Coolant temp gauge replacement (late petrol)

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937carrera
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Re: Coolant temp gauge replacement (late petrol)

Post by 937carrera »

https://www.thesamba.com/vw/forum/viewt ... e+resistor

Have you seen this - some more data for you
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937carrera
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Re: Coolant temp gauge replacement (late petrol)

Post by 937carrera »

itchyfeet wrote:What is the purpose of the capacitor?
its not fitted in my guage out of dash test.
coincidence?

Funny, I also found this http://www.vanagonauts.com/Warning-Light-Fix241.htm
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Re: Coolant temp gauge replacement (late petrol)

Post by bigbadbob76 »

Just spotted this as I've been away.

My gauge went wonky after I did the engine re-build.
1st off it didn't read anything.
New sender fitted, it went full scale.
Found the sender wire was resting on the exhaust header, melting the insulation and earthing it'self.
Sorted that and all was well.

in your case-
The way the gauge works, the only way I can see you getting full scale is if you short the sender wire to earth, this could be the capacitor breaking down or a melted wire like mine.

I also got random flashing LED problems with normal gauge readings before I fitted Chris's replacement dash PCB. I think they were level sender wiring issues in the blue flexi PCB.
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Re: Coolant temp gauge replacement (late petrol)

Post by itchyfeet »

I took this broken gauge (that I thought I'd fixed) appart and removed the PCB so now I have just the gauge with two wires and no PCB

As the resistors I have are perhaps too small I tried some Aluminium clad resistor then some rheostats, using a 10V DC bench supply I measured the current throught the gauge and I'm still getting a higher current through with the sensor in hot water ( about 84 is around 100ohms) than with a fixed resistor of 100 Ohms.

:shock:
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Re: Coolant temp gauge replacement (late petrol)

Post by bigbadbob76 »

Just to rule out a possibility.... your sender is a thermistor right?
When testing my thermoswitch for the carb heater (also mounted in the thermostat housing) I've seen resistance with a digital mulitimeter during the switching phase.
This resistance doesn't show with a good old AVO 8 meter that actually drives some reasonable current through the switch.
If someone's put a thermoswitch in where a thermistor should be you'd get a full scale reading after a few mins as the coolant heats up to the switch temperature.
You'd then have a short to ground when the switch closes.
Just an Idea, but maybe worth checking.
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Re: Coolant temp gauge replacement (late petrol)

Post by itchyfeet »

Right sanity check tonight I have spend more than 10 hours so I'm calling it a day, I don't normally give up but It will have to wait for another time or not at all.

I set out trying to get a more accurate gauge and get the level sensor working which I could have done in much less than 10 hours of hot sunny weather.

I can fit a VDO sender for the VDO 52mm gauge and the level sensor works I just need to use a different wire for the VDO gauge and keep the original wire for the level sensor.

I Put the dash back tonight.

Thanks for all the help.
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itchyfeet
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Re: Coolant temp gauge replacement (late petrol)

Post by itchyfeet »

For anybody interested this is the gauge with PCB removed

ImageP1100642 by Paul_Barr, on Flickr
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937carrera
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Re: Coolant temp gauge replacement (late petrol)

Post by 937carrera »

Most unlike you Paul, you must be really hacked off with it.

Did this bit from the Samba thread make sense to you ? (or anyone else)

Seems that the current in the thermistor is heating it up and leading to high readings. I think that elsewhere there was comment that you needed a minimum 1 W resistors to avoid heat varying the measured resistance in them

kkuemmerle wrote:After giving my cooling system a full overhaul (all new hoses, new radiator, new thermostat, new temp sensor) I too had a jut a bit too hot reading. I tested my thermostat in hot water all seemed good. I tested my sensor in hot oil all seemed good. I tested my gauge with the recommended resistors all seems good. I ran my van idling in the driveway and monitored the resistance of the sensor with a multi-meter all seems good, using the temp vs. resistance curve I got with my hot oil testing the coolant was just right and the fan was cycling on at just the right temps. So what could be wrong? All the pieces work correctly when tested separately but not when plugged together. I think my problem is self heating of the temp sensor, the power applied to the temperature sensitive resistor for the gauge to work is heating up the sensor causing the gauge to read a bit high. The multimeter applies much less power to read the resistance then the gauge electronics. My theory is that in order for the applied power to be dissipated the sensing resistor ( pretty sure it is a 10,000 ohm thermistor) needs to be touching the metal part of the sensor that is in the coolant stream. I am guessing when sensor are made if the thermistor a bit away from the metal and then gets potted in the epoxy is is insulted from the coolant stream and any power applied to the thermistor heats it up a bit. To test the theory I built a buffer circuit that applies 1/100 the power to the thermistor and then amplified the output of the thermistor by 100 to get the correct input to the temp gauge. Viola all is working now. My gauge reads right in the middle. Below is some data from my testing.

Resistance of sensor vs temp of hot oil
Image

Graph of sensor resistance vs temp of hot oil with exponential curve fit.

Image

Graph of sensor resistance vs temp of hot oil near operating point of engine.

Image

Using a few resistors to load the gauge and measuring the current through the resistors the gauge seems to be about 55 ohms.

With 10 volts going through the gauge and the sensor at about 85 ohms (between 90 and 95 C for the coolant temp) the current in the sensor is then 10/(55 + 85) = 71 mA. The power in the thermistor is then 0.071^2*85 = 0.433 watts.

