the battery, it must have some bearing on the no volts measure at F23, even though there was 12v at socket 30 (Now we know that the 30 pin was not in the pump relay).

Note that faulty DME relay operation will shut off the Pump relay.

[Cea123]

Javier,

Thanks for respondng, but please take a look at the reply I sent to "genphreak" at 4:59 on yesterday. I appreciate your commenting ( in red typeface) on the message previous to this reply, but the message I sent to genphreak speaks to being on the right path as opposed to the everything else prior to my 4:59 message to genphreak.

[Javier]

Yes I saw that message thus my first response at 11:12 am. Then I commented in red at 11:52 to your first post with the knowledge of originally mixed relays, because there seems to be some issues at the DME relay (30 pin socked should be 12V no mater what).

Up to know, presuming you Checked Ok the voltage at B+ post, I mean those low voltage readings ( is a low as 1.7 and 3.65) proved to be a reading procedure error, what you seem to have is an issue with the pump relay, if Relay ended being Ok, you may have issues with the DME relay or the DME, as told you, if pump relay is Ok, the debugging takes you to another step. Let's verify first that the B+ post voltage stands solid, and the pump relay is working fine.

[Cea123]

Okay, I'll check the things you've outlined as soon as I get back to the car. I'll keep you posted.

Thanks!

[Javier]

Just noticed that in your car, the B+ post is inside the box besides the pump and DME relays, see drawing 7100.0-01 figure 2 in your schematics. Guess you knew it, but!

[Cea123]

Yes, I knew that but thanks anyhow!

[genphreak]

... when I jumper 30 and 87 the fuel does come up to the rail.

Well it would need >1V to do that. What are your confirmed readings now of the FP relay and at the pump itself?

If you have fuel in the rail and the pump is proven to be working and you should be fine. Always best to test this with a fuel pressure gauge as it is common for circuit testing to be challenging. I've had the FP relay round an unexpected way, I suspect that M50 and/or M60 e34s (or perhaps just RHD ones) are all different to M20 and M30 powered cars. Is yours LHD?

Is the engine starting at all? Another thing you can have is a flooded engine if so, I found once on mine that the only way to start it was >30s hard cranking with the accelerator down wide open (WOT) the whole time, it eventually spluttered adn farted back to life. I suspect on thee that once the flood washes the walls of the chambers free of oil, compression reduces so much that the rich mix cannot combust at all.

[Cea123]

Javier,

Due to the death of an immediate family member, I haven't had an opportunity to resume the troubleshooting of my son's car, but plan to put some time into it this week. The last things that I tested were the fuel and DME relays and they checked out to be good (they click like new). But, I came across some information regarding the crankshaft position sensor (CPS), this info mentioned that the engine wouldn't start if the CPS is bad. But, as I said earlier in the thread that the engine will start momentarily when I spray some fuel directly into the throttle body. I intend to test the CPS just to be certain that its not bad.

On the other hand, just let me add that before the no start problem arose, my son's car overheated and as a result the cylinder head cracked, so my son purchased another cylinder head and had it rebuilt by a local machine shop. My son installed the cylinder and made sure that all TDC markings and VANOS adjustments were correct and manually rotated the engine two full turns to insure all was well the timing (TDC) markings. During this top-end rebuild, my son replaced the fuel pump for generally purpose, so after reassembly the initial attempt to start the engine was momentary and would only start when fuel was sprayed into the throttle body.


If you have anything else in mind that I should test, please let me know.

[Javier]

Sorry about your loss.

Your message brings a new whole story to this issue. Yes there are a lot of things that may be wrong. A wrong CPS will prevent spark and fuel pumping, guess you have spark. What about injectors, are they getting firing signals? Are all sensor properly connected? Have you any Check Engine warning? Have you confirmed already you have fuel pressure in the injectors rail when cranking the engine? First let's be sure the pump is working fine when cranking the engine with all gadgets in place. If not, let's be sure at least that DME is sending the Fuel Pump relay the signal to start the pump. As I have to presume you confirmed there were no pressure in the fuel rail when all this started.

[632 Regal]

Very sorry about your loss.

Momentary firing... This brings flashbacks of the old 850. Same situation I was chasing for half a year. In this case it was the anti theft all along. I assumed I was getting spark because it would "momentarily fire" after spraying it with starting fluid. That is what really messed my thinking up the whole time. I don't have any idea why it would fire at all because the anti theft is supposed to kill the ignition and fuel. There was no reason for the anti theft to go on but it just did and wouldn't shut off so I had to cut one small wire to make things work again.

I would second Javiers idea and make sure you have fuel pressure and second that you positively have spark when cranking.