Resistance <> Temp Chart for 78+ oil temp sender?

[silverlock]

Chasing down a possible oil temp sender or gauge issue and want to validate temp <> resistance of the sender.

Does anyone have the chart or the VDO part number?

P/N: 911-606-112-00 - https://www.pelicanparts.com/More_Info/91160611200.htm?pn=911-606-112-00-M100

I _think_ the VDO cross-reference is this one (200 deg C) but would love to get confirmation

[porschedude996]

I just went through a problem I had. Porsche (911 Air Cooled) used two scales on oil temp, top of scale 300 and 340F. The chrome plated sender is mated to the 90-300F, 140-300F or 120-300F top of scale same sender, while the brass version is paired to the 140F-340F top of scale gage. I measured my sender “chrome plated” and it measured 2.9k ohms at cold room temperature 65F. The constant is the top value, bass=340F, and Chrome Plated=300F top of scale.

[silverlock]

Useful, thanks!

911-606-112-00 is the 200C chrome-plated sender. Matched to my Style A gauge, which is original to the car.

Seeing abnormally low resistance values, even at 70F ambient. Going to sit down tonight and test the sender, harness, gauge sweep (min/max at infinite/zero resistance) part by part and see where the issue is. The NPM senders rarely fail but lower than expected readings are a sign, as wiring issues would typically cause a short.

VDO table 12 shows 20C/68F should read ≈ 3k Ohms with a +/- 13% tolerance per https://vdo-webshop.nl/en/temperature-sensors/453-vdo-oil-temperature-sender-200c-1-8-27-nptf-4103590949318.html - I think this is the right table based on your ≈2.9k Ohm reading?

[porschedude996]

Even though the thread form is wrong, the actual resistance may be the same. I can add a data point for you when warm. I need to warm her up. She has been asleep during my interior upgrades.

[silverlock]

A few other threads imply that the extrapolated values outside of the 50C (482 Ω) - 200C (14 Ω) endpoints are different. ≈ 1500 Ω at 20C — but I can’t find an official VDO table for that despite extensive searching.

[silverlock]

As an update: I measured a clear 2.99k Ohms @ 68F at the sender with the signal wire disconnected.

This matches the table helpfully provided to me in a DM, and other VDO charts:

| Temp (°F) | Resistance (Ω) |
|-----------|----------------|
|    36     |     6666       |
|    67     |     2840       |
|    74     |     2457       |
|    78     |     2265       |
|    89     |     1721       |
|    94     |     1518       |
|   110     |     1040       |
|   115     |      943       |
|   120     |      820       |
|   125     |      758       |
|   130     |      680       |
|   135     |      606       |
|   140     |      542       |
|   150     |      450       |
|   160     |      375       |
|   170     |      317       |
|   175     |      285       |
|   180     |      262       |
|   185     |      241       |
|   200     |      185       |
|   208     |      156       |
|   210     |      159       |
|   220     |      129       |
|   230     |      109       |
|   240     |     94.5        |
|   250     |      78        |
|   260     |      68        |
|   270     |      58        |
|   280     |      49        |
|   290     |      48        |
|   300     |      45        |
|   325     |      41        |

[silverlock]

Why I’m asking: I suspect my gauge is out of calibration, and is reading high. It could be a sender wire issue, but a short would usually peg the gauge to max.

Example: full cold start (overnight), style A gauge, ign on, needle starts at about ≈ 115F instead of the bottom (<= 90F). I don’t think this is correct: it should start at the bottom of the gauge in temps below 90F ambient.

• It then runs hotter than I’d expect (210F - 235F; never higher) despite having a big new CSF fender cooler, working (tested) thermostats & an engine oil cooler that was checked ~3k miles ago.
• It is the correct oil temp sender for an SC: the later chrome plated sender (P/N 91160611200)
• I’ve checked the sender connection at the sender itself, as well as reseating it and the ground wires at the gauge. Oil pressure is spot on and thus unlikely to be a bad ground issue at the gauge.

Current thinking: sending the gauge off to be checked. Photo below shows ign on + ambient ≈ 68F and the gauge reading high.

[evan9eleven]

I've been looking at this on my car just recently, so I'll add some data points. Not sure any of this helps you, but here goes:

• My 81 is running the temp sender that Pelican specified for it... part number not handy as I changed it over 10 years ago.
  
• My temp gauge was just refurbished by John Bell, backdated to the style "A" gauge with temp markings from 90-300F. (Highly recommend John's work by the way!)
  