My circuit applies 0.1 volts through a 55 ohm resistor to the sensor then amplifies the voltage at the junction of the resistor and the sensor by 100 and applies that voltage to the gauge to pull the right current through the gauge to move the needle to the correct location.
Last edited by 937carrera on 07 Aug 2018, 06:41, edited 1 time in total.
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Re: Coolant temp gauge replacement (late petrol)

Post by itchyfeet »

937carrera wrote:, you must be really hacked off with it.


Yep

And I was stuck and going around in circles, so the best thing to do when you are stuck is to walk away have a complete rethink and come back to it later, also I really do have better things to do it has taken all my spare time for 2 weekends.

I want to use the the van so I just put it back as it was with the 52mm gauge on the OEM sensor.

Using a spare wire I can quickly get the level sensor circuit working by connecting it to the dash gauge with no sensor engine end ( I checked this works)

I have ordered another sensor and I will tinker with this old gauge.

It occured to me that dunking a sensor in hot water may damage it.

Still reluctant to remove the tin top dash, the pop top dash oil LED, high beam lamp and battery LED all went intermittant and I had to play to get them working, best left alone IMO, I don't want towo broken dash pods.
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Re: Coolant temp gauge replacement (late petrol)

Post by 937carrera »

itchyfeet wrote: Yep

And I was stuck and going around in circles, so the best thing to do when you are stuck is to walk away have a complete rethink and come back to it later.

Yep, amazing how many times a nights sleep allows you to see things you didn't see before.

I have continued my education, probably old stuff for you but I found a great description of how the system works over on the Samba (link is to the description, not the first post)

https://www.thesamba.com/vw/forum/viewt ... c&start=14

and of course our very own Wiki with contribution by AngeloEvs

http://wiki.club8090.co.uk/index.php/Co ... ht_on_dash

Both articles comment about the sender units not behaving correctly if the coolant mix is out of spec (probably something to do with electrical conductivity of the fluid mix as conductivity is used as a test parameter in water treatment / boiler systems)

Were you testing with 100% water, maybe worth using a coolant mix, just to eliminate that variable, even though it seems it is only an issue with more than 60% antifreeze .

As another thought, independently wire your spare gauge up to the sender unit in the tin top. No disturbance of either dash that way.
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Re: Coolant temp gauge replacement (late petrol)

Post by bigbadbob76 »

itchyfeet wrote:It occured to me that dunking a sensor in hot water may damage it.

Mine was fine before the engine seriously overheated but had died after the re-build so you may have a point there.
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Re: Coolant temp gauge replacement (late petrol)

Post by AngeloEvs »

The T25 coolant warning system is essentially in two parts. The level sender part and the temp sender part. Either of these can cause the LED to flash, but theoretically, only the Temp sender part should influence the gauge needle position.

Removing Module 43 removes the level sender part of the system whch is useful in diagnosing erroneous LED flashing. Module 43 outputs a sine wave voltage to the level sender, if the amplitude of that sine wave exceeds a threshold value (due to loss of fluid) then module 43 outputs short duration negative pulses every few seconds to trigger the Small circuit in the gauge. These pulses are short duration, low frequency and should not significantly affect the gauge position unless left to do so over a long period of time.

However, module 43 could become faulty and affect the needle position but rare.

The temp sender affects the gauge position and if the resistance falls below a critical threshold then the gauge internal circuit triggers the LED and the needle rises.

You tested the gauge whilst fitted into the pod, on a bench, used dummy loads to represent temperature variations and the gauge, pod wiring, regulator output all now appear to be ok.

With dash re connected and module 43 removed the gauge swings full scale which suggests the fault lies in the temp sender part of the circuit. Either the temp sender, associated wiring has developed a fault or DJ engine has something in the coolant warning system that I am not aware of.

The clue seems to be that at some point previously a third party temp gauge was installed and the question I ask is why? If it were me, I would be happy with the dash and focus on the temp sender part of the circuit and associated wiring. Might be worth just re-looking at the Fuel injected 2.1 cooling system schematic for anything that has been overlooked.
Last edited by AngeloEvs on 07 Aug 2018, 09:02, edited 1 time in total.
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Re: Coolant temp gauge replacement (late petrol)

Post by AngeloEvs »

Also, check the temp gauge and fuel gauge wiring order in the 14 way edge connector. The wires can easily be removed and re arranged to suit earlier dashpods, possible that a mistake has been made in the past due to dashpod changes.
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Re: Coolant temp gauge replacement (late petrol)

Post by 937carrera »

...but I think Pauls position will be that he is testing the gauge / sender independent of the van wiring with a 10V regulator in place providing the correct voltage. The needle swing / LED only happens with senders (2 of them), not fixed value resistors. :?: :?: :?:
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Re: Coolant temp gauge replacement (late petrol)

Post by AngeloEvs »

937carrera wrote:...but I think Pauls position will be that he is testing the gauge / sender independent of the van wiring with a 10V regulator in place providing the correct voltage. The needle swing / LED only happens with senders (2 of them), not fixed value resistors. :?: :?: :?:

I would get a replacement sender, re do the test. One of the senders was tested on the bench and, at 80 degrees, had the gauge virtually full scale. Either the gauge is duff, the temp was higher than measured or the sender is the incorrect type/faulty.

Using appropriate fixed resistors is how VW check calibration and is valid as a method. The sender currently fitted could be unstable or insulation breaking down as temp and current increases. A dash direct to the sender with independent 10v supply and wiring, and isolated from the rest of the system wiring suggests sender, pcb foil or gauge at fault, I cannon think of any other possbility.
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