Wiring at Firewall

Specific conversations and info for the BMW E28 M5 and M535i.
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Syndrome
Posts: 128
Joined: Nov 23, 2021 3:46 PM
Location: DC

Wiring at Firewall

Post by Syndrome »

Hi all. The car is a 1988 M5. There is a metal bracket on the upper-center of the firewall that supports two plastic connectors. The top one has a black plastic connector, and the bottom on a white plastic connector. Both of the male ends of the connectors have crumbled. I know I can replace the female ends with new BMW parts (and they are pretty cheap). These are the ones with the spring that you push down to release the male end. Unfortunately, the male end is only sold with the wires they go to.

My understanding is that the top (black) plug goes to the crankshaft position (speed/reference) sensor, which is part 12141708618. I think that number was superseded by part 12141730773, but it doesn’t really matter. My question is, what does the white (the lower one) go to? I can’t seem to figure this out using realoem. BMW wasn’t much help either. Can you help me with a name and/or part number for the cable/wire I need? Thanks!
Fanclutchnut
Posts: 113
Joined: Jan 05, 2021 7:32 PM
Location: Westchester Co NY

Re: Wiring at Firewall

Post by Fanclutchnut »

It goes to the same spot on the bellhousing but are reading different references on the flywheel. One references crank speed, the other crank position. The sensors are the same and so are the plugs. However, they must be plugged into the correct harness spot on the firewall for the car to run properly. I will go through my docs and see if I can find out any additional info.

Incidentally, our plugs are crumbled and we don’t t have the firewall bracket. Anyone have a line on a spare?
Fanclutchnut
Posts: 113
Joined: Jan 05, 2021 7:32 PM
Location: Westchester Co NY

Re: Wiring at Firewall

Post by Fanclutchnut »

Here ya go, not S38 specific but I believe they apply

Step 8, Speed Sensor and Reference Sensor!

So, you are cruising along on interstate 10 in your Motronic Basic 528e, 533i, or 535i secure in the knowledge that Robert Bosch improved the Motronic system so that a coolant temp sensor won't leave you stranded like it did with the old L-Jetronic system in your E12 528i. Now is when you find out he added two new sensors that will leave your stranded. *Failure of either of these will stop your car dead, right NOW! Well, that's a little bit of an exaggeration, but not much. Please read on.

The Motronic Basic 528e, 533i, or 535i all have two sensors mounted on the bell housing. The first is called the speed sensor. As you may have guessed it reports engine RPM to the Motronic ECU. Failure of this sensor may or may not totally disable your car. It's supposed to limp along poorly constantly missing if this sensor fails. Reality may be different.

The reference sensor is more critical. Without it the car will not run.

These are both mounted on the driver's side of the bellhousing. The speed sensor is mounted above the reference sensor. Caution: the two sensors are actually identical, so it's possible to connect them incorrectly. If the speed sensor is plugged into the connector for the reference sensor, and vice versa, the car won't run. The speed sensor mounts above the reference sensor and connects to the BLACK plug. The reference sensor connects to the GREY plug.

Both of these plugs have the Bosch style connectors on them with the dreaded metal clip. It's probable that the connectors will have been damaged at some point in the cars life. Mechanics love to test these sensors, but they really don't like messing with the metal clips so they tend to force and break things. If the connector is damaged DO NOT try to cut off the connector and solder on a new one. You won't be able to do it. You will have to find another way to secure the connector (tape, zip ties, etc.), or replace the entire sensor.

The Speed and Reference sensors connect to the Motronic wiring harness at the crossover bracket. Watch out, these two connectors can be reversed so make sure the upper sensor connects to the black connector.



Thankfully it's very easy to test these sensors. Once again, get out your Ohm meter. Remove one of the connectors. Note that this time there are three electrical terminals present. That means three tests per sensor. The terminals are numbered 1, 2, and 3 from left to right as viewed from the front of the car. If your car's crossover bracket is missing orient the sensor so that the single tab is up, and the two tabs are down. Now the terminal on the left is number 1.

Measure resistance between terminals 1 and 2. You should have 864-1056 ohms. Between terminals 1 and 3 you should have 100,000 ohms minimum. Between 2 and 3, again, 100,000 ohms minimum. Now Bosch wants you to take this reading with the outside air temperature of 77F (25C). I find it doesn't make much difference, but if it's really hot or cold out and the sensor is a little out of spec, I probably wouldn't worry about it.

Now test the other sensor. The values we are looking for are the same because these two sensors are EXACTLY the same part number.

In the picture below I am testing the reference sensor between terminals 1 and 2. The sensor passes!

It's a really good idea to carry a spare sensor with you if you are taking a long trip. Since they are both the same you really only need to keep one with your spare fuses and relays.

Sharp observers will notice a position sensor mounted on the front of the engine of a Motronic Basic 528e, 533i, or 535i. This sensor has NOTHING to do with the running of the engine. It's not linked to the Motronic system in any way. It's used to hook up some device the dealers have.

If you have a 528e with Motronic 1.1, your car does not have the two reference sensors on the bellhousing. Instead it has a "pulse sensor" mounted at the front of the engine. Test the resistance of this sensor and look for an ohm reading from 486-594. You also need to verify that the tip of the sensor is within .7mm-1.3mm from the toothed wheel.
Machtig
Posts: 267
Joined: May 14, 2009 10:44 AM
Location: Kingsland, GA

Re: Wiring at Firewall

Post by Machtig »

So the wiring to the sensors and the crossover bracket plugs looks different than most other wiring on the car. Looking at the electrical manual, these engine speed and reference sensors have a yellow, black, and "shield" wire and the entire length of wire except an inch or so at the ends is encased in a hard rubber? sleeve. My question for anyone that knows is to install the new male connector I got (mine also crumbled), can I cut the rubber sleeve back about an inch? Does the shield wire connect to the sleeve? The new male connector I got is a little longer than the original one and I need to free up about an inch of the wire so they'll slide in.

Image
Machtig
Posts: 267
Joined: May 14, 2009 10:44 AM
Location: Kingsland, GA

Re: Wiring at Firewall

Post by Machtig »

I think the correct answer is replace the entire sensors as they come with the wire and new male connectors. Since I have new connectors I'm going to try to make them work and if unsuccessful I'll just replace the sensors.
///M
Posts: 74
Joined: Apr 17, 2009 11:21 PM

Re: Wiring at Firewall

Post by ///M »

The shield wire connector, the silver one in the picture is connected to the braid that covers the black and yellow wires. There is a foil covering the black and yellow wires and that is wrapped with a braid. The purpose of the braid and foil is provide a shield for the signals. When there is a joint like in your photo the braid is wound into a wire and connected to the crimped pin.
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