• Prior to the refurb I had the style "C" gauge similar to what you have. Mine also did not bottom out when cold, but only moved about 3-4mm between dead cold and fully warm.
  
• With a warm engine (checking block next to sender with an IR thermometer = ca. 60˚C/140˚F) with temp sender disconnected, reads 360 ohms. Same reading at the other end with sender connected and gauge disconnected.
  
• When connected the sender measures 156 ohms. Same reading at the other end with gauge connected. John told me that this reading with the gauge connected is not really useful, however.
  
• The temp reading at the block and on the gauge match, though the temp seems low in my case. Still running the trombone oil cooler. I have yet to see the gauge climb over 140˚F, however, I'll do more testing when the opportunity arises for a road trip.

How it was before:




Refurbed gauge with Style A temp scale, warm engine:




Reading 60˚C/140˚F with the IR thermometer on the block right next to the temp sender, matches the gauge pretty closely:

[Hokie6]

For comparison purposes, my '83 911SC shows the following scale vs. resistance:

3/3 ~ 5.7Ω
2/3 ~ 52.5Ω
1/2 ~ 115Ω
1/3 ~ 160Ω
0 ~ 500Ω (just starting to rise)

I had hoped to use the sender with my Megasquirt system to control the 15° and 30° switch functions, but it's too nonlinear to use for that purpose. It seems the gauge is also non-linear, and matches the sender's non-linearity. Hope that helps -

[Duc Hunter]

I just went through this after my car sat for a year (rust repair). When I fired it up my oil temp gauge was reading warning, and seemingly high. I used my IR gun to figure that out, and just ordered a new sender. Swapped it out and fixed my issue. I have heard from John Bell (rebuilds gauges out here) and others that the gauges typically work or not, it’s the senders that are the issue.

[silverlock]

I do have a brand new sender, but since the current one measures (at least at ambient) to spec, I’m hesitant to throw parts at the problem until I understand what’s leading to the high reading.

Need to bring my IR gun with me + cross-check resistance at sender with temp reading. Can be tricky to get a clean reading on polished surfaces.

[Hokie6]

Two tests you can try - use a hot water bath to quantify your sender. You can start with boiling water, and take resistance measurements as the water cools. If you want to get to the extreme points, use an ice water bath for lower temps and an oil bath for higher temps. The oil bath is a bit trickier due to the high temps, but I've been able to heat a small oil-filled container with a torch or small camping stove while taking readings. Best to do it outdoors and use glove/googles etc. Might be a bit more trouble than it's worth though -

To quantify your gauge, substitute a potentiometer in place of the sender (1k or so), and measure resistance vs scale. This should be a good way to quantify your system and will give you a better idea of where your problem lies.

[evan9eleven]

The guitarist in me loves the potentiometer suggestion!

[Duc Hunter]

Originally Posted by silverlock

I do have a brand new sender, but since the current one measures (at least at ambient) to spec, I’m hesitant to throw parts at the problem until I understand what’s leading to the high reading.

Need to bring my IR gun with me + cross-check resistance at sender with temp reading. Can be tricky to get a clean reading on polished surfaces.

Agreed on getting clean readings. I tried the sender itself, the housing around it, the oil lines near the fan belt that feeds the cam chain tensioners, and the feed line out of the block to the thermostat (exhaust plays games with this one), as well as the fittings near the thermostat. Good luck!

[silverlock]

To close this out: the chart I posted above is correct and I was just running hot.

Fixed this in two ways, on top of the CSF Carrera style oil cooler that we’d already installed:

1. Added the 3.2 Carrera shroud / stone guard + foam seal. This certainly helps force air into the cooler and is noticeable even before the fan comes on with a much slower temp ramp.
2. Installed a 7.5” SPAL fan (Patrick Motorsports kit) and the 195F SPAL thermoswitch + relay kit.

Gauge reads a tidy ~200F, even after some spirited driving or in ~1 hour of NYC traffic at 90F ambient. Wiped 35-40F off the oil temps and haven’t seen a reading beyond 210F.

The 195F (vs the OE 210F) thermoswitch + thicker CSF cooler really stack up.

[porschedude996]

Well done!

[trials935]

Will the 195F spal thermoswitch fit in place of the stock Carrera oil cooler switch in the right front stock oil cooler without modification of the threads? If not, anyone know of one that will with a part number?

[silverlock]

You need a 3/8 NPT to M14x1.5 adapter for the SPAL relay 195F kit. FCP Euro, Summit Racing, Patrick Motorsports should all have one.