Oil Temp Gauge Calibration

[Randy in Maine]

Has anyone seen this post from over at Pelican? I am not sure if this is really the way to do it or not. I often wonder how acurate the VDO gauges and sending units really are. I like Richard's thought of putting the sending unit in hot canola oil of a known temp and seeing what it measures at about 200 degrees.

Thoughts?


Calibrating Oil Temp Gauge & Sender

Note, this is not as accurate as sending you gauge and sender to a calibration shop, but is much faster and cheaper too. The VDO oil temp gauge in my race car was made in 1978 and when the engine blew at the CA speedway this past June I lost the calibrated sender. I tried 5 or 6 different ones including one VDO said was sure to be right on but thay all caused the gauge to read 44 degrees high. I measured the oil return to the dry sump tank with no air flow through the cooler and determined that 140 oil was reading 185 on the gauge and 180 oil was reading 220 on the gauge. So how to match them??? Use a variable resistor or pot (2k ohm) and connect it between the signal connection on the gauge and the sensor line. Set the pot to read 0 ohms and then start and warm up the engine and monitor the gauge temp and the actual oil temp and when you start getting over 120 oil temps, start adjusting the pot so the gauge will read the same. I once you are finished, usually at 180 or so oil temp and the gauge matches, shut off the car, unhook the pot and read the resistance it has. Get a resistor that is close and solder a pair of lugs on each pigtail and cover with heat shrink tubing. Install between the sender wire and the gauge and it should read pretty close not. Note that oil temp gauges do not have a linear resistance but go in steps but an increase of total resistance will make all marks on the gauge read closer. My setup needed a 800 ohm resistor to make the gauge and actual temps match. Good luck

[ratwell]

I actually got around to doing in last Sunday. See my disturbing reply at Pelican's 914 board.

View the difference. The number on the left of the thermometer is the oil temp. The number on the right is the alarm so ignore it. Every mark on the VDO gauge read 40 degrees higher on the digital thermometer as I increased the temp of the oil.

I'm pretty sure folks just slap these things in and debate the fact that the location is all that matters. The type of sender used is what matters equally and if yours is slow to respond like the VDO units then you're not going to catch high temps driving by the gauges unless you watch them constantly. Even if you do you'll only be able to guess the temp unless you foolishly push the engine harder and keep it there.

I'd like to find a solution to this problem that still uses the VDO gauge. Anyone?

I'm going to test the VDO oil pressure sender against a mechanical gauge as soon as I locate a gauge along with the correct fittings to have both hooked in at once (like Bentley shows). Hopefully it will perform better. Berg says otherwise.
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