In my quest to smooth out my idle issue, I wanted to remove and check the DME and VNC units.
The DME (Digital Motor Electronics or ECU in normal cars) and VNC (Vanos control unit) are both stacked in a little compartment at the back of the engine bay, under the cowling. There are two things that these units are known for; A, getting waterlogged in that compartment and B, cracking solder joints.
First, the coolant tank needs to be moved. This is clipped in at the back, and hooked into a tab at the front. The rear of the tank will just lift upwards out of its clip and then side toward the rear of the car to disengage the front tab.
The RH side of the panel is hooked into a little tab. You need to pull the LH side towards the front of the car, whilst holding the loom against the firewall (to unhook it from plastic panel), and then slide the panel to the left.
Then you have a big gaping hole with wires and control units in it. This is the point where you can tell if it’s been full of water in there or not. Mine, thankfully, looked pretty dry. A bit dusty if anything.
On the bottom, with the big connector, is the DME. Up top is the VNC, with a smaller connector.
I found removing the DME connector from the DME before sliding the unit forward, was a lot easier than trying to remove the DME first, as there is limited space due to the fuse box.
To remove the DME connector, the silver metal locking tab needs to be lifted up, away from the DME. This will allow you to tilt the wire end of the connector away from the DME. The other end of the connector is hooked into the DME and to disengage it you need to tilt the connector away from the DME until it slides out.
With the DME out and on the bench, I needed to disassemble it to inspect the solder joints. Normally this requires bending tabs on the bottom of the unit, but my DME has obviously been open before, and these tabs are missing.
Now, the DME3.3 used with the S50B30 is a two board setup joined by a ribbon cable. This is fairly straightforward, only complicated slightly by the fact that one layer of the boards has its own pins in the connector, and needs some specific conditions to remove the pins. The pins are the top layer
In the bottom corner of the boards there are two little plastic retaining clips on each corner of the board. These pull apart to release the boards, and then the boards must be carefully prised apart.
You need to separate them at least as far as the above photo, if not slightly further. Just don’t damage the flex cable between the two.
With the two boards apart, you need to carefully pry the layer of pins out of the connector. There are a couple of clips on the back of the connector that need to be undone (pre-broken off on my DME), and then using a little flat blade screwdriver, wedge it between the top of the plastic around the pins, and the lip just above them on the casing. If you have enough angle on the boards, this should allow the pins to pop out backwards
This is also the same procedure required to chip the DME, which I figure is why my DME has been open before. There were obvious witness marks on the tune chip socket. The chip that would be replaced is circled below
Anyway, that’s not what I’m here for (for now). What I wanted to check for, were cracked solder joints. Anyone who is a long time reader of mine will likely remember I fixed Nicks Vitesse by fixing solder joints in his ECU. This is the same thing I’m looking for here.
Reassembly of the DME is just the reverse of disassembly. Hook in the pins, press the boards together and then reinstall the casing and screws.
Next was to remove the VNC. This is a weirder mount; you need to slide the VNC towards the rear of the car, and then down, to get it out. The connector has a tab that slides across to unlock and remove it.
And as a reminder of how special these early M3s were, all the original VNC chips have a handwritten sticker on them. This is also the chip that gets replaced when you chip the car (both the DME and VNC need to be chipped).
I completely removed the board from the housing, but found nothing out of the ordinary here, so reinstalled and reassembled it. Refitting the VNC to its mount is a pain. It needs to slide backward and up, and then forward to lock it in.
The connectors and cover then go back on, and the battery can be reconnected.
The result of this work was… nothing. No change at all. Still runs and drives the same, but at least I know it should be more reliable in the future. I now also know how to remove the DME and VNC to chip it if those “Group N” chips on eBay tempt me too much.