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Discussion Starter #1
My neighbor's 2003 LT cooling fans won't come on. We tore into it with gusto. Here's what I found so far: Disconnecting the water temp sensor results in the little buggers spinning merrily away until, of course, I plug it back in, whereby they cease, causing the coolant reservoir to urinate on my hand. Coolant is full at both radiator and reservoir (to max). Massaged hoses until I became rather excited myself. A bit unsure about cap, but wondering now if the sensor might be bad. Any help with my homework will result in appropriate genuflection to the LT gods.
 

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I foind this some what related post:

T-stat opens @ 185, the fans kick in @ 221 and the High Temp Warning Light goes on @ 239.

For the early bikes that 221 is very near the red mark.

Each fan has it's own fuse. But both fuses are fed from one relay. The relay is controlled by the motronic "brain" and the brain is fed by the temp sensor at the rear of the cylinder head. The temp sensor down by the water pump feeds the temp gage on the dash.
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Discussion Starter #3
Thanks, Bob
I've seen those posts, and checked the fan fuses and the fan relay fuse- all good. The temperature gets almost to the red, then coolant starts peeing out of the reservoir cap. As soon as I pull the engine sensor plug the fans come on and the temperature begins to fall. Its almost like the temp sensor itself (the one in the cylinder head) is not doing its job. Could the fan relay be suspect even if the fans come on when the sensor is unplugged?
 

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If the fans come on when you unplug the sensor - it is the sensor. It sends a signal to the Motronic brain which in turn triggers the fan relay. There have been a few of these fail recently.
 

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BecketMa said:
I foind this some what related post:

T-stat opens @ 185, the fans kick in @ 221 and the High Temp Warning Light goes on @ 239.

For the early bikes that 221 is very near the red mark.

Each fan has it's own fuse. But both fuses are fed from one relay. The relay is controlled by the motronic "brain" and the brain is fed by the temp sensor at the rear of the cylinder head. The temp sensor down by the water pump feeds the temp gage on the dash.
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sounds like a bad temp sensor. everything else seems to be operating as it should
 

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Discussion Starter #6
Many thanks for the info. I plan to order the temp sensor today and will post the results of its replacement. Hopefully those fans will start singing as soon as the engine gets to temperature. One question- how necessary is the special tool 00 2 800? It looks like an open end wrench, possible offset. Also, does it assist in setting the torque value to 30 Nm?
 

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If you have the coolant resevoir overfull, And is sounds like it might be... Then when the engine heats up and the coolant expands, It's going to blow some out...Maybe reduce the coolant in the overflow bottle to minimum level....

You need to know if it's ever getting hot enough to trip the sensor...

If it's not getting to 221 degrees before you bail on it. The sensor is not going to trip, And the fans are not going to come on...

But there's not much difference between 221 and 239 when the light should come on...
I hate to advise anyone to overheat their engine to the point where the light comes on just to prove the fan sensor is bad...

I'd test that sensor somehow.. maybe put it in some boiling water. Or just replace it if it's not too expensive...

Remember water boils at 212 so you're going to have to get a full rolling boil going on it...Maybe put a little anti-freeze in the water to increase the boiling point...Use a candy thermometer to get beyond the trip point and then meter the sensor.. What was once shorted should now be open..

Good Luck

John
 

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Discussion Starter #8
Thanks, John
The reservoir isn't over full, it's right at the max mark. I watched the coolant level as the engine warmed to the first white mark on the gage. Shortly after that, the coolant level began to rise in the bottle. That is when I shut the engine down, to prevent the overflow.
 

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C-A-D said:
So by installing a switch in the wire going to temp sensor one can build a "force/manual" start of FANs? If I understood correctly? Tempting idea in the "just in case book"...
I don't know if I would do that as this sensor does more at the Motronic than just trigger the fans. It is part of the feed to the fuel MAP that goes full rich when cold. You would be better served to put a switch and diode into the fan relay if you wanted to "command" the fans on.
 

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ICXC said:
Many thanks for the info. I plan to order the temp sensor today and will post the results of its replacement. Hopefully those fans will start singing as soon as the engine gets to temperature. One question- how necessary is the special tool 00 2 800? It looks like an open end wrench, possible offset. Also, does it assist in setting the torque value to 30 Nm?
You could just use a crow foot of the right size as that is all that is. But an open end wrench will suffice and you can "guesstimate" the torque value.
 

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Discussion Starter #12
Spoke with a local dealer today. $80 for this sensor! Mechanics don't use and can't even get this "special wrench", from US or Germany. So much for high tech... I'm going to pull it and see if I can bench-test it, but it won't be for a few days. Gotta work sometime-
 

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A hair dryer will take you above 221 if you're patient. Connect the ohms meter, put the hex flat in the vise with the connection pointed down, and point the hair dryer at the end that goes in the engine and monitor it with an infared thermometer.
 

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If it boils as quick as you indicate, it sounds like a thermostat stuck shut. The water isn't circulating at all to get hot that quick.

You can put the therm in boiling water to see if it opens up. If I was a betting man, I'd say it won't open up in boiling water.

Just my 2 cents before you spend any money.

Vern
 

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Discussion Starter #15
Hmmm...
I had thought about the thermostat but didn't want to drain the system to remove it. Guess it wouldn't be that bad to take it out and look at it. One thing I did notice- the LEFT radiator heats up very hot before the RIGHT even gets warm. I think I read something about this in the service manual. Possibly indicative of a thermostat malfunction?
 

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Sounds like it to me. The T stat is half the price of the sensor. I have never really checked my radiators to see if they both got hot about the same time.
 

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Discussion Starter #17
Well it took a while to get the new tstat, but it's here. Froze it overnite and am trying to put the assembly back into the radiator housing. It is super hard to get into place, and I can't seem to get the clip into its final position. The tab is in its recess and the assembly (tstat and housing) seems to be bottomed out. ANy suggestions? Thanks for all help so far!
 

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Discussion Starter #18
Success. Fans now come on. Cannot say if it was the thermostat or the sensor, but it works! Thanks for all who contributed, as I will contribute to this wonderful site. It continues to be an invaluable resource for the LT owners.

Best,
Nick
 

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My coolant warning light is on as soon as I start the bike. The temp gauge seems to work normal, the coolant level is correct and the engine runs fine. What sensor is triggering the warning light?
 
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