Temperature gauge missbehaving

Hello,

I've just bought a Defender 90, 2004. There are a couple of small-ish problems that I need to sort out - but this one has me confused;

When I've been driving for a little while (long enough that the engine is up to temperature), the temperature gauge stays down in the blue/cold region of the dial. When I lift my foot off of the accelerator it rises to near the half way point on the dial. When I put my foot back down on the pedal it drops back down into the blue (or very slightly above it). While it's in the blue zone, it isn't at 'zero' (ie the gauge is showing something, just not very much).

My first guess was that the coolant was too low. It was a little low (under an inch below the mark) so I topped it up. I also cracked open the air bleed thing at the highest point and coolant dribbled out so I don't think there's air in it. No joy there.

I parked the car up and fiddled with the lights etc to see if anything electrical interferes with it. No joy there either.

Now the bit that got me confused; I put the handbrake on. This caused the gauge to rise from 1/4 to 1/2 way. I took the brake off and the gauge dropped. I did this 5 or 6 times to make sure - it's definitely connected.


I have searched the forum but not seen anything matching my problem that's been solved successfully.


Any suggestions?? I'm lost


Thanks

Barney

Start with the basics, check the earth straps between battery, gearbox and chassis. Strip them, clean them with some wire wool and reassemble them with some copper grease to aid the contact cos it sounds like your handbrake cable is doubling up as your earth cable

marmaduke said:

start with the basics, check the earth straps between battery, gearbox and chassis. Strip them, clean them with some wire wool and reassemble them with some copper grease to aid the contact cos it sounds like your handbrake cable is doubling up as your earth cable

Click to expand...

+1

marmaduke said:

start with the basics, check the earth straps between battery, gearbox and chassis. Strip them, clean them with some wire wool and reassemble them with some copper grease to aid the contact cos it sounds like your handbrake cable is doubling up as your earth cable

Click to expand...

+2

Thanks for the replies so far, but unfortunately it's not fixed yet.

I cleaned the battery, chassis and gearbox cable contacts as well as the strap that goes from the RHS chassis to the engine. The chassis cable to the ECU is clean and I re-attached a smaller wire that was coming loose from a fixing point on the bulkhead directly behind the engine.

Whereabouts is the temperature sensor? Presumably that will have a local earthing point nearby I could try to clean up too.


It's probably not connected but I found a pair of wires that had fallen off of somewhere whilst I was underneath. A yellow/green and a green/brown. I haven't worked out what they're for or where they go yet. I'll do so when I find a wiring diagram that I can understand


Thanks

Barney86 said:

It's probably not connected but I found a pair of wires that had fallen off of somewhere whilst I was underneath. A yellow/green and a green/brown. I haven't worked out what they're for or where they go yet. I'll do so when I find a wiring diagram that I can understand

Click to expand...

Ignore that bit... it goes to the reverse light switch in the gearbox


Fiddling with the gear stick appears to cause the temperature gauge to fluctuate too.

Can only speak for a 300tdi, the temperature sensor is at the front of the engine next to the thermostat, it's only a single wire and I believe it earths itself which is what gives the resistance thus moving the needle, the same as a fuel gauge

The plot thickens :5bwilly_nilly:

The temperature sensor has two wires coming from it - both go to the ECU. The sensor appears to be a thermistor, which seems ok.

Just to see what would happen I unplugged the sensor - it was a zero, obviously. I applied the handbrake and the needle moved! I also noticed today that I do not have a handbrake warning light - coincidence?!

One test I did do was to check the resistance between the drivers under-seat chassis earthing point and the metal case that the ECU is in. There was a small but measurable difference. With the engine running there is also a measurable voltage across the metal can and earthing point, which changes when the handbrake is applied.

I cant see in the diagrams whether the ECU is independently earthed via its case, or predominantly incoming earth wires from elsewhere. I'll add an extra earth strap anyway - it cant hurt (i hope).

Additionally I'll investigate the handbrake switch. I suspect there's a short somewhere.

Unless of course someone has some better guidance

Thanks!

Sounds like the handbrake switch, or a fray in the wire from/to the switch. Bloody lectrics!

Earths for deffo, go round with a grounded jump lead till you find the thing thats lacking

Earthed the ECU, prodded stuff with an earth lead, fixed the brake warning light and reverse lights.... nothing changes it.

I cannot explain the handbrake thing. The brake cable doesn't touch any cables, and nothing is frayed. I had a friend crawl under it to confirm - his response was "what you're saying is impossible!"

My next guess for what's wrong is the injector wiring loom. When fiddling with the ECU I unplugged it & noticed there is a small amount of oil in the red plug. Assuming that the sensor comes in on the red plug; given that the temperature sensor is a resistance based device, having non-conductive oil in the connector will surely cause an issue of some description. I can't exactly come up with a reason for it to be so pronounced, but it's the best avenue I've got at the moment.

Any way to check the old owner didn't upgrade the injector loom before I got it, and it's just old oil coming out? It's not pooled up under the seat like some people report when they get the problem.


Thanks