W124 OM 603 engine swap questions.

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Ron . B.

Active Member
Oct 18, 2007
I have been asked to put a OM 603 Diesel(N/A) into a W123 300 TD . So far it's been straight forward but i have struck a strange problem.I cant find the lead or position for a temp gauge connection. The 'new' engine is a 1989 Australian version ,no emissions or EGR parts ,with the electronic idle control .The cars owner dropped the engine into mine along with the remains of the engine loom.
I have modified most of the original loom to connect to the 6 cylinder glow plug relay etc. But although there are three coolant transponders, there doesn't appear to be one for the temp gauge in the Dash cluster. In the left front of the block left side is a single pole temp sensor ,with a green/blue wire which I would normally say was the gauge connection but it goes to a ELR relay (N8 or R18/1 in wiring schematics) . to Pin #8 marked TF.
Theres no rev counter or aircon cut out relay.
Also on the left side is a two pole coolant transducer with a white and a brown pair of wires which are connected to the glow plug relay.
On the front of the engine by the top hose is another twin pole transducer with a Violet/grey wire and a brown wire. The transducers are just temp switches and wont work the guage.

Is the dash temp gauge connected to the ELR module ? which pins?
Despite several hours looking on line,there doesn't appear to be any schematics which match the Australian W124 Diesels or the wiring layout hence me asking here where perhaps the UK got a similar version before 1989.
The single pole sensor you found is the one your looking for, IIRC there is another port for a sensor further forward of that one which you can put the w123 sensor into.
I found out what is going on .There are only three temp switches on this version, a one year only engine and only sold in Australia.
The temp sensor on the top rad hose location was a temp sensor for the air con module,controlling the Aux.Fans and preventing engine overheating in our summers. And, because this car has the original "manual" type air con it wasn't needed.I simply added a normal MB temp gauge sensor and have run a new lead to the gauge .Once I get this thing up and running I'll see if its accurate.:)

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