Oil temperature, dipsticks and sensors

[furgo]

Bear with me for the intro if you're interested in oil temp sensors and gauges, or skip to the dipstick part...

My bus came with a dipstick temp sensor, which by now I'm pretty certain has given up the ghost.

At some point while driving a few weeks ago I noticed the oil temp gauge was stuck on the far left, and it wouldn't move. After checking all electrical connections, I ended up removing the gauge and the sensor.

I connected the gauge to my 12V bench power supply, and replaced the sensor input with a 1K-Ohm potentiometer. I managed to move it through its whole scale (50 - 150 °C) roughly from 200 Ohm to 25 Ohm. The main issue here is that it's rather difficult to come across information or datasheets from VDO, but comparing these values with some resistance vs. temp tables I found online, they seem reasonable. At that point, I was not looking for accuracy, but just to see whether the gauge was working at all, which I concluded it did.

Regarding the dipstick sender, I've not come across any other one like that except for one single thread on the forums, but no other info whatsoever. It's a genuine VDO part, but with no part number I could find:

Not in the picture is the adjustable dipstick cover, which I incidentally just found next to cylinder #3. Sometimes one just wonders what POs were up to.

In any case, I did measured the sensor's resistance by heating up water (hence my measurements don't go beyond its boiling point). Comparing them with the datasheets I found, my recorded values are way off specs:

I thought thermistors were quite robust components, so I don't know what could have caused it to fail so spectacularly. I would have expected just some drift with age, or an open/short, but not this sudden change in the curve.

I cleaned up the contacts and double-checked the resistance at different points in the sensor, and I even took its case apart. Same result: at low temperatures the sensor goes in the K-Ohm range, where it should be in the Ohm band.

At this point, I'm just looking at replacing the dipstick. I know there is a better location to measure the oil temp by replacing or modding the inspection plate underneath the dipstick, but I might tackle that when I've got the time.

Being based in Europe, here are the options I found:

Type 1 oil dipstick sender (435 mm - 17 1/8" )
http://www.justkampers.de/5573-oil-temperature-dip...gines.html

Type 4 oil dipstick sender (615 mm - 24 7/32")
http://www.justkampers.de/000001-dipstick-oil-temp...gines.html

Notice that the sender seems to be dependent on the type of engine. Other than the different length, I do not know what makes a sender better for a particular type of engine. I phoned 3 different suppliers and none could tell me the reason why or the actual limiting difference.

My old sender did go about 330 mm (13") into the dipstick tube, so the Type 1 435 mm sender would do nicely. I can't imagine the recommended 24" Type 4 dipstick not getting in the way, so I'd rather not use it.

Doing some more research (did I say how difficult it is to come across some minimal technical info on VDO products?), it seems VDO produced these two dipstick senders at some point:

• X10-323-003-001 C - Dipstick-Type Oil Temperature sensors 435 mm
• X10-323-003-002 C - Dipstick-Type Oil Temperature sensors 615 mm

This (undated) VDO sender datasheet mentions them being removed from their product line. Neither did I find any info about them on the VDO site, so I assume that VDO does no longer makes them and the new ones available are just copies made from another company. The bit that mostly interested me though, was that they used the same sensor at the tip of the dipstick. So I conclude all these dipsticks are the same, simply with different lengths.

In summary, I think I'll just order the Type 1 short dipstick sender and I'll try it. But perhaps someone can enlighten me about what makes Type 1 vs Type 4 dipstick tubes so different? (I don't have a Type 1 engine to compare).

Thanks!
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'79 Westy, P22 interior, FI 2.0 l Federal, GE engine (hydraulic lifters)

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[busdaddy]

The tube diameter is the same, the difference is the lengths, you'll also find a difference on type 4's depending on where the dipstick is located (through shroud or on the top of the case). Here's what the through the shroud stick should measure: https://www.thesamba.com/vw/forum/viewtopic.php?t=446925
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[furgo]

[Wildthings]

I don't have a particular product to recommend, but a surface temperaturer sender attached to the top of the block would give you a very good representation of the oil temperature. No wires hanging off the bottom of the rig, nothing extra to deal with each time you check the oil, etc.

Would be a more than a bit hard to install with all the FI stuff in place though.

[kreemoweet]

I have one of those VDO dipstick senders, and it's in good working order. The resistance curve goes from about 600 (or more) ohms at room
temp to about 35 ohms at 250*F. I don't think any of the senders discussed above is for a Type I VW engine, because the lengths are all
WAY too long. The one I have is a perfect replacement for the stock dispstick, and the L1 length (as shown in spec diagram above) is 6.275 in. = 162 mm.

I'm sure VDO made those dipstick sensors to fit a wide variety of engines, using a wide variety of gauges with differing response curves. I don't think
there's necessarily anything wrong with the one you have - it just wasn't made for VW engines or the gauges usually used in them.

On my 71 bus with stock motor, I've had a number of different sensors/gauges installed in various places, and I've never had a significant discrepancy on them
from the temp measured down the dipstick tube with a lab-grade glass thermometer.
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[furgo]

[kreemoweet]

Oops, I missed that the gauge/sender combo had been working OK before.

My sensor curve is probably what is described in the VDO spec sheet you posted, and it's evidently intended for use with a 120*C(250*F gauge). That wouldn't be
applicable (with any sort of accuracy) to your 150*C (300*F) gauge. VDO makes/made sensors with a lot of different curves, and a lot of them have been discontinued. A lot of
sensor types also seem to have only, or largely, been sold only in certain areas of the world.
The sensor doesn't even have to be from VDO, as long as the temp/ohm curve is correct for your gauge. You can also insert precision resistors in parallel or series to "adjust" the
curve to better suit your gauge.

I assume you undid those binding screws inside the sensor head and checked for corrosion/hi resistance there? Tried using new wire with the sensor, to rule out invisible
strand damage in the middle of one of the existing wires? Is that connector at the end of the sensor wires polarized? Correct polarity is mandatory with those things, I believe.
_________________
'67 bug: seized by the authorities
'68 bug: seized by the authorities
'71 kombi: not yet seized by the authorities

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[furgo]

[furgo]

The new dipstick oil temp sensor arrived and I've been doing some tests. First, the sensor itself:

• I cannot see any "VDO" inscription on it (the original had it stamped). Neither I can see it offered on the VDO site. Thus I conclude it's a reproduction of the original VDO X10-323-003-001, no longer available. Update: (*)
• Length L2 (bottom of the case to bottom of the rod) is 350 mm, length L3 (top of the case to bottom of the rod) is 435 mm.
• In any case, slightly longer than the stock dipstick, as expected
• The quality is acceptable. I might do a slight mod by replacing the felt bit by cork, as otherwise it will quickly become an oily mess.

These confirm what I guessed in the other posts, most importantly, that dimension-wise it is a direct replacement for my old sensor.

I did some measurements in the 120 F - 212 F range (50°C - 100°C) and the combo sensor dipstick and VDO gauge were quite accurate and responsive when comparing readings with my digital multimeter in temp mode. I had a go at measuring higher temperatures by heating oil, but it didn't work that well (too little oil, also it heated up too quickly for accurate measuring). In any case, the sensor seems to match my old gauge, which is good.

I'm going to add the min/max marks to the dipstick sensor and keep the stock dipstick as a backup only, as it's a bit of a pain to keep changing dipsticks any time you want to measure the oil level.

I did not manage to find any part number or specs for the (Type 4) dipstick, so I ended up measuring its dimensions. Here they are in case someone finds them useful. Update: I believe the part number is 021-115-611 E

Or in imperial: