Temp Gauge Mystery Update

[Mybikesacx500]

Rather than dig up my old post, the story so far:-

My temp gauge developed the habit of working fine for 20 minutes driving, then the needle would go past full scale, right up to the top.

I replaced the temp. sender, as my useless faulty multimeter measured it as very low resistance

Here's the update

I got a replacement (dull needles) full instrument cluster from Benfur, swoped the main cct board into my cluster . and put it back in the car

20 minutes later, the temp gauge goes to full scale, Hmm, so ok, its not the temp sender, or the main cct board.

I put the replacement cluster, with my original cct board in the car, reasoning the fault was still in the original cluster, and that both cct boards were good, and I wanted to prove this

by seeing the temp gauge behaving itself.

20 minute later...... yep, full scale again

SO, the fault is not the temp sender, nor is it anything in my original, or replacement cluster. (It's not a short in the cable from the sender, pulling it of the sender drops the needle)

My assumption was the wire from the sender went direct to the instrument cluster, but it seems it must be going via some other bit of circuitry that develops too big an output, but still drops to zero when the connector from the sender is disconnected, seems unlikely, but it's the only explanation that fits, unless I am missing something.

The warm sensor measures 110ohms, the voltage back up the sender connector is 10.8V, when I then reconnected it, the gauge showed the correct (cooling engine) reading, and the voltage at the (connected) connector was 6 v

Confused? I am Any comments gratefully received

Points arrising- the cowl round the instrument cluster has 3 very tight clips along the bottom edge, its a straight pull towards you.

Cleaning the inside of the cluster plastics makes the instruments brighter, as does carefully wiping the needles themselves

[Ruud]

Hi Mate,

Not a clue about temp issue :)

But it was nice meeting you at the Scottish gather today :)

[ROYT]

Rather than dig up my old post, the story so far:-

My temp gauge developed the habit of working fine for 20 minutes driving, then the needle would go past full scale, right up to the top.

I replaced the temp. sender, as my useless faulty multimeter measured it as very low resistance

Here's the update

I got a replacement (dull needles) full instrument cluster from Benfur, swoped the main cct board into my cluster . and put it back in the car

20 minutes later, the temp gauge goes to full scale, Hmm, so ok, its not the temp sender, or the main cct board.

I put the replacement cluster, with my original cct board in the car, reasoning the fault was still in the original cluster, and that both cct boards were good, and I wanted to prove this

by seeing the temp gauge behaving itself.

20 minute later...... yep, full scale again

SO, the fault is not the temp sender, nor is it anything in my original, or replacement cluster. (It's not a short in the cable from the sender, pulling it of the sender drops the needle)

My assumption was the wire from the sender went direct to the instrument cluster, but it seems it must be going via some other bit of circuitry that develops too big an output, but still drops to zero when the connector from the sender is disconnected, seems unlikely, but it's the only explanation that fits, unless I am missing something.

The warm sensor measures 110ohms, the voltage back up the sender connector is 10.8V, when I then reconnected it, the gauge showed the correct (cooling engine) reading, and the voltage at the (connected) connector was 6 v

Confused? I am Any comments gratefully received

Points arrising- the cowl round the instrument cluster has 3 very tight clips along the bottom edge, its a straight pull towards you.

Cleaning the inside of the cluster plastics makes the instruments brighter, as does carefully wiping the needles themselves

If the gauge goes to hot, the resistance at the sender should be low approx 25 Ohms, according

to the manual.

If all that checks out, are you being misled by a faulty thermostat stuck shut, and the gauge is

showing correctly?

Just a thought....

[SteveLS400]

but it seems it must be going via some other bit of circuitry that develops too big an output, but still drops to zero when the connector from the sender is disconnected, seems unlikely, but it's the only explanation that fits, unless I am missing something.

My next step was going to be replacement cluster - so you've probably saved me the hassle, thank you.

It does seem logical that there is some other impediment. I can't see this circuit needing a relay; so what could it be?

Equally confused.

[Mybikesacx500]

Hi Rudd,

yes, the 1st Scottish meet went well, I thought, decent venue, no rain, good chat about each others cars, hope you get your heater sorted.

Here's a thought, put a garden hose in the engine fill point, take the hose off at the heater valve, make sure water is coming through both the removed hose and the

valve, that should shift any air bubbles

Hi Mate,

Not a clue about temp issue :)

But it was nice meeting you at the Scottish gather today :)

[Mybikesacx500]

With the engine warm ie 15 min after a run, the reading was 110ohms, which seems about right, I think. This had the gauge reading full scale, but after a bit of measuring, it was back to 1/3 reading, beats me..I did start wondering if the engine really was overheating, but the needle is kinda erratic as it looses the plot, just doesn't seem to be a result of a real temp change

Rather than dig up my old post, the story so far:-

My temp gauge developed the habit of working fine for 20 minutes driving, then the needle would go past full scale, right up to the top.

I replaced the temp. sender, as my useless faulty multimeter measured it as very low resistance

Here's the update

I got a replacement (dull needles) full instrument cluster from Benfur, swoped the main cct board into my cluster . and put it back in the car

20 minutes later, the temp gauge goes to full scale, Hmm, so ok, its not the temp sender, or the main cct board.

I put the replacement cluster, with my original cct board in the car, reasoning the fault was still in the original cluster, and that both cct boards were good, and I wanted to prove this

by seeing the temp gauge behaving itself.

20 minute later...... yep, full scale again

SO, the fault is not the temp sender, nor is it anything in my original, or replacement cluster. (It's not a short in the cable from the sender, pulling it of the sender drops the needle)

My assumption was the wire from the sender went direct to the instrument cluster, but it seems it must be going via some other bit of circuitry that develops too big an output, but still drops to zero when the connector from the sender is disconnected, seems unlikely, but it's the only explanation that fits, unless I am missing something.

