I know there’s loads of threads posted about faulty temperature gauges, well now it’s my turn. I’ve read all the other ones and I’m not finding many that seem similar to my problem and they don’t really have conclusions.

My gauge sits at the bottom end and only moves up slightly after driving for a while. But when I pull up and put the clutch pedal down the needle suddenly jumps up. I got connected to the ECU and it seems to have the temperature correct, though I’ve not had a chance to see what it’s like when it gets up to temp. But it started at the ambient temp and started increasing slowly with the engine running, and was unaffected by putting the clutch down. I’ve had it all disconnected and been doing continuity tests between the cables and the body but everything seems to look good. No routes to ground once the cables are disconnected at each end. I thought it might be a grounding issue but I’ve got no other problems. The other dials share the same ground and they’re all fine.

Im thinking about hooking the ECU pin to the ground and seeing what the multimeter says but I’m not sure what I’m looking for. I’ve even had the ECU open expecting flit find loads of oil but it was absolutely clean in there. Can anyone help me out? I don’t want to change the gauge because they’re so expensive.

Old truck and old wiring TD5? you don't say.
Way out is to fit an after market capillary gauge that reads in numbers degrees. No wires or electrics involved except the instrument lamp. Durite gauge for £40ish

Run an earth from the (neg) side of the battery to the passenger seat rail.
Probably not the gauge itself.
Deffo sounds like wiring.
TD5's sem to have a lot of them.

Alternatively as @tottot said, fit a capillary gauge..

I’ll consider buying the other gauge but I’d rather try and fix this one first. I always wonder if fixing one electrical issue may fix other problems I haven’t yet noticed.

How exactly does it the gauge work? It looks like it has a live and a ground, then anofher route to ground through the ECU cable. Is that right? When the temperature goes up I’m guessing there’s less resistance on the ECU cable to ground?

Plagerism rules ..
TD5 coolant-temperature-sensor feeds to the ECU, not the gauge.
The coolant-temperature-sensor is used (along with the fuel-temperature sensor on the pressure-regulator, and the intake-air-temperature sensor in the air-filter-casing) to let the TD5 ECU make decisions on fuelling, EGR-volume, boost-limit, glowplug-energisation etc.
As a bit of a sideline the ECU sends a composite-of-various-sensors signal to the dashboard temperature-gauge.
__

So not a simple as a 300tdi engine at all which does alter the resistance relative to temperature and deflects a needle, a voltmenter across a thermistor...
Which rather points to the output of the ECU or the gauge itself.

It is like the battery light on my car. Last year it came on and within a couple mins systems started shutting down including the electric power steering. Thing was like the TD5 system light only came on when batt voltage dropped to a very low level as it is controlled by the ecu. The alternator had stopped charging some time before but there was no indication this had happened, unlike older vehicles where light would come on as soon as it stopped working.
Did not make it home, car now has aftermarket voltmeter fitted.

check coolant connector for clean contacts

I’d ruled out a bad earth as I’d checked the continuity but now I’m thinking it may be a poor contact somewhere. I only have a couple of crocodile clips electronics type cables lying around so I ran them between the battery clamp and the negative and the needle immediately jumped up a little bit. I’ve already cleaned up the ECU to body contact but I think the body to battery contact may be poor, wherever it is. I have an earth strap I’d used elsewhere but I don’t think it’s needed where it is. I’m going to move it from there to between the body and the gearbox to battery connection. I’ll see how it goes.

not strictly defender (P38) but land rover use same items on various models

I’ve cleaned up the chassis earth that’s along the earth strap connected to the gearbox/transfer box. This made a big difference and the temp gauge went up about a quarter of a way after about 5/10 minutes of driving.

But for some reason it’s still being affected by the clutch. I watched it a bit closer and it seems to be the revs that are affecting it. As I drive between gears it slowly goes down until I hit the clutch to change gear and it suddenly shoots back up. I sat outside my house and gave it a bit of a rev and watched the temp gauge go down. I know people fit aftermarket tachometers to the TD5 so I’m going to figure out how they do that and see if somethings making contact somewhere. I’ve previously had oil in my loom so I wonder if that’s still giving me problems somewhere.

*Edit*
Actually thinking about it, it’s probably much more likely that it’s the alternator linking it to the revs right? The revs go up and the alternator runs faster. Seems to make sense.

I’m going to link a multimeter up to the pins on the back of the gauge and see what’s going on there, see which of them change with the revs. I’m also going to check the resistance between the earth on the ECU and the negative terminal. I have a spare earth strap now so if I need to I’ll run something from the ECU box to the chassis.

have a gander https://www.landyzone.co.uk/land-rover/rev-counter-for-4-cylinder-petrol.386612/
not sure I covered diesel

Thanks mystery, after looking at that installation document I'm assuming that it's not some weird non-existant tachometer issue.

So late last night I found that the resistance between the battery negative and the ECU earth point was around 2 ohms. I had a quick Google and it says that I should be seeing less than 0.5 ohms. This evening I ran a wonderful new earth from the ECU seat box to the chassis and now I'm getting 6 ohms resistance. I'm assuming this isn't actually the case and I'm making some mistake with my measuring, I'm not an electrician. If anyone wants a blood donor ask someone else because the mosquitoes drained me dry.

I also found that there's an earth somewhere between the ECU and the brake switch. I don't know if that's causing me problems but I guess I'll need to solve it.

*EDIT*

I was chatting this problem through with a close friend of mine (ChatGPT) and was politely told I was an idiot for not disconnecting the battery before I tested the resistance. I couldn’t be bothered opening the seat boxes and disconnecting the battery to test so I just took it for a drive. Lo and behold, it’s completely fixed. The temperature gauge went up to nearly half way before I got back and didn’t budge when I hit the clutch or anything. It might’ve been in my head but I think it was running a little better as well. After owning it for the last three and a half years it’s nice to have a working temperature gauge finally.

So for anyone reading this, the solution was to fix the earth to the ECU. In my case I ran an earth strap from the bolts that hold the seats in, to the chassis and cleaned up the chassis earth near the battery.

glad you fixed it you have to compensate for the lead resistance when on ohms scale and really you cant beat an old Avo mark 8 analogue meter that calibrated in my opinion and of course remove any voltage source

So what I said in post #3 then ? "Run an earth from the (neg) side of the battery to the passenger seat rail."

miktdish said:

So what I said in post #3 then ? "Run an earth from the (neg) side of the battery to the passenger seat rail."

Click to expand...

Yeah. Sorry to not say thanks but I did read your reply and took it on board. I ran a cable from the negative to the chassis but I didn’t have enough cable to get all that way and only had one short earth strap that I was recycling so I needed to find where the actual problems had cropped up and fix them.