Fuel pump issues 1985 e23 745i

[Tirantekk]

This is my first time asking for help on here so I really dont know what im doing.
The fuel pump in my 745 was bad and the car had been sitting a while. I replaced the pump and checked the relay and it seemed to be in good working order. When i turn the key the engine turns over but the fuel pump gets little to no power. Metering the fuel pump wire connection shows that it has very low fluctuating voltage but i cant seem to figure out why. There is hardly any info out there on these cars so any help i can get it welcome. This car is very important to me and i just cant give up on it.

Thank you

[bmwdirtracer]

Welcome to the forum. Absolutely, you have a very special old Bimmer. Don't worry too much about there being little available information on this model: this is really only going to come into play when you're dealing with the turbo and related items; for basic info, refer to the 735i.

You mention that you "replaced the pump".... but which pump did you replace? And where exactly are you monitoring the voltage? And exactly what is the "low fluctuating voltage"?

Going entirely from memory, you need to test the green/violet wire from the fuel pump relay, to the two fuel pumps.

Make sure, when you test voltage, that you're using a perfect ground. Also, make sure to test that voltage with the fuel pumps disconnected, one at a time, in case one of them is short circuited.

Remember that ~12.5 volts will only be delivered for ~2 seconds at each "key-on", OR PERHAPS ONLY FOR THE FIRST TWO SECONDS OF ENGINE TURNING.

Here are the diagrams for a 735; this will get you very close, for this issue.:
http://wedophones.com/Manuals/BMW/19...g%20Manual.pdf

[Tirantekk]

I replaced the in tank pump and checked the voltage with a meter by connecting the meter to the plug for the pump and putting the the key in the start position (i had help) the voltage read only 2-4 volts but at a previous time with the same test read nearly 8 if i remember correctly.

Im still learning everything i can. A lot. Thank you.

[bmwdirtracer]

You should check for spark, to see if the computer is even trying to start the car. If you have spark, then we help you find out why the relay's not delivering power to the pumps. If there's no spark, we'll help you figure out why the DME (computer) is unhappy.

[Tirantekk]

There is spark, that was the first thing checked when i got the car.

[bmwdirtracer]

Okay, good job!

Now, you need to check the inputs into the fuel pump relay; terminals 30 and 86. These are a .75 mm red/white wire, and a 1.5 mm red wire, going to the fuel pump relay. The 1.5mm red should be hot at all times (12.5+ volts), the .75 red white should go hot when the ignition is in "on" position, also ~12.5 volts.

[Tirantekk]

Alright, ive checked both and like you said the red was 12.5 the whole time and the red and white went 12.5 with the key in the correct position.

[bmwdirtracer]

Okay, great. Now, let's find out which side of the relay the problem lies.....

Easiest next step is to jumper the terminal 30 of the fuel pump relay female plug to the terminal 87, then see if the car starts., or check for 12+ volts at the green/violet wire at the pump.

Now, keep in mind that I don't know how good you are, so if that description is inadequate, please do not try this. If you accidentally deliver power backwards to the ground from the computer, you'll ruin the computer.

[Tirantekk]

Given the risk I will wait to do that, i have a mechanic friend helping me this wednesday so i will tell him this and he will be able to do it instead. I will get back to you then. Thank you very much for your help thus far.

[bmwdirtracer]

I've taken a closer look at the wiring diagrams. The green and purple wire is the fuel pump hot. The relay feeds this wire, which feeds Fuse number 11, which feeds the fuel pump. Test to see if fuse 11 is hot with 12+ volts on both sides, while the key is turned to start. Remember that this may only last for ~2 seconds, so you'll probably want an extra person. If the fuse gets 12+ volts both sides, but the pump does not, the wiring is bad between fuse box and pump. If the fuse gets hot on only one side, the fuse is bad. If the fuse does not get hot either side, we'll backtrack to the relay, to see if it's getting a ground from the DME.

[Tirantekk]

Ive checked now and the fuse does not get hot either side.
I ran the same test on another fuse to be sure i was doing it correctly and it worked. Now what?

[Tirantekk]

I jumped the fuel pump relay and now the fuel pump runs. There was an issue getting it to the rail so i bypassed the secondary pump temporarily just to get the car going and find other issues. The fuel now makes it to the engine but the car will not start. It sounds the same as it did with no fuel, no sputtering at all. I checked for spark again and didnt get any so i got another plug for cheap but it doesnt fit right. Pushed into place i got one spark from the new plug but nothing more and only once.

[bmwdirtracer]

Okay, now we have a better diagnosis. The computer is not trying to start the engine. Forget all about the fuel pump; return everything there to how it was.

You are now looking for spark.

At the back of the intake manifold area, do you see a bracket of some sort holding two fair-sized, identical rectangular plugs? One end of these wires will go to the bellhousing of the transmission.

