temperature sensor disconnected

[alexk]

Hello, I recently bought a galaxy 1998 td glx (my first mpv) and the prev owner said it was all running fine but that it had started going very slow up hills. It seemed to drive fine for me on return journey but on following day it was reluctant to start (fired but kept stalling). After 5 mins of trying it started (with cloud of white smoke) and continued to run fine and started again easily while warm. I noticed the temp gauge was not rising at all during driving and checked the engine bay. I think I found two temp sensors: one near the radiator and another mid-way along the top hose which was disconnected. The ground wires coming off this seemed a bit degenerated (copper wire showing in places). I reconnected this sensor and then the engine did not want to start at all!! Also, the heater plug symbol only stayed on for an instant and there was a beep from the dash after each attempt to start. I disconnected it again and the engine started. When warm, I reconnected the sensor and the engine would start again although same thing happened with glow plugs (no beep though). Also, i noticed that the temp gauge on the dash is working when the mid-way sensor is connected.
What do these two sensors do? And what is likely to be the fault with the starting? Many thanks for any replies 

[Mirez]

Firstly - Welcome!

The earlier TD isn't my speciality however the ECU knowing the engine temperature is fairly critical to how any diesel engine starts. Normally, VW have two sensors - one for the ECU and one for the instrument cluster. From what you're describing though, it all seems a little strange! It would be an idea to take some photo's of both sensors so we can ID them for you

[alexk]

Here is a pic of the engine bay. The sensor marked 1 is the one that is not disconnected.

[Chrispb]

I would imagine the one nearer the engine is for engine temperature gauge and ECU with the one in the pipe for electric fan/air con
can you check the colour of each sensor and how many pins each has, as there are a few options.

[alexk]

The one nearer the engine is green and the one near the radiator is brown. Both have 4 pins

[Chrispb]

The green one is for ECU and gauge 059919501A

The brown one is for cooling fan/air con 357919369E

[alexk]

Thanks for that clarification. Does anyone know why the engine might not start or give full glow plugs when the green (ecu) is connected? It must be working to some extent as the gauge comes back online when I plug it in. Could a faulty ground be causing the problem as I did notice the two ground wires look a bit degenerated?

[SilverBeast]

I understand that the four pin sensors have two temperature sensors built in. One for the ECU and one for the Temperature Guage.  (My MKII has a two pin sensor that does both, and when it was faulty the guage only read at higher temperatures).  If the ECU is not getting the correct temperature it may think the engine is warm, not use the glow plugs at all  and not deliver enough diesel, hence struggling to start.  There are different sensor versions available, which can usually be identified by colour) so you need the correct one for your car if you change it.

[alexk]

OK, thanks. Any idea why the engine starts when sensor is disconnected (ecu getting no reading at all maybe)? Should I plug it in and try to get a vag com reading?

[Chrispb]

Yes connect any disconnected leads and scan for codes

[alexk]

Hi, I ran the vag-com with vdcs-lite under the autoscan for galaxy and the only fault that came up was "exhaust gas recircultion valve (N18) 31-10 open to short or ground - intermittant"
Should I be trying another setting other than autoscan?

[Chrispb]

Hi
Autoscan will only work if you have the registered version of VCDS but will be listed for Sharan not Galaxy.

So If you have the free version you need to select engine then scan, then repeat for the other modules (note not all modules will be relevant to your model)

The fault you have mentioned (N18 valve) this controls operation of the EGR valve, this may also be disconnected, faulty or faulty wiring.

[alexk]

Ok, thanks. Tried the other modules and am getting "no response from controllers". I'm not much further on in discovering why the engine won't start with the green temp sensor connected! Unless this is somehow connected to the egr valve... How can I check the wiring to the egr valve please anyone??

[Chrispb]

So with the sensor connected do you get any faults from the engine module

[alexk]

With sensor connected the only fault I'm getting is the one quoted above re the EGR valve

[Chrispb]

The white smoke issue could be a clue that the timing is not spot on and as various imputs like crank & cam position sensors and temperature sensors send info to the ECU assuming the basic is within limits.

[alexk]

So I guess I should just replace the sensor regardless of absence of faults from vag com? Btw,I did a full scan for modules and all that came up was engine, cent elecs and exhaust system... Is that about right?

[Chrispb]

Probably not, have just read again your original post and am more convinced you have a timing issue, these engines do suffer with the crankshaft sprocket keyway wearing which causes the timing to go slightly retarded.
disconnecting the temp sensor and probably any other sensor would cause the engine to run in basic settings mode.

[alexk]

Ok, thanks. Will i prob need a n sprocket then? Or will adjusting the timing belt suffice? What's the best way to judge whether it is indeed a timing problem then?

[Chrispb]

Only by inspection, when was the belt last changed? or is it due to be changed anyway.

[alexk]

Current mileage is 127k and was changed at 102k

[alexk]

Is the recommended mileage of 60k for timing belt change?

[Chrispb]

Fords do say 60.000 but you should also consider the idlers and tensioner pulleys, was they done aswell.
I would certainly have no hesitation in starting off with a new belt and pulleys and of course the crank sprocket.

[alexk]

Hello again, my mechanic had it for a couple of days and said he didn't think the timing was out. I don't know if he actually opened it up but he said that he thought that there would be white smoke all the time if the timing was out and not just on starting. I'm going to replace the temp gauge and see if that improves the starting. With regard the slowness up hills could the problem be a choked up fuel filter maybe?