Wiring issue? - temp gauge stuck on max

[Ribfeast]

Hi guys, a few days ago I ran some wires through the firewall using a coathanger to pull it through. Ever since then the temperature gauge pegs onto Hot when I turn the Ignition to ON (maybe I damaged a wire or insulation). The car still drives fine (and isn't overheating), but I'd like to find the fault if possible.

I've tracked down the wire that sends the signal to the temperature gauge, it is a brown one, that goes via a white plug/socket underneath the glove box. I took some voltage measurements with the ignition set to ON, and wrote down what pins and wires I got those voltages on. If someone has a free moment (and a RHD MX83 cressida which has an analog dash), are they able to unplug the white socket and take some voltage readings for me? I am assuming the readings would be the same on a 1JZ cressida compared to a normal one.

Here's the readings I got:


http://ribfeast.no-ip.com/1JZAnalogGauges.pdf

Any wiring diagrams of Cressidas would be really handy. What I'd love to know is if the temperature gauge is a voltmeter, or if it is an ohm-meter, or where it derives its signal from. I'd love to know what voltage is meant to be going down the brown wire when the car is idling at operating temperature, and whether all the pins in my diagrams are meant to read 12V when the ignition is turned to ON.

Car is a 1JZ cressida btw, 1989 GLX.

Any help would be greatly appreciated

[Cool1]

The temp sensor just works on resistance to gnd. So if your guage has gone fully max, you have the sensor wire shorted to ground.

[oldcorollas]

edit: BAH, beaten by seconds....

oh right.. it was about an earth, had to be Cool1

usually temp sensor is a variable resistor, so that it gets voltage from loom and grounds it...
can't remember off top of my head if hot is low resistance or high tho.... i think it is low resistance, so you may have a short on the engine side of temp sensor wire?

[mrshin]

The only other thing is, I've had issues with temp gauges when there's other earthing faults - i.e. the wire between the gauge and the sensor is fine, but the earth to the instrument cluster is questionable.

[gianttomato]

Most sensors are high resistance when cold, low resistance when hot. I say you have shorted the sensor wire to ground.

[Ribfeast]

Does that mean the temp gauge is basically an ohm-meter? Would the ECU put out any codes at all if the sensor is shorted to ground?
Even when I completely unplug all the plugs from the ECU it still pegs on max temp when I turn the ignition to on.
Just trying to work out if the brown wire is being shorted to 12V, or shorted to ground, it seems like 12V, as I have 12V on that wire when the white socket is plugged in and the ignition is on.
Everything else on the instrument cluster works great - tacho, auto lights etc.

[sideshow]

the brown has to be earthed
if its the right brown one

[stradlater]

The sensor works very similar to a bi-metalic strip.

Basically, as the heat increases, the sensor offers less and less resistance.. If your sensor has some how failed and is earthing against the engine somewhere, then you'll get 0 resistance and a constantly HOT temp reading..

Best you be pulling out a multi meter.

Oh, and on the ECU part...

The ECU has it's own temp sensor.. There's two. One for your temp gauge in the dash and one for your ECU.. The two are completely independant.

[Ribfeast]

The sensor itself should not have failed, as I didn't get near it at all while I was doing my work.
Those voltage readings I took above are when the plug is pulled out of the socket, and the igntion is switched to ON. The brown wire on the socket is definitely the right one, as the temp gauge goes up when I apply some voltage to it.
If the brown one is meant to be earthed, where does the temp gauge get its voltage from to move the needle? I just find it suspicious that the brown wire on the socket has 12V on it, it seems like something has shorted the 12V rail onto that wire. I was just hoping that someone else with a cressy coul unplug that white socket and take a reading on their brown pin with the ignition on, to see if they get 12V too, cos I reckon it shouldn't be present.

[Ribfeast]

OK I just unplugged the green plug from the temperature sensor at the front of the engine, and the temp gauge still goes up. So that rules out the sensor right? :-/

[oldcorollas]

whats the resistance between the brown wire (from loom side of plug) and ground?

edit: and then unplug temp sensor and take reading again....

if you ground the 12V ont he gauge side, does the gauge go up? (maybe try briefly)

if the sensor gets no voltage, then changing resistance will do nothing.... ie gnd - resistance- gnd = nothing useful...
but GND (engine) - resistance(sensor) - 12V (from gauge) = circuit

[Ribfeast]

All fixed! The brown wire had a break in it, just where it comes through the firewall. Ran a fresh wire directly to the temperature sensor and now it sits on zero (car is dead cold). I won't know for sure until I drive the car, but I reckon this has nailed it. Thanks for all your help everybody!

[sideshow]

dont know where u got brown as the temp sender
the 1j has a yellow colour i think for temp

[oldcorollas]

I've tracked down the wire that sends the signal to the temperature gauge, it is a brown one, that goes via a white plug/socket underneath the glove box.

............

edit: my bad at least he found a wire that went to the gaughe.. and was the one that had a prob with it

[Ribfeast]

Well the temp sensor at the front of the engine has a brown wire and a blue/red wire coming out of it, no yellow. And the wire that I found broken in the loom was brown...

The break was a bastard to find though! I only found it after I had routed a fresh wire through the firewall to replace it lol