Tapping into EDS harness on 1993 300D to control turbo
 

Hi everyone! Just bought a 300D a couple weeks ago and have been going through it while it sits on blocks waiting for the driveshaft support bearing to arrive from the USA.


As we all know the EGR has to go, the manifold is filthy. I want to try something different from the standard actuator swap though.


I do electronic controls and have been measuring the signals that the EDS manages, and everything is there to control the turbo electronically without needing to override it with a mechanical actuator. All I need is a microprocessor (Thinking ESP32 with BLE for readout) and a couple MOSFETS. Benefits should include:


- no need to pull the turbo or order specific parts
- EDS can supply clean power, idle control, tachometer

- easy adjustment and measurement of boost
- can run boost curve instead of fixed pressure for lower EGT
- boost can be set to track MAP instead of gauge pressure for high altitude
- air flapper can be used for overboost or runaway safety
- optional output for water/propane injection that can track boost pressure


So the question - because the plug appears to be stuck pretty good in the stupid EDS module and I don't want to trash it in the cold :uneasy:


The manual shows there is a jumper available to tap into this cable for troubleshooting. Are the plug ends available somewhere to make something myself, or can the factory plug end be disassembled to access the pins from the back?


I would like to do something aesthetically pleasing rather than run a razor knife down the harness cable and start splicing and taping.

Interesting the EDS module just controls idle speed and emissions. Unplugging the connector once you know how to release the squeeze clip lock is actually quite easy. If it were me I'd be tempted to ditch the innards and replace it with your computer of choice. I may even have a spare EDS so you can swap it in and out till you get it right.

I figured there was a trick to that connector, but I didn't want to snap something. Couldn't find any info on it online.

I never considered just putting my own computer inside the box, I like that idea. I guess I assumed it couldn't be opened in a non-destructive way. I'll have to get it indoors on the bench and see how to disassemble it. Worst case I think with it out of the car, there's just no boost and no idle-up on startup. Maybe no tach?

I drove the car home with no boost (vacuum lines had snapped) and was keeping up at the speed limit so that wouldn't be a big deal while working on it.

I thought that the one valuable function it has left is translating the ring gear signal to the tach signal for the dash. Not sure what that sensor is, if it's just 5v Hall effect, or if it's a coil type that requires amplification. Definitely I could manage idle speed control myself as well.

Yes it converts the ring gear counter to the tach also, oh and also handles over boost cut out.

How are you supposed to release that connector? The module is kind of stuck down there by the hinge, awkward to examine it closely.

Can it still handle overboost if the EGR has been deleted? Or does it give up entirely on the whole turbo system? Lots of people saying that when you do the actuator swap you lose overboost cutoff, since there is no overboost valve on the ALDA on these cars.

At least overboost is easy for me to handle, just cut the power to the changeover valve pair and the vacuum source for the wastegate will slam the "pressure control flap" shut. When I was doing some testing in neutral, that appears to be exactly what it does when boost exceeds the setpoint (appears to be about 5 psi under no load)

Under the wire harness at the end of the connector is a release tab just push that and lift up on the harness/ connector. The other end of the connector will disconnect when the lift high enough on the other end.

The EDS opens the egr valve and monitors MAF expecting a reduced volume of air. If a reduced volume is indeed detected boost is allowed. A diode can be used to fake the MAF reduction.

Subscribed. I haven’t changed my 1991 om602 to a pressure actuated wastegate. This seems like a good idea if it can do telemetry and control.

This one is getting back-burnered for awhile as my driveline parts, glow plugs and such have arrived, especially as the EDS is currently working with the EGR installed. Going to get the car back on its wheels and driving, first.

However I'll update when I make some progress.

Dieselbenz, that was the other simpler idea I considered, using the EGR signal to pull down MAF with a diode/resistor etc. As MAF is the only way for it to know the EGR did anything. I didn't find any records of it, so figured it would be easier to re-engineer from scratch than to fool around with different values in the hopes that it would work. Is this a documented fix?

I have used this on several different models including the 602.

Perfect, thanks! Surprised people are swapping actuators when there's a simple fix like this to keep the factory equipment.

I think I'm still going to build the controller though because it'll be nice to not have to trust a black box system. Your resistor fix though, will let me put sensing on the I/O and try to extract a factory boost map while I'm driving (unless you have one of those in your back pocket too :D).

I'm curious to see how the flap and EGR interact. There seems to be a common myth that the flap is used to draw vacuum for EGR... but the block diagrams don't support that. The flap is part of the P2 control loop, and the EGR valve loop is independent. And as you say an error causes a shutdown of the entire system.

As the flap and wastegate are either/or, there's no way to engage the flap while spooling the turbo. So I can't see it being used as a part of the EGR without killing boost pressure entirely - it must be for limiting overboost only.

Did you wire up a harness to do this? If so, how did you get the OE pins and connectors? From a donor car? Or did you just solder resistors in series?

I had cut the MAF wire closer to the MAF on a 602 but on the 648 it's done by the SAM/ fuse box.

Progress! The connector is indeed quite easy to remove if you just pull the tab the right direction.

I took the module indoors and popped it open on the bench. It's not sealed at all, just 4 T10 screws at the corners will open it up.

Good:
- Very easy to implement Dieselbenz's fix internally!
- Easy to grab regulated 5v power
- Easy to cut traces to take over outputs
- Lots of room for an ESP32 on the top side of the board

Bad:
- No discrete output transistors to easily drive by cutting traces. They look to be on the covered Bosch daughterboard. The low voltage gate drive signals are probably available between the CPU and daughterboard though, making a full trace and reimplementation a summer project unless I wanted to build a more complex PCB.
- Windowed EPROM, despite the sticker, I wouldn't leave this board outside of the box for too long if you want it to keep working.

I would have implemented Dieselbenz's fix this evening and unplugged the EGR valve except for one issue... I promised a friend I would pick her up for a ride in my Mercedes tomorrow :D

I also have to confirm pin numbers with a meter.
- EGR output has to be 18
- MAF, the 602's sensor does not have 5 pins, and it's not documented on the drawing which resistor is the thermistor and which is the vane. I am suspecting pin 22 from the drawing, but will have to check with a meter.

I'm not sure if I want to do that 12v pullup or just put it to 5v, as the MAF input appears to be limited to 0-5v. However the diode will only conduct when the EGR output pulls it down, making the actual pullup voltage fairly irrelevant as long as it's higher than the MAF output. Either 5v or 12v should work.

Was 12v just used for convenience, or was it needed because the ECU checks the valve is floating up to 12v in between pulses or something?

Was 12v just used for convenience, or was it needed because the ECU checks the valve is floating up to 12v in between pulses or something?

Likely would work with nothing as a pull up but I don't like things to just float. I had a pin out for the connector at one point but it went with an engine I had sold.

Wow good info.