R129/M119 Engine Wiring

[JCW1]

And now a question if anyone can help.

In the atached picture of my original loom, the two connectors that can be seen, does anyone know what do the sensors that they are atached to do?

The cylindrical plug attaches to a sender with 4 pins and the rectangular connector to a 2 pin sender. The reason for asking is that the plugs are molded to orientate the connection in one position but the senders don't have a matching profile so can be connected in any position (I hope that makes sense!)

Does it matter which way they are connected?

 

[Doodle]

The large one is your water temp sensor. Pretty sure it's handed (operating on hazy memory of changing mine), I certainly recall the pins and sockets being numbered on both sensor and connector.

I'll check mine tonight if need be.

 

[JCW1]

Good point I'll get my magnifying glass out.

Thanks Doodle.

 

[Druk]

Possibly the smaller one is the fan switch if it fits into the water jacket beside the larger one. I thought the larger four pin had a locating nib so it couldn't be connected wrongly?

 

[JCW1]

Possibly the smaller one is the fan switch if it fits into the water jacket beside the larger one. I thought the larger four pin had a locating nib so it couldn't be connected wrongly?

The plug on the end of the loom has a cut out so that it could only be connected one way (also he pins are numbered. Unfortunately, the socket on the end of the Sender does not have a matching internal shape and the pins are not numbered.

Maybe I have a non-genuine part.

I suppose the question I have, is whether I can start the engine with this disconnected or if it will do any damage if it is connected incorrectly?

 

[Doodle]

The ECU relies (in part at least) on the engine temperature reading to tell it when to come out of cold enrichment. Without it connected you'll be pouring fuel down the cylinders, same as when the sender fails.

 

[JCW1]

The ECU relies (in part at least) on the engine temperature reading to tell it when to come out of cold enrichment. Without it connected you'll be pouring fuel down the cylinders, same as when the sender fails.

So just out of interest, if the plug wasn't connected (or connected incorrectly) would the enrichment be noticeable when driving? Like back in the day, driving with the choke out.

The reason I ask is that I took the plunge and connected the plug thinking that I would be able to tell if it was wrong.

The engine fired up first time and after about 30 seconds, settled down to a smooth idle.

I have since made 4 trips totalling 100 miles and have to say I can't notice any running issues. The car pulls cleanly when warmed up and the exhaust doesn't smell rich. I guess the way to confirm would be to get it on an EGA and check the readings but maybe I have just been lucky.

The most important thing from my viewpoint is that 'limp mode' seems to have been consigned to history:bannana:

 

[Doodle]

Unfortunately the M119 is pretty good at hiding the symptoms - even with an AFR as poor as 11.5 there was nothing outwardly amiss. The giveaway was when it guzzled a quarter of a tank in 60 miles.

EGA is the only way to be sure.

 

[JCW1]

Just to close this out, for time pressure reasons, I wasn't able to take the car to get the emissions checked so I decided not to drive it, for fear of doing more damage due to overfueling.
However the car needed a new MOT so I took it in last week. Emissions were well inside the limits with CO at 0.02% and HC at 8ppm, both at idle.

Looks like I got lucky with the sensor plug position so I have now marked it for future reference.

 

[RichardF]

Thanks for the useful post and write-up. It'll give others the confidence to sort out there looms, good to hear of reliable and helpful suppliers.

 

[JCW1]

Thanks. It's the best thing about Forums, in my view, Mutual help and support.

Now to move on to fixing the roof issues!!!!

 

[E55BOF]

I remember this thread; it's where I found out about Si-Leck. They did my engine loom; good guys.

The water temperature sender is the one that takes the two-pin connector; or at least, it is identical to the part I bought off eBay as a water temperature sender for the M119. As the plug can go on either way round, I assume it doesn't matter, or MB would have designed it so it could only fit one way.

Does anyone know whether the temperature gauge in the dash is an analogue display, or does it take its signal from one of the ECUs - presumably the base module?

 

[JCW1]

I don't know the answer to your question. The Coolant Temp Sensor on mine is certainly the one with 4 pins and this is definitely connected to the ECU, as Doodle states, it supplies information to control the cold starting/running enrichment. However there are, from memory, two other two pin senders in the same area and maybe one of those is for the gauge.

If you don't get any helpful answers here, you could always phone Simon at Si-Lec as he seems to know his stuff.

 

[hotrodder]

Yep, usually 3 temp sensors/switches. The larger round one is for engine management, a two pin aux fan switch (the fans are cycled according to A/C refrigerant pressure and/or coolant temp) and usually a single pin for the temp gauge
All of the sensors are just thermistors (resistance and thus voltage varies with temp), https://www.google.co.uk/search?q=m119+b11/2 has a couple of hits that explain the ecu 4 pin temp sensor
Sender Unit, coolant temperature (FACET , 7.3151): MERCEDES-BENZ Parts - Bestpartstore.co.uk 4 pin,
Sender Unit, coolant temperature (ERA , 330333): MERCEDES-BENZ Parts - Bestpartstore.co.uk 2 pin
Sender Unit, coolant temperature (ERA , 330104): MERCEDES-BENZ, VW, SSANGYONG, DAEWOO Parts - Bestpartstore.co.uk single pin

 

[E55BOF]

Good grief! So I've bought and fitted a water temperature sensor when I meant to fit a coolant temperature sensor. No scope for confusion in that nomenclature, is there?

Oddly, though, when I took the car for a short spin after replacing the blue two-pin, though the coolant temperature gauge was still reading as though the thermostat had failed, the car seemed to run rather better!

I'll try disconnecting the two-pin, and see what if anything happens. I'll do the same if there's a single-pin switch. As for the four-pin, I'll get another one and change them over, too, and again, see what happens.