Temperature Sender what do I need

[FITZ]

I have a LR90 that was originally a 2.25 petrol it now has a 3.9 V8. What temperature sender do I need to give the correct reading on the original temperature gauge. The 3.9 sender shows the as max on the gauge when the engine is only warm.

[western]

My 110 parts book shows PRC2506

for the V8 carb engine maybe it'll work on your 3.9efi & should operate OK with the existing temp gauge

[Quagmire]

I think the v8 and 2.25 engines used the same gauge, from when i microcatted the difference so should read the same- as western notes thats on a carb v8 though.

My 90 was a 2.25, when i went to carb 3.5 i just connected v8 sender as was fitted to my 2.25 gauge and it reads ok, well the needle sits dead centre when up to temp anyway...

[Dave W]

You need the gauge off the 50th Anniversary model or NAS spec.

The older carb V8 sender is a different thread to the EFi one so they can not be swapped unfortunately. The last couple of vehicles I've done I've changed the gauge to the one above, it isn't cheap though.

One option I'm investigating at the moment is to use a TD5 sensor. A TD5 gauge gives pretty much the same response as the V8 carb and 300TDi gauge, hitting the red at about 70 degrees C. The TD5 sender is just short of 12mm thread dia and the EFi one is just short of 16mm thread dia. I'm off to our local engineering place later in the week with both senders to see if they can supply an adaptor or a blank that I can drill and tap to take the TD5 sensor as that might be a cheaper option than buying a new gauge.

[FITZ]

Guys thanks for all the replies.

A bit more info,,,,,the 3.9 EFI engine had a sensor that fitted and showed correct on the original 2.25 gauge but after the car boiled over because I had the electric fan switched off (muppet) it cooked the sensor and the x-fan switch.

I had it fixed and a few other jobs, the guy who did it the work fitted a standard 3.9 temp sender and binned the cooked sensor. From that I know there is a sender that fits and works. The original sender worked fine with the original 2.25 engine gauge before I cooked the sender. So the hunt continues, please let me know if you know what sendor works in the 3.9 with the LR90 2.25 petrol temp gauge.

[Dave W]

Just an update on this in case it's of use to anyone else. I ended up doing a kitchen experiment with a pan of water, a digital thermometer, a TD5 coolant sensor and a 3.9 EFi sensor (2 of them actually so I could double check the readings).

This is the result.. (sensor value in ohms)

Temp....TD5.....3.9....3.9(2)

10......3900...1198

20......2700....810

30......1810....543

40......1213....360

50.......877....254....230

60.......636....173....184

70.......454....118....128

80.......336.....84....89

85.......298.....76....72

90.......252.....63....60

95.......224.....56....50

100......190.....50....30

The second 3.9 sender was only measured from 50 as that was what the water had cooled down to when I started with that sender !

I then checked the gauge itself by connecting a variable resistor in place of the sensor on the vehicle and finding out where the cold mark, centre and hot mark were.

The cold mark (the dash just above the blue area) comes in at 185 ohms

The centre comes in at 106 ohms

The hot mark (the dash below the red area) comes in at 63 ohms

The first thing this showed me was that the TD5 sensor was a none starter, the TD5 ECU must use a PWM signal or a resistor ladder to control the gauge based on the sensor input as using the sensor direct won't register on the gauge until the coolant is over 100C

I'm not sure why the two 3.9 sensors were so different but I'm guessing tolerance differences as they are from different manufacturers, maybe one of them is after market or maybe LR changed suppliers. They are, however, at their closest around the important 80 to 90C area.

Looking at the gauge requirements and the sensor output it looks as if adding a resistor inline with the sensor of around 30 ohms would put the gauge in the right ball park. I'm going to fit a trimmer pot inline with mine so I can make fine adjustments to it to compensate for any change in the sender unit as there is about 5 ohms difference between the senders I have at the important temperatures. I'll use the second sender as it has a bigger swing when the temperature heads above 90 degrees so will give a more obvious gauge movement if the engine starts to go over temperature.

[Aragorn]

Have you considered buying a proper aftermarket guage that has the actual temperature graduations and its own sender?

It would be far more accurate, and its good to know your engine is at 95c rather than "3/4 thru the white"

From those measurements above, a V8 running an 88c stat would likely be sitting at the "hot mark" most of the time, which isnt very intuitive, as you'd expect normal operation to be in the middle.