E24 chassis Motronic 1.3 upgrade notes

Update: this problem is resolved. TL;DR version: the E24 chassis harness has two start signal wires for the DME. The E34 engine harness only needs one; they’re spliced together internally.


I’ve been struggling with a few little details on my hybrid E24 project. Basically, it’s not starting. Cranks all day, but not even a hint of fire. So what’s the deal with the car not starting? I’ve tried everything, can’t get power from the main relay!!!!

In situations like this, I prefer to run through all my actions again, basically starting from scratch. Just in case you’ve run into a similar situation, here’s the nitty-gritty details of this project:

  • The chassis was built 2/86, originally had an M30B34 with the Motronic 1.0. Those cars had the old-style square C101 (fusebox plug).
  • Many drivetrain parts came from an ’81 Euro 635CSi donor. I also borrowed that car’s C101 fusebox connector and pigtail. Believe it or not, the Euro plug has a few connectors missing (dynamic oil pressure) and a few others are moved around (static oil pressure), compared to the NA (US) market version.
  • The ’81 gearbox & 12# flywheel are in this car too, running Motronic 1.0 is still an option.
  • The ‘newer’ engine is an M30B32 from an ’82 NA 633CSi, with the old-style everything, LOL.
  • The engine wiring harness is from a late model ’89 NA 635CSi. This is an upgrade from the previous Motronic 1.3 harness, salvaged from an early E34 535i.

I’m assuming that the problem is definitely with the new harness, but it never hurts to run a few quick tests. My early troubleshooting steps didn’t turn up anything: fuses all OK; battery tested 12.3VDC; primary relays tested out OK. The DME, AFM and ignition components all ran great with the last engine, it’s unlikely they’re the problem. The major sensors (CTS, AFM, CAS) all tested out OK; although the crank sensor resistance was a little low at 450 ohms. I’ll replace that crank sensor later, but it’s unlikely to cause a no-start.

Next, I jumped both primary relay sockets to test the power segment of the harness. The main relay circuit immediately powered up the ICV and other ancillaries, so that’s OK. The fuel pump circuit powered the pumps right up, I could hear fuel gurgling & purging air out of the fuel rail. Since those systems are working, I can eliminate most of the auxiliary systems. That leaves the DME, wiring harness, and start circuit. Two likely alternatives: the DME either isn’t getting a start signal, or isn’t powering up the car properly.

So…off to the engine wiring harness we go. Electrical troubleshooting really isn’t fun, but it is critical for projects like this one.

The E24 harness definitely stands out as the problem, but I didn’t want to give up on it. The E24 is a little different from later M30 cars. Unlike the E32 7-series & E34 5-series, most E24 had the battery up front. E24 versions of the 1.3 harness have the power segment of the harness connecting to the battery directly, up at the fuse box. Later versions of the 1.3 harness run into a tidy little E-Box at the passenger strut housing. Thus, using later E32/E34 harnesses in the E24 require a thick +12VDC wire snaking all the way back into the glove box and some way of mounting the relay block inside the car. The E24 harness also uses different bulkhead plugs, so it fits into the cowling perfectly, no need to cut and glue old connectors onto a different harness.

I finally gave in to inevitability and decided to completely re-wire the C101 fuse box pigtail from scratch. I’d use the chassis section of the E24 B35 engine wiring harness instead of the E34 version (fuse box square C101 –> new round C101). When I compared the two parts, differences were immediately evident! Both C101 chassis side connectors are physically the same, but the E24 version had one more terminal installed, it’s completely populated. So off to Google I went for more basic research.

I found a critical detail about my exact situation in this specific post; part of the invaluable & magnificent B35 swap thread at My28.com:

As an addendum to those using the e24 B35 harness, who also are more inclined to use this chart by color and not pin number, there are a few differences between the e34 harness wire colors and e24 harness.
#2 = BU/VI (same as e28)
#7 = large GN (same as e28)
#8 = small GN *This needs to be spliced into #7 above or you get no + to represent “key on” to the ECU, meaning no start.
#10 = BU/WT (same as e28)
And some of the unused wiring is different than the chart, but aren’t necessary to note.
Since a lot of people are more visual and rely on colors in a schematic, it’s good to know these differences.

So….while the E24 chassis harness is more or less the same as the E34…there is one critical difference with that #8 wire. I replaced the E34 pigtail with the E24 version more or less in the exact same way, except I spliced small green #7 & large green #8 wires together. Feast your eyes on a solid half-hour’s work:

I popped this new harness into the car, and IT FIRED RIGHT UP!!! I’m a happy fella right now! I’ll shorten & properly splice all the wires later, to finish it up and get it off my to-do list.

Quick note: I do not recommend those inline crimp-type splices for permanent use, especially in areas exposed to water intrusion like the engine bay. Leaving this jury-rigged wiring in place is asking for trouble, especially on a mission critical part like the primary engine wiring harness. These types of splices are at best a temporary fix. They have their place though: easy & quick changes to the harness, moderate water resistance, and relatively high mechanical strength. In this case they are far better than wiring nuts, vampire taps, etc.. Best used only for testing. They have many problems: nearly impossible to crimp properly, sizing is difficult, wire tends to flex and fatigue, especially smaller-gauges, and both ends are open to the elements even if you’ve crimped the plastic ends properly closed. Electrical problems are maddening enough to track down…even with high-quality OEM connections, why make your life more difficult down the road? More importantly, put yourself in the next owner’s shoes…it’s no fun to track down problems with someone else’s shoddy work.

The recommended method is an inline braid, soldered, with heat shrink tubing to protect the joint. I’ll also stagger these splices to keep the harness from having a large bulge, then finally wrap it up with some nice 15mm or 1/2″ fabric friction tape.