Does the VDO CHT Gauge use a K or J-Type Thermocouple? Options

[frank_c]

As the title states, I'm trying to determine if the VDO CHT Gauge uses K or J-Type Thermocouple (TC).

Backstory:
My '76 2.0 has a VDO CHT gauge installed by a PO (photo 1). I know there are better options out there, and I plan to replace it with something better the next time I drop the engine, but for now my main goal is to to get the VDO back to a functional state.

The PO installed the thermocouple at the stock temp sensor location @Cyl #3 on the 2.0 head. In addition, the "Cold Junction" for the TC wiring was in the engine bay near the distributor. So, in an effort to provide a more accurate temperature reading, when I had the engine out last winter, I:

1. Replaced the old TC with a K-Type TC located under the #3 spark plug (photo 2)
2. Used a special K-Type connector in the engine bay and extended the K-Type TC wire all the way to the gauge (cold junction at gauge).

The gauge needle did not move upon my initial test.

To ensure the gauge was still good, I hooked it up directly to the old TC that I removed and heated the TC with a heat gun. The needle moved after I swapped +/-.

I then re-connected the new TC wire I ran, this time swapping the +/- terminals, started the engine and confirmed that after a few minutes the needle moved (yay).

Apparently, per the below Samba thread, some VDO CHT gauges have their +/- terminals swapped, so that solves that mystery:

https://www.thesamba.com/vw/forum/archive/i...t-173868--.html

But I noticed something else during this process which is why I'm asking about the TC type. I purchased K-type TC and wiring because what I read up until that point indicated the the VDO gauge used a K-Type TC. The K-Type TC wiring is color coded Yellow(+) and Red(-) [ see chart below]. However the wiring on the old TC I removed is Red(ish)/White. And according to the TC color chart, Red/White is a J-Type TC. To confirm, I checked and found that the White(+) lead of the TC I removed is indeed magnetic. I then found this thread on Shop Talk Forums stating that the VDO CHT actually uses a J-Type TC, not a K-Type as most commonly thought:

https://shoptalkforums.com/viewtopic.php?t=144947

If it turns out that the VDO gauge does indeed use a J-Type TC, then since I already have the K-Type TC, connector, and wiring installed, the easiest path forward for me is to simply purchase a K-Type gauge (which may pose a new challenge). If it uses a K-Type, then I should be good for now.

Sorry for the long post. Looking forward to hear what folks have to say who have been down this path already.

[VaccaRabite]

K type.
And replace it NOW with something that is temp compensated. Most of us use Dakota Digital.

Depending on the ambient temps your cht can be off by a lot - to the point of your better off without it, IMO.

Zach

[FlacaProductions]

https://www.dakotadigital.com/index.cfm/pag...prod/prd347.htm

[Superhawk996]

Ah if only life were so black and white.

Once upon a time (80s) I had an VDO with a J-type red and white thermocouple

I’ll leave the whole cold junction discussion for another time.

If OP truly wants to keep K-type thermocouple and wiring - then go for the Dakota. The cold junction compensation is a bonus.

Note: negative lead of K-type (red) is Alumel and that is also highly magnetic. The magnet test is useful for determining the negative lead of either a J-type or K-type thermocouple but isn’t useful for determining one type vs the other.

Note 2: color codes for thermocouples are not standardized internationally.

[frank_c]

Once upon a time (80s) I had an VDO with a J-type red and white thermocouple

Hmm, curiouser and curiouser. The date code on my gauge is 7/86. Now I'm not sure what my gauge is (J or K). Why would the PO have used a J-Type TC (assuming the red/white is correct) if it was a K-Type gauge? In addition, the gauge did show temperatures in appropriate CHT range with the old TC.

If OP truly wants to keep K-type sensor and wiring - then go for the Dakota. The cold junction compensation is a bonus.

Given the number of unknowns here, I think you guys have convinced me to just move to a new gauge. I was trying to keep with analog gauges, but the DD looks like the way to go. Does anyone know if the Micro 1000 uses a K-Type TC?

https://www.aircraftspruce.com/catalog/inpa.../CHT_gauges.php

Note: negative lead of K-type (red) is Alumel and that is also highly magnetic. The magnet test is useful for determining the negative lead of either a J-type or K-type thermocouple but isn’t useful for determining one type vs the other.

As usual, Superhawk is correct. I was going by the chart I posted that listed only the J-Type + lead as being magnetic, but I just went an checked and the K-Type is also magnetic. Thanks. So the old red/white TC I removed may or may not be J-Type. One more unknown.

