Well, now I'm not too sure the Disco's actually overheating anymore... At first, I thought the electric fans were shutting off when the AC was on driving the temp guage to just shy of the red line because sometimes when I run out when I run out to see when it's showing hot they are both running, but sometimes they aren't & the fan has stopped running after the Disco is shut off when it's warm. When the guage is showing hot & if you turn the AC off, the temp guage drops to normal in a second or two & stays there, or even more confusung, is if I keep the AC on while driving the guage is all over the place normal, just shy of the red line, in the middle of the guage, to normal again; you pick. I thought maybe there was an air bubble around the temp sending unit, so I ran the heater while driving it for an hour with no change. While I had the heater on, the guage never went past low normal, turned the AC on and the temp guage shot right up to the red in like 2 seconds. Now I'm new to the Discovery, but there's no way the water is heating up or cooling that fast. Anybody ever seen anything like this before??

 

Yes...fan clutch, water pump, radiator needs rodded...what have you done thus far?

 

If you have gauge movement in just a few seconds, your high school physics learned you well, the engine is just not quite that big.

1. Remove the gauge wire from sensor, and clean to bright metal. Remove all gunk. It is the one on the left in pix. The square top one on the right is the two wire one that signals the ECU.

2. If you have truck warmed up, engine off. If you turn to position 2, but don't crank - observe gauge. Does changing AC controls make gauge move around? Should not, as you are not changing the water flow or the load on the engine. I guess a gauge could be picking up stray current from a short in the wiring harness.

3. If you can get your hands on a scanner or ultra gauge, it can read the other sensor directly from ECU, and give you more exact readings.

4. Test you fans for correct direction. Cupped side of blades toward block. When truck is at idle, paper towel should stick to the grille. When truck is off, position 2, switch on AC. Electric fans should come on, and that paper towel should suck toward grille. My PO wired fans reverse, when AC was on it subtracted air flow. And he had the belt on wrong, so water pump was running backwards.

On a D1, the electric fans should run whenever AC is ON, and not cycle on/off.

5. Fan clutch is the old reliable - when cold, should spin with resistance, release and it may go 1/4 - to no more than 1 turn. Feels like peanut butter inside. When warmed up, turned off, should still not spin more than 1 revolution. Freewheeling means no cooling except when at highway speed. At 60 mph, you don't need a fan. You need a fan at idle and slow.

6. The way to remove an air bubble is to use a thermostat with a jiggle device or a 1/8 inch hole in the flange, locate at 12:00 position so any steam pocket that tries to form is passed on through the system. $10 and you can have a 180F stat. Stat spring goes inside the block.

 

I knew my 150K in student loans wouldn't steer me wrong. Couldn't duplicate that kind of engine temp difference in that short of time with a flame thrower & liquid O2. Will try everything you suggested tomorrow & let you know. Thanks for the direction!!

 

And Keep the AC OFF until you finish all that. Also check out your hoses. You do not want to have to tear it down twice and waste all that green anti-freeze do you?

 

I'd lean towards a bad fan clutch. Feel on the front/face of the clutch - do you feel a very slick sheen of liquid? If so, it's leaking and needs replacement.

 

Another check would be to unplug the gauge sensor (single wire) - and see if you still get readings changing on gauge when AC turn on / off, etc. From the RAVE:


of the sensor is approximately 17 ohms.

So a short to ground or gunk on the connectors at the wiring from gauge sensor to instrument cluster would have the effect of making it seem hotter. If you have access to a scanner it would be good to compare readings, OBDII port uses a separate sensor thru the ECU on a D1. On a D2, just one sensor and ECU drives gauge.

 

I would bet sensor issues.

 

Update... Pretty concerning update on my temp guage issue. Took the Discovery for it's first long drive today & surprisingly the 65 mile drive on the interstate at 70-75 MPH with no AC resulted in a steady(normal) reading. Drove home after dark & as soon as I turned on the lights the temp shot right up to the high side of normal & bounced around for the entire drive back. Now the wierd thing was I had to stop at the red light at the end of the exit ramp & with my right turn signal on, the guage bounced towards the high end of the normal in perfect beat with the turn signal. Turned it off, and it slowly went back down. Turned it back on & away it went again.... What does the guage, fan, AC, lights & turn signal have to do with each other??

 

A loose ground or an electrical short, mashed harness, skint wires, rats gnawed it, etc. I'll look on the drawings and point to some spots to check.

OK. Try this - unplug the gauge sender (that is the single wire sensor near the thermostat on top of engine block - the one with square top next to it has two wires and is for the ECU). Gauge should read minimum with truck running. Flip on lights and turn signals and AC and see if the gauge moves any. If it does, that indicates a short from one wiring system to another, can be a connector full of green corrosion, etc.

If it does not move, reconnect gauge sender. Problem returns. The sender is at the "end" of the circuit and connects to the engine block which should be chassis ground (or "earth"). So by unplugging the sensor you have broken the "earth" for that circuit. Other circuits (like cluster lamps) have and earth point as well. It may be loose and they are trying to use the earth path supplied by the gauge sender.

Check for engine block ground by using a battery jumper cable from negative (-) black battery post to clean metal area on the engine. If problem goes away the jumper from engine block to frame needs attention. Also check cluster ground point under kick panel, Earth 201 (E201).

After that I'm for removing dash cowl over cluster and having a look at connectors and back of cluster for crud on circuit board, etc.