Incorrect Temperature gauge reading

[puddin]

Evening All

Yet another temperature gauge question, a bit long winded, please bear with me.

The temp gauge is sitting just shy of 3/4, there is no evidence of actual overheating, the engine "seems" to be at the correct temperature, fan kicks in and out as it should, however gauge reading is a worry and needs to be fixed.

Using the gauge engine coolant sensor wires I started by placing a 100 ohm resistor in place of the coolant sensor, this drove the needle to about 1mm to the left of dead center. Bingo..... Duff Sensor, I thought....

So, I removed both sensors and compared values, I got vastly different readings from both so I decided to replace both sensors, originally with GSF units and have again replaced the gauge sensor with a Meyle unit from VWHeritage. Replacing the sensors didn't help.

I next looked at module 43, I tried removing it and swapping with replacement. No difference. Time to look at the gauge.

The gauge unit is receiving 10v from the dash voltage regulator. I have removed, cleaned gauge and contacts to the printed membrane.
I have also tried disconnecting the sensor wire from the dash pod multiplug and running a new wire directly from the lug of the gauge to the sensor, got same reading.
The gauge seems to work fine in other respects, the LED light flashes at startup and will also flash if the coolant level sensor is disconnected.

Sensor values
The resistance values of both coolant sensors at ambient 18 degrees are pretty close at around 1575 -1580 ohms.
At what I believe to be normal engine operating temperature both sensors are reading 107 and 105 ohms, again pretty close.
Left running for another 10 mins or so and the fan eventually kicks in, at which time the sensor reading is 95 ohms, increasing to 108 ohms shortly after fan stops.
The impedence of the sensor earth to chassis is ~ .4ohm
The values lead me to believe both sensors are working correctly.

To re-cap, 100ohm resistor in place of sensor works fine, however a sensor at ~100ohm shows overheating?????

Any advice gratefully received, It makes no sense to me.


Kind regards
Tony

P.S - Coincidentally (or not?) issue seems to begin when I replaced Water pump (GSF) Now, I spent days checking for airlocks, tried different thermostat (New Brickwerks unit) etc before looking at the gauge operation.

1987 1.9DG.

[ghost123uk]

Tony, just to let you know your question is not being ignored. I guess no-one has any suggestions as you seem to have covered all the likely suspects very thoroughly (esp the sensors which do vary by manufacturer)

From all the tests you have done it seems the only thing left to suspect is the gauge itself, but it seems too much of a coincidence that it happened at the SAME TIME as you changed the pump, (though from the symptoms it is very unlikely to be the actual pump causing it). Fan belt not slipping ? (shooting in the dark !!)

(p.s. = control box 43 is only for the low coolant sensor circuit so it will not affect the gauge reading.)

[puddin]

No worries Ghost, any help and feedback is always appreciated.

Cheers

[MartinR]

My temp gauge sat at the 3/4 mark for a good two years, and despite me eying it with some suspicion there were no other problems or symtoms - like you fans kicked in and out etc. I then had reason to strip out the entire dash area and cleaned up all contact and earths on reassembly. Since then temp gauge has settled far more comfortably, just nipping the right hand side of the light.

So in my case, though there was nothing obvious, looks like it was connection cleaning was the issue.

[puddin]

Thanks Martin,

I'll probably take another look at the gauge itself at the weekend, may even try and recalibrate it with some additional resistance as a last resort!

Tony

[bushtrack]

Hi All !

Just joined the group and am really pleased that I'm not alone with the temp guage quandary...was starting to blow my head apart as I'd done pretty much everything there was to do to get the temp reading down with no avail. Will do the same with the connections and hope that will cure my problem!

Only hope I can be of help in the future for other problems with 25's.

Quick tip on bleeding the system...drill out the bleed screw fitted to the rad with an 1/8th drill bit about a 1/4 inch from the head and then cross drill. Saves scolding fingers and makes bleeding easier if youre doing by yourself.

Cheers all

[AngeloEvs]

The only thing I can add is that the rad fan is controlled by its own sensor so the temp gauge will be registering higher than normal at this point. A sensor suspended in a pan of boiling water should read about 100 ohms and the critical resistance which triggers the LED to flash is 50ohms. I would be inclined to try a 82 degree stat that also have a small bleed hole in the flange (or drill one yourself) and see how it influnces the gauge reading under normal driving conditions ( I have had one for the last five years and the gauge sits below the LED). Too me it seems like a coolant issue especially since this has started since replacing the water pump.

[dansimpson]

My gauge used to read too low, just on the white bit, I suspected the sensor, which when changed did not alter the reading. When I checked with a meter, the bit where the sensor goes on the thermostat housing to the rest of the engine showed a resistance of about 10 ohms, not alot I know, but when I connected a separate earth lead from there to the engine block, the gauge rose to just under halfway.
Just picking up on the changing water pump you mentioned, as mine had been done before I bought the van. Since then i've had the thermostat housing off, and since I put it back on the gauge now sits just above the red LED. I wonder if sometimes the resistance to earth may increase if maybe some sealant gets on the mating faces??
I dont worry too much about it, as the fan doesnt kick in unless we sit in traffic for a couple of minutes, so I've just got used to the "new" normal reading.
I'm not totally sure what it should read when normal, as my car reads about 3/4 when up to temp anyway.

[Evsi]

Mine is the same on my JX.

Again, only sat at 3/4's after the engine was rebuilt. I also tried everything to correct it including drilling bleed holes in the thermostat, but it's stayed the same. Engine seems fine though and i've been to France and back since.

[Epiphone]

Hi

ive lost my coolant twice and had it replaced twice, and each time different temp readings

last time, the garage put in to -50 degrees, so I reduced it down by emptying the header tank a few times. and the temp did drop, but still sits high.

Bought a oil cooler and will install a temp gauge with that.

Might put in a lower degrees opening thermostat also

My bus runs well and have been on many holiday. But im also learning and playing also

[puddin]

Thanks for all the comments, I have decided to live with it until the holiday season is over, but I'll be keeping a close eye on it until then.


Kind regards

[puddin]

Update on this issue as I finally got around to this again.

I have never been happy that the gauge is sitting high.

I have now replaced almost everything in the cooling system (Temp Sensors, Dash Gauge, Radiator, Coolant, Water Pump, Thermostat, Thermostat housing) , I also had to rebuild the engine top end (Lesson Learned: Be very careful of the crappy Radiator Fan Earth )

I went as far as to mount on a plywood board a Voltage regulator and spare gauge to replicate the dash which I connected directly to the battery.

After all this the situation was still the same, gauge sitting at 3/4 in normal use.

The final step was to install a 80c thermostat, which GSF sorted for me (Part 176BM0120) happily this has now returned the gauge to the correct position.

I haven't (currently) got the enthusasim to continue the investigation as why I now need an 80c stat, everything was fine prior to the first waterpump change, maybe the new pump struggles with a partially open 87c stat?

Thanks for all the help along the way.
Tony

P.S
Got the cooling sorted then went to garage and filled up Petrol tank - Got home and found the tank has begun leaking from a corroded seam...... What a great hobby!!!!!