1974 Mercedes 450SL

[merz450sl]

Just wanted to check back in after completing the MegaSquirt conversion from D-Jetronic on my 1974 Mercedes 450SL. I could never find much on the web about converting one of these, and most of the threads that I did find seemed to evaporate just as they were nearing the end of the process - couldn't tell if that meant that they weren't successful, of if they were having so much fun driving that they never came back to report the results. I can understand the latter now, the car has been terrific to drive since I got it back on the road at the beginning of August. (I use MegaSquirt for fuel only, not spark)

I bought the MegaSquirt bits early in November 2014 after the D-Jet finally gave up the ghost and I couldn't find anyone to work on it for less than the price of the car. By early January I had finished building the ECU and it was working well according to the JimStim... then I had to wait for weather to hook it up to the engine. First start was on April 12, with the ECU on a table next to the fender. The biggest difficulty was the tach signal from the distributor, which was very erratic. The main thing that improved that was adding a 15V Zener diode between the coil and the line to the ECU; later added pointless electronic ignition to the distributor which also seemed to help and gave a better spark. I was able to use the original Bosch/Mercedes temp sensors for air and coolant, but had to make some metal parts to adapt a Chevy TPS to the Mercedes throttle body. I had bought a wideband O2 sensor with a nice gauge that matched the SL's instruments, but ended up not using it .... for some reason it wasn't communicating well with the ECU and was messing up the tuning instead of improving it, so the tuning guys ended up tuning the engine as open-loop and not hooking up the O2 sensor. The car runs great this way, so I could have saved that $250 and been happy without the sensor.

It took a couple of months (between rainstorms) to route the wiring harness through the car, mount the ECU, relay and fuse block to the passenger side kick panel. With my first-guess settings on TunerStudio, the car ran well enough to drive it up and down my street once or twice but I wouldn't want to take it out on the road. We happen to have a shop in town (SR Performance) that is not only experienced with MegaSquirt but also ex-Mercedes master mechanics (and also has a dyno), so I trailered the car over to them for the final tune. In addition to the tuning, they installed the O2 sensor (didn't know yet that I wasn't going to use it) and the pointless distributor kit.

The car now starts the first time a plug fires, and runs at least as well as it ever did on its factory system. Gas mileage in town is not much different from before - about 9 mpg - I never get it out onto the highway enough to check mileage there (these topped out at about 14.5 highway when new). Being open loop, they set it a little on the rich side to favor reliability over economy, and I might be able to tweak it a little leaner ... but I'm not doing that just yet, the car runs too well to want to mess with it until I've just enjoyed driving it for a while longer.

If anybody is trying to do one of these, I'll be happy to share my experiences - my email is rick.oleson@gmail.com. I still probably don't understand more than half of what I've done, but it was enough to get it working. The illustrated version of the story is at https://www.flickr.com/photos/26262745@ ... 0166253696 , and some day I hope to post a more detailed account on my website for those who come after.

rick : )=

[toehead]

Congrats, and beautiful car!

Those D-jetronic systems are very interesting. Not nearly as wild as the K-Jetronics though

[Carlg]

lovely looking car