Temperature Gauge Goes to Red on Engine Start before Continuing Past Red

[chrisAB]

Hi,

New to the forum! Glad this community is here,

My Carinas temperature guage starts at half way (normal level) at two clicks but then rises to the red when the engine starts. After about 30 seconds it will go to the top of the red and continue beyond as the engine heats up

I've tried disconnecting the 'coolant temperature sensor' pictured with the green connector but the same thing happens as above. I have also removed and tested the thermostat.

What could be causing this and how could I further diagnose? As far as I know the guage is fed from the sensor so how can it rise if the sensor is disconnected?

Any help would be greatly appreciated

[Greybeard]

I think, firstly I would try a continuity test from the sensor to the gauge. Also ensure the gauge is earthed ok. This may be harder than it seems if the gauge is on an instrument cluster PCB. In that case go to the PCB plug.  If that checks out then try a new sensor (scrap yard?) before replacing the Instrument cluster.  I don't think this is an easy fix, but try the sensor first because it's the easiest and cheapest to do. Good luck.

[Greybeard]

Correction! The other side of the gauge will probably go to 12v, not earth. Don't know what I was thinking!

[chrisAB]

Thanks so much for your quick reply,

I just got the multimeter out again to test continuity as you advised and measured the voltage across a few parts with the engine on - it read 19-21v!!

So I now suspect it is the alternator that is feeding an overly high voltage into the gauge and pushing it's reading sky high.

Looks like a new alternator is needed.

Hopefully this will help someone else if they get a similar problem..

[Corollanutter]

If those are the real voltages seen then it has fried your Battery too. I'm surprised it hasn't popped your bulbs. 

[APS]

The gauge temp sensor should be the one with a single wire going to it (the sensor with two wires is for the ECU). Disconnecting the connector from the gauge sensor should give you a zero (cold) reading on the gauge. Earthing the wire in the sensor connector should give a max (hot) reading on the gauge. 

Essentially, the temperature gauge is an Ohm meter that just indicates resistance to ground. The hotter the engine, the less resistance in the sensor between the sender pin and ground.

If the two tests above are true, then you most likely have a faulty sensor with too little resistance. It's worth trying a new one. They are cheap and pretty straightforward to replace.

 

[flash22]

You have 2 sensors there, one is coolant temperature sensor (ECU - green sensor) and the other is engine temperature sensor (Gauge - grey sensor)

you may have another sensor in the front of the block

 

you need to check your voltage at the Battery off and running

[chrisAB]

Thanks so much for advice thus far all, that makes things clearer.

I have changed the alternator which now reads 14.4v - one bulb had indeed blown. However this has not resolved the gauge problem.

I ordered a new gauge sensor (grey connector) and installed as @Greybeard suggested and performed the test @APS suggested. Pictures attatched of the gauge position in 3 circumstances with the key turned two clicks and the engine warm.

With gauge sensor disconnected the gauge reads about half

With gauge sensor connected it reads past the red

With gauge sensor earthed it reads past the red

I didn't get pictures but if I start the engine from cold it moves from the half way point up to the just below the red, then goes above the red as above after a few minutes.

Any ideas would be very appreciated. Could it be a fault with the gauge itself?

[flash22]

i would remove the gauge cluster, unplug it and plug it back in a few times (cleans the contacts), dust and dirt can build up in/around the plugs and sockets, the contacts get tarnished and can cause all sorts of issues

[chrisAB]

Fixed! Hopefully this helps if someone has the same problem.

I had to remove the instrument panel by removing steering wheel, airbag, plastic surround etc following the Haynes manual.

When I got the panel out I removed these 8 screws (visibly removed below) to take off the temp/fuel gauges from the main panel.

The panel here:

The problem was that a terribly soldered resistor had become fully disconnected from the heat as visible in the following image. These four resistors define the position of the gauge.

I resoldered the resistors and improved soldering on other weak joints and now it is working properly. Thank you everyone for your help!

[flash22]

oh a real old school single sided pcb, its got a little warm, 26 years of heat cycling causes the odd dry/cracked joint

[APS]

1 hour ago, chrisAB said:

The problem was that a terribly soldered resistor had become fully disconnected from the heat as visible in the following image. These four resistors define the position of the gauge.

Ha, I was just about to suggest, either a poor connection (as Bob said), or a broken PCB/component. Considering how they've mounted those resistors it looks like they expect them to get hot. Indeed, very old-skool. But it works. 

Nice work and thanks for sharing!

[Cyker]

Woo well done for figuring that one out!!

And also thanks for taking the time to document it!