Oil temp gauge/sender question

I had the same trouble when I used a drain plug sender on my beetle. It never did register more than 120 degrees. Apparently the airflow across the base of the engine prevents the sensor from giving a correct indication. I switched to the dipstick sender and obtained more believeable results(180-190 on a cool day, 210 on up for highway/hot days).

Chris

Chris, those readiings are right inline with my temp readings *with* the drainplug sender....i have in the past had both bad guages and senders that did not register correctly...even new ones...

What is the hottest temp that you have seen and still run. The reason is that mine has been running between 250 and 280. It seems to be way to high but I have never used an oil temp gage before. I use castrol 20-50w oil and have used it in everything that I own since about 1973. It is supposed to be able to take the heat and not break down like some other oils.

if your guage is accurate you should *never* see 280F....i say this because if you by chance see the 250 you weren't paying attention and shutting it down around 240-250 MAX

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Dear Karls,

The basic answer is 'no,' assuming the length in question is limited to the distance from the engine to the dashboard in a bug or bus.

Prior to installing any instrument it's a good idea to verify that it works and that it is reasonably accurate. With oil temp senders the usual method is to provide proper electrical connections, such as attaching a ground lead to a spare sump plate into which the VDO type sensor is installed, and to then suspend the sump plate in a pan of oil. A thermometer of known accuracy measures the temperature of the oil as it is heated and the readings are compared to those on the VDO gauge.

For the purpose of the test the sensor is connected to the gauge by fifteen feet of wire, wound into a coil for ease of handling. For your own information you may wish to insert a longer length of wire to give you some idea of the potential difference in the readings. You will find that for wire gauge of #16 or larger, there is no change even with

100 feet of wire.

However, you should also be aware of the fact that Volkswagen issued a number of Service Notes with regard to the VDO instrument cluster installed as an after-market item by many VW dealers, warning them that the instruments did NOT accurately reflect the state of the engine.

With regard to the sump-mounted oil temp sensor readings, with the vehicle in motion the temperature was about 100 degrees Fahrenheit LESS than the core temperature of the oil in the sump, and as much as 150 degrees less than oil in the valve galleries. That's because the VDO after-market sensor is in contact only with the relatively cool, stagnant layer of oil under the strainer and the sensor is exposed to the air flowing under the vehicle when in motion.

When required to provide a fully instrumented engine, Volkswagen installed the oil temperature sensor at the inlet to the oil pump, thus bathing the sensor in an active flow of oil drawn from the center of the sump.

In the same vein, sensing cylinder heat temperature (CHT) with a ring-type thermocouple installed under a spark plug also results in a wide variation of temperatures from one engine to the next due to variations in torque, the condition of the spark plug air seal and so forth. When ACCURATE cylinder head temperture is required, as with the later fuel-injection systems, Volkswagen provided a threaded lug adjacent to the #1 exhaust stack. A Service Bulletin explained how to fabricate a copper gasket with a similar lug, allowing early-model heads to be used on the later-model, FI engines.

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When fitted with instruments, the oil temperature gauge on VW industrial engines had no numerical markings. The gauge was divided into three arcs: Green, yellow and red. As mentioned above, the sensor was installed at the inlet to the oil pump.

By comparing the guage to a calibrated Stewart-Warner oil temp gauge I found the green arc (ie, normal oil temp) went to about 220 degrees, yellow to about 270 and red beyond that.

As a point of interest, the normal oil temperature of a 2180cc engine installed in my 1965 bus is about 250*F. Given that the oil galleries are running hotter than the temp measured at the pump, since compounded motor oils (ie, 20W-50, etc.) begin to break down at about 300*F I found it best to run straight-weight oil (ie, 30W for the cooler months, 40W for hot weather).

(Regardless of your instrument readings, if you see varnish beginning to build up in the valve gallery near your exhaust valves it's good evidence that the temperature in that region [which is the hottest part of your engine] is a above the break-down point of whatever oil you are using.)

-Bob Hoover

Thanks Bob for the best info that I have ever seen on oil temp's and what to watch out for. It was written so that I could understand what it was all about.

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Thanks a lot Bob, Your info has really cleared up my doubts.

I'll run some tests to see if everything is working well.

Thanks again, Karls

Hi! Locating the sender at the sump drain plug hole will give lower temp reading than what the oil temp is at the heads or at the cylinders.

I have a VDO cockpit oil temp gauge. I use this special multiple adapter from CB Performance for my VDO oil temp sender.

The multiple adapter screws into the case where the dash oil pressure warning light sender goes.

The oil temp sender and oil pressure switch sender screw into the multiple adapter.

I get the same readings as when I used to use the VDO oil temp dipstick.

I did not like having to always be extra careful with the oil temp dipstick.

I measured the oil temp at the dipstick hole with an air conditioning thermometer. It looks like a dial gauge with a metal stick on the bottom. When it read 180 degrees F, the VDO oil temp pointer was on the 1 at the 180 degree F mark. So I think my VDO oil temp gauge reads a little cooler than what the oil temp really is.