I had a split in the thermostat housing, so I decided to renew everything associated with coolant. The new overflow tank has a sensor in it that is apparently malfunctioning, since the warning is always on my info center, as well as illuminating a red warning light. Has anyone fiddled with that sensor at all? I pulled the wire connector to short across the terminals, and the warning turned off, as expected. How does that sensor work? Does it have a float in it? It seems to have a swivel on the underside if the tank where the wires connect. Relevant? Immaterial?

 

This particular section of the forum is for 2003-2009 XJ's and they use a different overflow/expansion tank set-up so you're unlikely to get any help here. Post in the right group.... maybe 308

 

Thanks, RD..I think this is the right spot for my '05 XJ8L, pictured in my signature.

 

You could install the old sensor from your old tank if it was working before you changed the tank. I just had a look at the one in my new spare tank and it is not a "float"

 

As Six Rotors suggests, if needed you should be able to transfer the sensor from the old tank to the new tank. The sensor slides into a vertical tube molded in the tank. Inside the tank is a donut shaped float that slides up/down the outside of the tube to indicate fluid level. So, two things could be happening:
- the donut float is stuck in the bottom position on the new tank
- the sensor in the new tank is no good


You may need to pull the tank back out to see what is going on.

 

As soon as I realized the problem, I checked the trash can where I tossed the old parts, and it had been emptied. Raced out to the dumpster, clean as a whistle. Too late. Since it's brand new, I figured I could get a replacement from Rock auto, I just wondered if there was something about the sensor that I might have bollixed upon installation. Definitely has to come out to fish out the part, since it's down under a big rad hose.

Thanks for the insight.

 

The expansion tanks in the 2004+ XJ8's don't have a float and the sensor is built into the tank. All we do when installing them is to plug in the connector at the bottom.

Also we're warned not to fill them beyond enough to cover the slightly raised bump/hump on the bottom of the tank directly below the cap. I had to replace the original when I found a split in the tube that the hose fits that was ejecting the tiniest stream when the system got hot. Almost microscopic and I was lucky to see it at all. My first replacement crapped out after about 3 years so I bought another one from Parts Geek.



I too had an ongoing battle with the warning light coming on and it took several months before I found yet another micro-leak in one of the runners in that octopus of a thermostat housing. It was one of the plastic type and it left virtually no trace but was losing coolant enough to trigger the low level warning every couple of days. Ended up that I had to replace the entire housing which was a bear because of the way it tucks under the intake manifold along with the various difficult to maneuver wire clips that hold the connections together.

 

I took a look at my old 2004 expansion tank (which came new with the car), and it is exactly as I explained it. They definitely do include a float and sensor. There is a donut shaped float within the tank that rides up and down on a tube - about a 3/4" range of motion. The sensor runs up inside that tube. The sensor is attached to the electrical connector, and can be easily removed with a quarter turn, then pulled straight out.


New tanks do come with the sensor and float, so nothing should need swapping over. I doubt you could replace the float separately. I think I've heard on this forum of a few others having problems with the float/sensor in replacement tanks, as I've also read of malformed top nipples for the top hose (which seems to be the usual failure point of these tanks).

 

I've never really looked at those tanks (I've had to replace 2 of them) beyond where to mount them and where to plug them in. When you look down through the filler cap all you see is the bottom of the tank at the SHALLOW end where there's no evidence of a float much less how much water is really in them. You're told to only put enough in to cover that slight hump directly below the cap so you'd think that wasn't very much reserve coolant available. Of course the DEEP end of the tank where the float is has a lot more where you can't really see it.

Both of mine failed at the vent tube at the top just below the adjustable 'pressure' cap and I suspect that they fail due to the vibration, weight of the hose and tension as the hose gets pushed around by less than careful owners and techs, and general lack of strength itself.

 

When you open the cap and look inside there is a stepped white piece that says Min for the lower step and MAX for the higher step. I keep the fluid about 1/16" over the top of the Max step. What is the coolant level on your car right now with regard to the two steps?

 

The coolant level is definitely above the min mark, I check it constantly. I plan to pull the tank Thursday to see if I can shake it loose or something. The car sat at the body shop for 10 days before I actually got to drive it, and now the Rock Auto return guarantee period has expired. Gotta make it work now.

 

It needs to be above the max mark. With it just over the min mark, on my car I get a low coolant message

 

I haven't any of the more common failures. But, there's obviously something wrong with the float. The low coolant message is on often but there are no leaks (none for as long as I've owned the car). The light won't come on if the car is parked and at normal temp. But if I drive on a freeway, up a hill, down a hill, it will come on and then after few minutes, it'll go off. Give it a few minutes and it'll go again.

If I drain enough fluid to empty the reservoir, can you see the float looking from the cap's opening? Or, is it off to the side too far?

 

It's too far on one side to see. I sent mine back for a replacement and the new one works correctly, though it has a quick trigger. If mine is anywhere below the max mark, I know it will come on. I apparently have a leak in the hose under the throttle body since everything else has been replaced and there's no sign of obvious leakage.

 

Check connector CP1 for corrosion
It is located under the headlamp washer tank and is exposed to water ingress

This plug has the coolant sensor, aux and IC pumps

If the sensor looses the earth connection you will get a low coolant warning
IE when the sensor is covered 0v is present at the IC

Also check the earths behind the headlamps as the is where the sensor earth point is

You can check the wiring by removing the sensor harness connector and shorting the plug. If you still get a low coolant warning the issue is at the earth or the CP1 connector. If it goes out you have a dud sensor

Cheers
34by151

 

Thank you!