The warm sensor measures 110ohms, the voltage back up the sender connector is 10.8V, when I then reconnected it, the gauge showed the correct (cooling engine) reading, and the voltage at the (connected) connector was 6 v

Confused? I am Any comments gratefully received

Points arrising- the cowl round the instrument cluster has 3 very tight clips along the bottom edge, its a straight pull towards you.

Cleaning the inside of the cluster plastics makes the instruments brighter, as does carefully wiping the needles themselves

If the gauge goes to hot, the resistance at the sender should be low approx 25 Ohms, according

to the manual.

If all that checks out, are you being misled by a faulty thermostat stuck shut, and the gauge is

showing correctly?

Just a thought....

[Mybikesacx500]

Right, so mine works for a while, then goes full scale when connected to sender, and drops down when disconnected

Yours wont move when connected, but goes full scale when grounded.

Both issues would suggest there is something more than a bit of wire between the sender and the gauge

How about the Lexus engineers put in some kind of "averaging" circuit to keep the gauge reading steady but indicative of the actual temp?

Circuit diagram, anyone?

Are the circuit diagrams on line anywhere?

but it seems it must be going via some other bit of circuitry that develops too big an output, but still drops to zero when the connector from the sender is disconnected, seems unlikely, but it's the only explanation that fits, unless I am missing something.

My next step was going to be replacement cluster - so you've probably saved me the hassle, thank you.

It does seem logical that there is some other impediment. I can't see this circuit needing a relay; so what could it be?

Equally confused.

[Ruud]

Hi Rudd,

yes, the 1st Scottish meet went well, I thought, decent venue, no rain, good chat about each others cars, hope you get your heater sorted.

Here's a thought, put a garden hose in the engine fill point, take the hose off at the heater valve, make sure water is coming through both the removed hose and the

valve, that should shift any air bubbles

Hi Mate,

Not a clue about temp issue :)

But it was nice meeting you at the Scottish gather today :)

Will give that a try, thanks

[ROYT]

Right, so mine works for a while, then goes full scale when connected to sender, and drops down when disconnected

Yours wont move when connected, but goes full scale when grounded.

Both issues would suggest there is something more than a bit of wire between the sender and the gauge

How about the Lexus engineers put in some kind of "averaging" circuit to keep the gauge reading steady but indicative of the actual temp?

Circuit diagram, anyone?

Are the circuit diagrams on line anywhere?

but it seems it must be going via some other bit of circuitry that develops too big an output, but still drops to zero when the connector from the sender is disconnected, seems unlikely, but it's the only explanation that fits, unless I am missing something.

My next step was going to be replacement cluster - so you've probably saved me the hassle, thank you.

It does seem logical that there is some other impediment. I can't see this circuit needing a relay; so what could it be?

Equally confused.

Just to add to the confusion re Temp. gauge mystery, I have looked at the wiring diagram.

It shows a yellow and green wire from the temp. sensor to the gauge VIA A RESISTOR on the board.

This diagam is marked 'combination meter for Europe' and has wiring colours.

HOWEVER further on in the book there is a large fold out NON COLOUR section marked 'combination meter for RHD' this shows the yellow and green going straight to the gauge WITHOUT the resistor.

PICK ONE!!

[SteveLS400]

Right, so mine works for a while, then goes full scale when connected to sender, and drops down when disconnected

Yours wont move when connected, but goes full scale when grounded.

Both issues would suggest there is something more than a bit of wire between the sender and the gauge

How about the Lexus engineers put in some kind of "averaging" circuit to keep the gauge reading steady but indicative of the actual temp?

Circuit diagram, anyone?

Are the circuit diagrams on line anywhere?

but it seems it must be going via some other bit of circuitry that develops too big an output, but still drops to zero when the connector from the sender is disconnected, seems unlikely, but it's the only explanation that fits, unless I am missing something.

My next step was going to be replacement cluster - so you've probably saved me the hassle, thank you.

It does seem logical that there is some other impediment. I can't see this circuit needing a relay; so what could it be?

Equally confused.

Just to add to the confusion re Temp. gauge mystery, I have looked at the wiring diagram.

It shows a yellow and green wire from the temp. sensor to the gauge VIA A RESISTOR on the board.

This diagam is marked 'combination meter for Europe' and has wiring colours.

HOWEVER further on in the book there is a large fold out NON COLOUR section marked 'combination meter for RHD' this shows the yellow and green going straight to the gauge WITHOUT the resistor.

PICK ONE!!

Its right about the wire colour!

I can't see why the LHD & RHD would be different on something as (in theory) simple as the temp gauge. The idea of a failed or failing resistor does have some merit. I will give this some thought, although I'm not demolishing any more of the car until I've solved the radio issue.

[Mybikesacx500]

Thanks Roy for having a look at the diagram, that's my theory of something between the sensor and the gauge out the window, if all there might be is a resistor on the instrument PCB . I have tried variations of 2 sensors and 2 instrument boards and 2 instrument clusters, and the problem is still there. Stumped for the moment

I emailed the PCB makers, asking for a diagram of the instrument cluster board, but not to surprised to have had no response

It shows a yellow and green wire from the temp. sensor to the gauge VIA A RESISTOR on the board.

This diagam is marked 'combination meter for Europe' and has wiring colours.

HOWEVER further on in the book there is a large fold out NON COLOUR section marked 'combination meter for RHD' this shows the yellow and green going straight to the gauge WITHOUT the resistor.

PICK ONE!!