These are the engine speed and reference sensors; their plugs are known to crumble, and the sensor go bad, over time. Do you see these two plugs? Do you see any sign of crumbling, or electrical tape holding them together?

(This is just a hunch; there are many other things to check, but this one's an easy start......)

Whatever you do, if and when you see these plugs/ sensors, do not accidentally switch the plugs; they are identical.

[thejlevie]

I agree with Chris, but I suspect there is still a fuel delivery issue. The secondary pump is the high pressure pump and it will be required for the engine to run.

[Tirantekk]

I have found the plugs and there doesnt seem to be any visible damage to them. There is a small ziptie around the neck of one of them but i think that is more for identification as well as, either a "W" or an "E" written on one plug and a "B" on the other. They are not identical because i think one if them must have been replaced as its casing is a darker plastic.

[Tirantekk]

I have found a disconnected plug near the computer. I was poking around to see what was there and i found it. It is a red and white wire and a brown wire in a plug together but i cant seem to find where they would connect to something else

[bmwdirtracer]

Do you have a multimeter to test the resistance of the two crank sensors?

[Tirantekk]

Yes I do have one. I cant say I know how to test the sensor but with quick instruction i could do it. Ive pulled the high pressure pump as well and it doesnt work when i checked for power at the pump previously it had it so i will be replacing the pump shortly. If the sensor is bad where would i find replacements and is it worth replacing them anyways.

[bmwdirtracer]

Let's deal with fuel pump testing first:
If the pump gets OVER 12+ volts power and a good ground, and does not make noise and vibration and pump fuel at that time, the fuel pump is bad.

If there is no power at the green/purple wire at that pump, or there's a bad ground, you cannot test the fuel pump.


To test the crank sensors:

It is very important that you understand that you must never do a resistance / ohms test on a circuit that has power.

Please read that again, and commit it to memory.

Resistance is measured in Ohms. There are also things called KiloOhms, which means one thousand ohms.

The crankshaft sensors have a plastic connector plug: one side of this plug goes to the sensors in the bellhousing, the other side connects to the car's computer.

When you unplug this connection, you must remove the metal spring clip, first. If you just wiggle and pull the two parts of the connector, you'll likely break parts of the sensor's plug. Actually, you'll probably break them, whatever you do, but you have to test the sensors, so you've really GOT to do this.

Anyway, TEST ONE PLUG AT A TIME ! DO NOT unplug both sensors at the same time unless you're very damned sure you've marked all 4 parts, made a picture of your marks, etc.

You are testing the sensors, not the car's wiring harness, so therefore work on the part of the plug that goes to the transmission bellhousing.

Here's where I get unscientific, but trust me.

Each plug has three pins: #1, #2, and #3. Therefore, you really have only 3 possible combinations at each plug: 1 to 2, 2 to 3, and 1 to 3.

Connecting your multimeter to any given 2 of these pins, you'll get a reading of essentially nothing.....or infinity, or some number of ohms. If you find, in your three tests per plug, ~zero or infinite, zero or infinite, and ~1100 ohms, plus or minus ten percent, then the chances are, that sensor is okay. If either sensor is not okay, the engine will not try to start.

[Tirantekk]

Alright here are the results ive gotten and they seem strange but i have checked and im pretty sure i have done it right.

Plug A:
1-3) 268.6kΩ
1-2) 301.8kΩ
2-3) 161.5Ω

Plug B:
1-3) 13.45kΩ
1-2) .447Ω
2-3) .433Ω

[Tirantekk]

I realized i have likely checked the ohms of the wrong part of the plug.
NEW RESULTS

A
1-3: open loop
1-2: open loop
2-3: 987Ω

B
1-3: open loop
1-2: open loop
2-3: 971Ω

[bmwdirtracer]

That's close enough that we can say the resistance values are fine. It does not mean that the computer is getting the info it needs, though. When you are holding the key in start position, and the starter is turning the engine, does the tachometer needle move at all?

Is the computer mounted firmly in its stock location?

[Tirantekk]

No the needle doesnt even flinch and yes the computer was mounted in its original position. I have since removed the pannel so i could check for issues.

[bmwdirtracer]

One of those two crank sensors reads a single tab on the flywheel (for engine position). The other counts the teeth, as they go by (for engine speed) The tab has been known to break off, though it's not common.

Those sensors are common failure items though, even if the resistance reading is good. I am not generally one to throw parts, but my experience with old M30's would make me buy at least one, to try in each of the two positions.

https://www.fcpeuro.com/products/bmw...or-12141708618

How long have you owned the car? Did it just quit running one day?

Check whether your coil has power at the + terminal with the ignition on.

[Tirantekk]

Is there any way you would recommend of removing the crank sensors. I have removed the screw but the sensor still seems very firmly set where it is. It twists but doesnt come out at all. I want to make sure there is no secondary attatchment that will break if i just tug on it. It was my grandfathers BMW he was the first owner. It has been sitting for the last couple years but its still in great condition. When he passed he gave it to me.