Speed input to ECU, halving tach signal.

[Italian_Tuneup]

Hello.

I'm having a hard time wrapping my brain around this one. I understand the 404 DME needs to get a speed pulse signal for proper operation. From all the wiring diagrams I have seen, most swaps pull this signal from the cluster to feed the ECU. I understand later cars likely did this to their Motronic units as well, but as I have found, my 12/84 build date car is Early-Early and has many traits not common to the early and late cars usually discussed in the swaps. I am not finding a speedo signal leaving the cluster. Is it possible to split the lead from the reed switch in the diff? This seems it would cause voltage issues and piss off the cluster or the ECU. Or maybe I am missing something totally here. If anyone else has dealt with a really early E30 in this regard, let me know.

Also, with the tach signal... Garagistic quit making their module. I have seen reference to using a Dakota Digital signal converter, but the Dakota Digital website specifically says it won't work on "most" european cars built after the 70's... so will it work?

[Gregs///M]

The signal coming from the speed sender in the diff is considered pulsed current signal. The signal coming out of the speedometer, is slightly modified "conditioned" signal so I recommend using that. Your early model cluster does have speedometer output, from the green plug at the rear of the speedo. This is the same plug that feeds the cruise control and convertible top. That is where I would tag the speed output signal.

Im sure there are other companies that make tachometer interfaces besides Garagistic and DD. The speed signal is pretty basic, simple, almost square wave which is commonly used on many cars. I'm sure most modules that are designed to half the signal will work with the standard signal coming out of the speedo.

[JGood]

I've been using the Dakota Digital converter on my m6x swapped 10/84 325e for the past 8 years/45k miles. Hooked up to the green connector on the back of the cluster. Installed it in 5 min, set it up, haven't touched it since. It's accurate from idle to redline. I never understood why anyone ever messed with that sloppy looking little circuit board thing that still required a 4 cyl coding plug anyway.

[Italian_Tuneup]

I've been using the Dakota Digital converter on my m6x swapped 10/84 325e for the past 8 years/45k miles. Hooked up to the green connector on the back of the cluster. Installed it in 5 min, set it up, haven't touched it since. It's accurate from idle to redline. I never understood why anyone ever messed with that sloppy looking little circuit board thing that still required a 4 cyl coding plug anyway.

Good to know, I will pick one up. Thanks for the info everyone. This all just reminded me too that I still need to send my automatic 404 DME in to get reprogrammed too. Phwew!

[JGood]

This all just reminded me too that I still need to send my automatic 404 DME in to get reprogrammed too. Phwew!

The 404 DME has a removable chip, the DME itself doesn't get programmed. You just buy a chip and swap it out. That's only if you desire though, it's not really necessary. Many people with m60 swaps are running an auto chip with manual transmissions, because there were so few manual 540i's that had 404 DME's.

[Italian_Tuneup]

The 404 DME has a removable chip, the DME itself doesn't get programmed. You just buy a chip and swap it out. That's only if you desire though, it's not really necessary. Many people with m60 swaps are running an auto chip with manual transmissions, because there were so few manual 540i's that had 404 DME's.

I've read some old posts where folks running automatic DME's had issues with revs hanging during shifts, etc. That's massively annoying. THe 404 I picked up is out of a E34 530i manual, which I believe still needs a different chip anyhow.

[Italian_Tuneup]

Back to these speed and tach signals... I am trying to find what line feeds the tach signal into the cluster. I can't find a pinout anywhere for any of the IP plugs. Any guidance on that. Also, back to the speed input for the ECM, I am deleting my OBC and replacing it with a clock. I noticed in the ETM that the OBC receives a speed signal. Can I divert that off the old harness going to the OBC to feed my DME input?

[Gregs///M]

The speed and tach signals are in the ETM. Speed is brown/red wire in the white plug and tach is black wire in the blue plug. If you are deleting the OBC, then yes you can reroute the speed signal input to the DME.

[Italian_Tuneup]

The speed and tach signals are in the ETM. Speed is brown/red wire in the white plug and tach is black wire in the blue plug. If you are deleting the OBC, then yes you can reroute the speed signal input to the DME.

Thanks! I appreciate it, hopefully this is my last wiring related question. Hope to test fire this weekend!