CHT Gauge Wire

[RussellK]

Anyone know if adding a section of wire between the guage and sender impacts the reading? It's a VDO so I know the reading is a off a bit already. I want to move the guage and need a little more wire.

[Gypsie]

I did and it is possible that it has affected the reading. A good solid (ie soldered) connection with the same gage wire is key.

I use the gauges ( I have two vdo's, one for each side) as a trend indicator. I have checked several times with an IR sensor and found the gauges read a little high (about 50 deg. according to the IR sensor.).

There are some that avoid the gauges if'n they don't know if they are spot on. I can understand that, but I figger that knowing if the temp jumps very fast can be helpful. Maybe more than not knowing anything but how hot the dipstick is after you pull over to check.

[vdubyah73]

the vdo will read higher and higher as out door temp goes lower and lower, and of course it will read lower and lower as the weather warms up. don't have one on the bus, but do on my chico performance racing 2275 in the buggy. the cpr heads are mini D ported with 12mm spark plugs instead of 14mm because of larger valves. i have been using a 14 mm sender because thats what i have. i figure it's off by 100º in the summer and close to 50º off in winter, yes i take rides in the winter. runs 200º + summer and and have seen 300º in the winter. should run 300-350º at 70º outdoor temp. the the vdo is only good for spotting a trend or spike. if your spark plugs show good color when checked go with what the gauge reads as normal and if it's 50º outside add 20º to the gauge reading, if 90º subtract 20º. if you suddenly see a 400º plus spike and you're pullin' a long grade or a trailer ya might wanna lift your right foot or downshift.
i was towing my buggy with my bus with the old engine, keeping up with the big dogs. heard the detonation rattle and slowed right down, it cooled down quickly and ran fine till i was ready to rebuild it. these engines will communicate if you listen.

[tristessa]

Get yerself some K-type thermocouple wire and run that all the way from the gauge to the sender at the engine. It's cheap enough if you shop carefully, and your gauge'll be more accurate when you're done.

http://www.ebay.com/itm/Thermocouple-K- ... 3a6ec51073
(plus several other listings)

[RussellK]

Why does it read higher when its colder outside?
Thanks for the thermocoupler wire tip. I'll check my local hardware store that guy can get anything.

[vdubyah73]

something scientific about disimilar metals and temperature differentials generating minute electrical currents.

[RussellK]

something scientific about disimilar metals and temperature differentials generating minute electrical currents.

Got it It just does

[Amskeptic]

something scientific about disimilar metals and temperature differentials generating minute electrical currents.

Got it It just does

The gauge head on the vdo makes a comparison as it "reads" the signal from the sensor. The needle movement on the dial assumes a baseline of 70*.
Therefore:

IF IT IS WARMER THAN 70* AT THE GAUGE, IT READS LOW
A real 86*, for example, is read as 70* so an engine that has heated up 300* will read 370* when it is actually 386*.

IF IT IS COLDER AT THE GAUGE THAN 70, IT READS HIGH
A real 35* at the gauge is read as 70*, so an engine that has heated up 300* will read 370* when the engine is actually at 335*

THIS IS STUPID.

COLD-JUNCTION COMPENSATION
The Dakota Digital gauges have a heater inside the gauge head that makes a consistent baseline temperature regardless of ambient. This allows the Dakota Digital CHT gauge to read an actual temperature of the engine to within 2% under any environmental temperature.
Colin

[reluctantartist]

As I understand it, on the vdo gauge, the thermocouple wire is the sensitive piece so if you add to the existing regular wire it should not effect the readings. That said be sure to use at least the same gauge wire or bigger for what you are adding so you do not add much resistance. The thinner gauge wire will have a higher resistance. I use mine for trending. You can't rely on it for an accurate reading.

[SlowLane]

Go browse Omega.com to learn more than you probably ever wanted to about thermocouples and to find good prices on thermocouple wire and connectors.

IIRC, the VDO uses J-type wire, not K-type. The differences are explained on Omega's site. J-type has white insulation on one wire and red on the other, while K-type has yellow and red. Note that in thermocouple-world, the red is the negative wire.

The VDO reading is dependent on outside temperature because it is uncompensated, unlike the Dakota Digital or MGL Avionics gauges, as noted by Colin. Richard Atwell also has a pretty good write-up on the VDO gauge at ratwell.com