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Mon Mar 04, 2019 10:11 pm

I did a test on my VDO gauge today, it is reading about 20 degrees hot from what the actual oil temp is from the dipstick. It is a brand new gauge and sending unit mounted on right side of engine block (not off the bottom of the oil sump). The builder of my engine installed the sending unit.

My question is can you calibrate one of these gauges or is it more than likely a bad sending unit? Or maybe I installed it incorrectly? It seems to slowly move upward as the engine temp increases. I have not been very impressed with VDO, also had an issue with the dual post oil pressure sending unit. Maybe I just have bad luck....

Mon Mar 04, 2019 10:50 pm

Wow! Only 20°? That's pretty good, actually..

These gauges are for comparison use only. These are not scientific instruments. And your guy made no mistakes installing it, that's how they are.

If you really want to try, I found this On Pelican Parts:

"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"

Hope that helps.

Mon Mar 04, 2019 11:20 pm

Throw it in a pan of boiling water and see what it reads, or use a multimeter with a temp sender wire down the oil dip stick to compare.

Mine reads 25 degrees hot-just use it as a reference or look at Dakota Digital or some of the gauges made for planes (see aircraft spruce).

Tue Mar 05, 2019 5:12 am

I've never trusted my gauge... it came with the car (which I've owned since 2002) so I have no idea how old it is.. but if it's accurate, even in the dead of a Maryland summer, my oil rarely gets above 120*F.. I think I've seen it go to 130-140 twice...

It does just barely move off of the 120 mark (the lowest mark) so I know it is reading SOMETHING, but I don't trust it..

Tue Mar 05, 2019 6:15 am

tattooed_pariah wrote: I've never trusted my gauge... it came with the car (which I've owned since 2002) so I have no idea how old it is.. but if it's accurate, even in the dead of a Maryland summer, my oil rarely gets above 120*F.. I think I've seen it go to 130-140 twice...

It does just barely move off of the 120 mark (the lowest mark) so I know it is reading SOMETHING, but I don't trust it..

I sure hope it reads about 60-70 degrees cold! I'd be worried about driving around with only 120 degree oil.

Tue Mar 05, 2019 8:09 am

Bashr52 wrote: tattooed_pariah wrote: I've never trusted my gauge... it came with the car (which I've owned since 2002) so I have no idea how old it is.. but if it's accurate, even in the dead of a Maryland summer, my oil rarely gets above 120*F.. I think I've seen it go to 130-140 twice...

It does just barely move off of the 120 mark (the lowest mark) so I know it is reading SOMETHING, but I don't trust it..

I sure hope it reads about 60-70 degrees cold! I'd be worried about driving around with only 120 degree oil.

it measures a range from 120-220 IIRC.. so anything less than 120 is pegged to the left.

I do run 4 thermocouples, one for each spark plug, and today on my way to work, it was 22*F outside, 15 minutes to work and my CHTs were about 110 or so..

Tue Mar 05, 2019 8:26 am

74 Thing wrote: Throw it in a pan of boiling water and see what it reads, or use a multimeter with a temp sender wire down the oil dip stick to compare.

Mine reads 25 degrees hot-just use it as a reference or look at Dakota Digital or some of the gauges made for planes (see aircraft spruce).

I used a meat thermometer down the tube and this is how I discovered it was 20-25 off. I just fired up my new 1915 yesterday and took it for its maiden voyage. It ran great but my temp after 20 mins was showing 220ish on the gauge. It was only 60 degrees outside and I thought it was running warm for such a cool day. I called the builder of my engine and he recommended using the meat thermometer as a reference to see how far off the gaugei is. I also have a multi meter with a temp sensor I am going to try.

I wish I would have known these electric gauges are so terrible. I would have installed mechanical gauges. I guess I will just factor in the 25 degrees when looking at the gauage.

I also have a VDO electrical oil pressure gauge, are these just as bad for showing actual oil pressure?

Tue Mar 05, 2019 8:33 am

Who was the builder? You have to remember that the engine will run a bit hotter during the break in portion. What car are you running it in and do you have enough air entering the engine compartment? I am assuming dual carbs which require more air.

Yes, if you are using a dual pole oil pressure sender it is common for the idiot light to come on at higher psi's-go to aircooled.net and pull up the vdo oil pressure sender and read. It is common for people to use a tee and use a stock VW oil idiot light sender as well as the vdo pressure sender.

Tue Mar 05, 2019 8:33 am

those gauges are good for trends only. Knowing where "normal" is on the gauge regardless of the number, and then checking to see if its "above" normal

Tue Mar 05, 2019 4:50 pm

Fman wrote:

I used a meat thermometer down the tube and this is how I discovered it was 20-25 off. Snip....

When was last time meat thermometer was calibrated?....

Dale

Tue Mar 05, 2019 6:20 pm

Mine reads 40* higher than actual. I used a calibrated candy thermometer and a RayTek infrared.

Tue Mar 05, 2019 6:33 pm

If you want an inexpensive way to check the calibration and temperature of gauges or your engine these are very accurate

https://www.amazon.com/Tempilstik-28012-Surface-Temperature-Indicator/dp/B01IWOBN38

Otherwise thermal imaging is fun.

Tue Mar 05, 2019 10:50 pm

74 Thing wrote: Who was the builder? You have to remember that the engine will run a bit hotter during the break in portion. What car are you running it in and do you have enough air entering the engine compartment? I am assuming dual carbs which require more air.

Yes, if you are using a dual pole oil pressure sender it is common for the idiot light to come on at higher psi's-go to aircooled.net and pull up the vdo oil pressure sender and read. It is common for people to use a tee and use a stock VW oil idiot light sender as well as the vdo pressure sender.

Engine was built by Chico Performance Racing in So. Cal...engine was broken in and dyno tested before shipping it to me. I just finished installing my oil cooler today and wired it with a toggle on the fan just for some peace of mind, especially during the warmer time of year. The issue with my oil pressure was on the sending unit (which is a dual pole VDO unit) was the tiny pin hole on the sending unit got debris in it and completely showed zero oil pressure on gauge with the idiot light on. Once I cleaned it out the oil pressure returned to normal and idiot light was off.

@DaleM yea no calibration on the meat thermometer, so it could also be incorrect. I will be using my multi meter temp wire next time which should be pretty accurate. My gut feeling is the gauge is just off, which obviously is not uncommon and more the norm for these gauges.