[Superhawk996]

Once upon a time (80s) I had an VDO with a J-type red and white thermocouple

Hmm, curiouser and curiouser. The date code on my gauge is 7/86. Now I'm not sure what my gauge is (J or K). Why would the PO have used a J-Type TC (assuming the red/white is correct) if it was a K-Type gauge? In addition, the gauge did show temperatures in appropriate CHT range with the old TC.

I want to be really transparent here. In the 80s I purchased my gauge used. I don’t know whether it originally came with J-type but I had no reason to believe it had ever been changed out (purchased from acquaintance out of his Bug).

After using it for years (almost 10) the thermocouple stopped working. It had developed a rust like oxidation on the iron wire. I had to clean the wires. Retwist and reinstall a new spark plug ring. That is why I believe it was J-type. My head temps always seemed reasonable and plausible so I never even thought about it. But as we discuss this who’s to say that in that era (or later) VDO could have been using K-type with red and white wires which could have been European sourced?

But here’s the thing. When you get down to it J-type is more sensitive than K and will read hotter if the gauge was intended for K-type.

Type J Sensitivity: 55 micro volt per degree Celsius
Type K Sensitivity: 41micro volt per degree Celsius

But either type will “work” (ie moves the needle as temp increase).

You would have to check the gauge and thermocouple calibration to determine the type with absolute certainty.

Lots of unknowns when dealing with old parts. It very well could be that VDO changed somewhere in production or that someone swapped parts. That’s why I said if only life was black and white.

Maybe John Bell has some history he can add. @jbell959

I just recently purchased another used CHT from John that I assumed I will have to calibrate to be sure. I figured I’d start with a K-type thermocouple wire because that’s what I have on-hand. Unfortunately I haven’t done that yet to be know definitively what I now have. But like you I want the VDO analog look. More than I care about absolute accuracy. At the end of day these old VDO’s are best used for a general trend, not absolute numbers.

If it helps you and you want to keep the VDO - I’ll calibrate mine this weekend and post the results and a how-to.

[Montreal914]

More countries chart.




For the gauge there is also this cost effective option.

https://thesensorconnection.com/products/ch...eter-gauge-kits

[frank_c]

I want to be really transparent here. In the 80s I purchased my gauge used. I don’t know whether it originally came with J-type but I had no reason to believe it had ever been changed out (purchased from acquaintance out of his Bug).
...

Lots of unknowns when dealing with old parts. It very well could be that VDO changed somewhere in production or that someone swapped parts. That’s why I said if only life was black and white.

Well, the guy over on Shop Talk Forums was rather confident that his was a J-Type, so perhaps there are a few of these J-Type variants out in the wild.

Because of all the unknowns, last night I placed an order for the Dakota Digital gauge. Once it arrives, I'll compare its reading with the VDO gauge to see how close they are (taking cold junction temperature into account).

I also just remembered that my Klein multi-meter has a temperature function. I checked and it uses a K-Type thermocouple - so I'll also measure the temperature with the Klein to have another reference point. Not sure if it is temperature corrected.

If it helps you and you want to keep the VDO - I’ll calibrate mine this weekend and post the results and a how-to.

I would be good to know the procedure in case my VDO is significantly off and I decide to either move back to it some day, or decide to sell it so that I can state that it has been calibrated. Thanks for offering to post the procedure - certainly no rush.

[frank_c]

More countries chart.

Thanks for posting the chart.

Well, this thickens the plot. Below is a photo of my old thermocouple. In my original post I mentioned that the leads were Red-ish/White. As you can see, they are more like Brown/White, but there was no such color combo in the chart that I had, so I assumed it was really Red/White. But the chart posted by @Montreal914 does show a Brown/White code for a T-Type thermocouple.

More unknowns. Hopefully I'll get the VDO gauge type mystery resolved once the Dakota Digital gauge gets here and I run some tests.

[barefoot]

You can do a bench calibration on what you have. Pull the tc from under the plug and test it at room temperature, then immerse it in a small dish of boiling water & read gauge
I did this test on my setup using a gauge with cold junction compensation & found it
read correctly.

[stownsen914]

I was just researching this recently, as I have an older VDO CHT gauge that I'll be putting in a 911 racecar. I found conflicting info on whether the VDO uses J type or K type, but settled on it more likely being J type. I wanted to purchase the correct wire and connectors. I gave up and bought a new VDO thermocouple that came with 15 feet of extension wire lol. For what it's worth, the new VDO lead consists of red and white wire.

It kinda makes sense to use J type for CHT as the typical temp range for J type is lower than K type (but well above the 600 deg max on the gauge), and I noticed that K type wire seems to be more expensive.

