FJBen
SILVER Star
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- #2,661
Hi
I have my external gauge (Motometer, which is the low cost brand of VDO) plugged to the OEM temp sensor. The reading might not be baselined, but the relative reading is extremely accurate. On a cold start, I can even notice the thermostat starting to operate: Temp goes up, then temp goes down 3° as it opens and cold coolant from the radiator goes in circuit, which makes it close a bit, goes up again, opens again ...
Eventually it levels at 82ish °, which is pretty close to my thermostat spec.
I'm very sure I'd notice a significant coolant loss, as no media around the sensor would certainly cause an instant temperature deviation of some sort.
My cluster gauge is 'working', but it anyways barely ever exceeded the middle position, even not when the engine definitely ran hot. So I don't mind to just disconnect it and switch the sensor to the external gauge.
Probably you can also plug your gauge to the OEM sender and see what it does?
Cheers Ralf
Great info. Thanks!
I would also assume the loss of coolant would trigger some movement on the factory gauge but judging by what others have experienced, you might not see it or it might be so subtle it would be ignored.
So you got me thinking about the factory gauges we have. The specs for the senders work off of resistance.
My Auber gauge can read a NTC (negative temperature coefficient) thermistor. Which means as temp goes up, resistance goes down.
According to Auber:
Only the 50°C to 150°C temperature sensors made by VDO can be read by Aube’s automotive gauges (SYL‐1813 and SYL‐1813‐MR). This category of VDO sensors are thermistors with negative temperature coefficient (NTC). Their resistance values decrease from 323 ohm to 18 ohm as the temperature increases from 50°C to 150°C. Auber’s gauge will measure the resistance value of the VDO sensor and convert it to the corresponding temperature value.
Here’s where it gets vague. It says converts to temp and they give this chart:
So at 50c: 122F
Oem = 226ohm +/- 33
VDO = 350ohm
So my OEM sender would show around 60c/140F (ish) so 10c hotter maybe…

On the high end: 115c/239f
Oem sender: 26 ohms
VDO: 40ohms
So the Auber gauge would read 26ohms as 139c/282f. Yikes. While reading hot is better than reading cool, I need to see if there is an offset I can program or maybe even a resistor added in to change that value?
I’m not electrically smart enough off the top of my head to figure that out since it’s a greater amount at higher resistance.
That or just add in a sensor in the radiator hose that the Auber reads correctly
And for all of you that don’t read my thread for the articles…
Also found this photo pop up on my phone from a few years ago. I still like the 255s and oem rims.