Well I never thought I would be looking into this ECU but I didn't have a choice! The Motronic system seemed superb and was digital! However an old 12v Senator (see Irmscher page!) dropped in to my lap, and a rather special one at that...it wasn't running very well.  The system on the car was LE-Jetronic which is analogue, no micro-processor and so no fuel maps to play with! The ECU had been modified by Irmscher to run the increased capacity engine (they kindly place a little Irmscher sticker on the outside to tell you :-)

In fact the system comprises of three control units linked to each other. There is a fuel (injection) ECU in the usual place (drivers kick panel) and is the LE-Jetronic brain, a separate idle control ECU (not shown) and a separate ignition control ECU (not shown) are under the bonnet. There is no lambda sensor on this system, the engine code is 3xNE and not C3xNE so has no catalyst converters either. As far as I am aware all the 12v catalyst converter cars, and hence ones which have a lambda sensor (C3xNE and C3xNE engine codes) use the Motronic M1.5 system and not the LE-Jetronic system. 

There is hardly no information on the old L-Jetronic system available on the web. A couple of books cover the subject but not in great depth.

From what I can gather the L-Jetronic system first appeared in the 70's a the successor to the D-Jetronic. The L-Jetronic uses a air-flow meter whereas the D-Jetronic uses a MAP sensor. Both are analogue (no microprocessor). The LE-Jetronic system is an later and enhanced version for the European market (hence the E), it is mirrored by the LU-Jetronic for the USA. LE-Jetronic sometimes has a lambda closed loop circuit. I think this is a standard feature on the LU and optional on the LE.

The problem with my Irmscher was that the engine ran very rich all the time. In fact, so rich, it would splutter and try and die at idle and belch out black smoke under throttle. I checked all the connectors first. Measuring the coolant temperature sensor revealed nothing strange, it's a 2 pin device but actual contains two standard NTC (negative thermal coefficient) thermistor with a common ground though the body of the sensor. One thermistor serves the LE-Jetronic the other serves the idle control ECU. This means that when checking it resistance with a DVM. one probe should be connected to a good "engine" ground the other to each pin (different to the Motronic ones which are measured straight across the pins).

Looking at the 25pin connector at the ECU I noticed that there was quite a bit of corrosion. The windscreen had been leaking and allowing moisture to gather around the ECU. This was cleaned out using RS solvent cleaner (safe with these plastic connectors as it's basically iso-alcohol based) and a spray of contact cleaner. The same treatment was given to the wiring harness connector. The result: Still running extremely rich.

All the injectors were removed and sent to HGL Motors for a re-furbish, and flow test. They came back with a clean bill of health and some new pintle caps fitted.

The Air-flow meter is a flap type similar to that used on the later Motronic system. The unit contains a air-temperature thermistor but it is not connected up in this Jetronic system. There is also no "5th" injector or warm up injector to worry about (an option on L-Jetronic).

I was lucky enough to own a spare ECU so I swapped this over. The result, smooth idle, and a much better mixture, although probably a little on the lean side as the ECU was from a 3.0l engine and this was the 3.6l Irmscher engine. I wouldn't mind if this system had a lambda sensor as it would at least compensate, but the ECU has no way of knowing it's mixture setting!

Time to open the ECUs and see what Irmscher had been up to in making there "special" ECU.

From what I can see the only two changes are the addition of a resistor (just visible on the underside PCB shot above) and a replaced resistor (R352) shown below.

This shot also shows one of the 5 "calibration" resistors mounted on stand-offs. It doesn't look like Irmscher has changed these, the solder looks "first run". From reading the pages by "The Finn who disappeared" it would appear that these resistors probably set things like rev-limit, fuel cut-off speed and temperature related enrichment. From looking at the resistors there seems to be coloured paint applied to the wire ends. My guess is that when these ECUs were being manufactured by Bosch they would go on a diagnostic rig and the operator would select resistors for each ECU to match the application or component tolerances.

Anyway, the "Irmscher" resistors were added to the spare ECU (75kOhm replaced R352, and a 22kOhm is bridged under the module) and the engine now runs superbly with a smooth, purring idle and no black smoke due to overfueling!

LINKS

http://users.exis.net/~audserv/Injection.html

L-Jetronic page