[stownsen914]

I just wrote VDO tech support to ask them which type they use for CHT. Will update when I hear back.

[stownsen914]

Update: I heard back from VDO, and I think there may be confusion even in VDO lol. They initially responded to my inquiry by recommending pyrometer extension wire (a pyrometer is an exhaust gas temp gauge, and the extension wire they sell is yellow which is standard for K type). When I reminded that I am asking about CHT, not EGT, the tech rep seemed to admit a mistake, and provided a technical drawing of their CHT thermocouple, which includes part numbers for individual components used to make it. A couple interesting findings:
- It indicates a "heater wire" used to make the thermocouple that is manufactured by the George Instrument Company, with a part number that starts with J. I think this is a clue.
- The 14 mm eye terminal that goes under the sparkplug appears to be a standard eye terminal just opened up to 14 mm (Amp part # 321165)

I put a call into George Instrument Company. Their tech guy is out today. Will try again next week and report back.

[frank_c]

You can do a bench calibration on what you have. Pull the tc from under the plug and test it at room temperature, then immerse it in a small dish of boiling water & read gauge
I did this test on my setup using a gauge with cold junction compensation & found it
read correctly.

Good suggestion, but it will have to wait until the next time I drop the engine as I'm not too keen on removing that piece of cooling tin with the engine still in the car.

[frank_c]

Update: I heard back from VDO, and I think there may be confusion even in VDO lol. They initially responded to my inquiry by recommending pyrometer extension wire (a pyrometer is an exhaust gas temp gauge, and the extension wire they sell is yellow which is standard for K type). When I reminded that I am asking about CHT, not EGT, the tech rep seemed to admit a mistake, and provided a technical drawing of their CHT thermocouple, which includes part numbers for individual components used to make it. A couple interesting findings:
- It indicates a "heater wire" used to make the thermocouple that is manufactured by the George Instrument Company, with a part number that starts with J. I think this is a clue.
- The 14 mm eye terminal that goes under the sparkplug appears to be a standard eye terminal just opened up to 14 mm (Amp part # 321165)

I put a call into George Instrument Company. Their tech guy is out today. Will try again next week and report back.

Thanks for posting.
This thread is getting more interesting by the day. I'm very curious to hear what the tech says.

[Superhawk996]

This thread is getting more interesting by the day. I'm very curious to hear what the tech says.

Any idea where that crazy sensor you posted came from? I don’t believe that bolt with a thermocouple in it is from VDO.

I very well could be wrong but I thought VDO only offered the ring terminal type that goes under the spark plug.

Assume it goes into the head at the CHT boss but that implies you are running carbs and not stock fuel injection that would use a completely different CHT sensor from Bosch.

The plot thickens (IMG:style_emoticons/default/biggrin.gif)

[Montreal914]

Agreed, if I recall ~35 years ago when I installed one bought new on my Bug it had a ring terminal with about 18"-24" long leads which meant the connection to copper wires was happening in the engine bay via a white plastic connector.

[friethmiller]

(IMG:style_emoticons/default/popcorn[1].gif) Following along as well, here. I recently purchased 2 new Westach Analog CHTs. They called for the J-type connector.

[barefoot]

You can do a bench calibration on what you have. Pull the tc from under the plug and test it at room temperature, then immerse it in a small dish of boiling water & read gauge
I did this test on my setup using a gauge with cold junction compensation & found it
read correctly.

Good suggestion, but it will have to wait until the next time I drop the engine as I'm not too keen on removing that piece of cooling tin with the engine still in the car.

Don't you have enough lead to pull the TC out thru the spark plug hole ?
Might be tough to get to it probably, need a long needle nose pliers ??
BTW the gauge I'm using came from China for about $15 and works quite well (IMG:style_emoticons/default/rolleyes.gif)

[frank_c]

Any idea where that crazy sensor you posted came from? I don’t believe that bolt with a thermocouple in it is from VDO.

I'm pretty sure the sensor is not an off-the-shelf item, but rather something that the PO fabricated. He is a retired jet engine engineer, and I found many such fabricated parts on the car - and lots of safety wire (IMG:style_emoticons/default/smile.gif).

Assume it goes into the head at the CHT boss but that implies you are running carbs and not stock fuel injection that would use a completely different CHT sensor from Bosch.

Correct, the old sensor was attached to the boss on the head for the FI CHT. And yes, I'm running Webers.

The plot thickens (IMG:style_emoticons/default/biggrin.gif)

Indeed - I thought this would be an easy one reply thread.