VDO cylinder head temperature gauge

[DNHunt]

I just installed a VDO cylinder head temperature gauge in my center console. The sender is a large screw terminal with 2 wires that fits between the head and the spark plug . One connects to the ground on the and one is the signal. There is no 12V connection except throught light circuit and this is switched and notcontinuous with the gauge itself. The gauge is hooked up and works fine.

So, how does it work? I must be missing something. With no continuous current it can't use resistance like the cylinder head temperature sensor in the fuel injection system. Could the gauge have a bimetallic strip in it like some thermostats? How would the heat be transferred through the 8 or so feet of wire?

I just don't get it,

Dave

[JWest]

Found on the web:

Accurate measurement of temperature is one of the most common and vital requirements in industrial instrumentation. It is also one of the most difficult objectives to achieve. Unless proper temperature measuring techniques are employed, serious inaccuracies of reading can occur, or otherwise useless data can result. The thermocouple is by far the most widely used temperature sensor for industrial instrumentation. Its favorable characteristics include good inherent accuracy, suitability over a broad temperature range, relatively fast thermal response, ruggedness, high reliability, low cost, and great versatility of application.

A discovery by T. J. Seebeck almost 150 years ago, opened the way for modern thermoelectric circuitry. In 1831, Seebeck discovered that an electric current flows in a closed circuit of two dissimilar metals when one of the two junction is heated with respect to the other.

In such a thermocouple circuit the current continues to flow as long as two junction are at different temperatures. The magnitude and direction of the current is a function of the temperature difference between the junctions and of the thermal properties of the metals used in the circuit. This phenomenon, is known as the Seebeck Affect.

The conductors can be of any two dissimilar metals, and when the hot junction is heated the current flow can be observed on a milliamp meter. If the position of the hot and cold junction is reversed, current will flow in the opposite direction.

In fact, a thermocouple circuit will actually generate a measurable, low voltage output that is almost directly proportional to the temperature difference between the hot junction and the cold junction. A unit change in this temperature difference produces some net change in the voltage.

Edit: didn't want to say I wrote this - I found it on the web.

[maf 914]

James, very nice explanation.

Two questions please. 1. If the cylinder head is the hot source, what is the cold source? The air or engine block? 2. If the current generated is dependent on the difference in temperature, is the gauge reading relative (i.e. the difference in temperature of the hot and cold sources) or absolute?

Thanks,
Mike

[Dave at Pelican Parts]

1. The engine bay air is the temperature measured by the cold end.
2. Not sure, but I would guess that they picked a number for "average air temp" and biased the reading by that much.

--DD

[jamcleod]

thermocouple would be 2 dissimilar metals connected together. heat to that junction creates a voltage which powers the meter.

[Zeke]

Originally posted by Dave at Pelican Parts
1. The engine bay air is the temperature measured by the cold end.
2. Not sure, but I would guess that they picked a number for "average air temp" and biased the reading by that much.

--DD

We had these types of head temp guages mounted on the steering wheel of racing karts. In that case, they were digital LCD's. Of course, the LCD was powered by a battery. I was told that the safe head temp range would move up on a really hot day and move down on a really cold day. This would offer some validation to what DD says.

BTW, I've been seriously considering using one of these from a kart on my car. My kart guage has a exhaust gas temp readout, head temp read out and RPM, all displayed simultaneously.

[pbanders]

Originally posted by Dave at Pelican Parts
1. The engine bay air is the temperature measured by the cold end.

The "cold end" of a thermocouple is the reference junction, which is inside of the DMM or meter. Take a look at the diagram for a "standard thermcouple" on this web page:

http://www.xco.com/How_it_works.htm#Normal%20thermocouple

I don't know how the VDO HTS is actually configured, it may be that the reference junction is in the engine compartment, but most other thermocouple meters have the reference junction at the meter.

[DNHunt]

The VDO kit has 2 wires approximately 12 inches long leading from the large screw terminal that goes under the plug. These are insulated with what appears to be fabric coated with a hardened resin (reminds me of the wiring in some old tube set radios and such). The wires run to a duplex solderless receptical from which 16-18 gauge wire runs through the center tunnel to the back of the gauge. The instructions infer that these wires should not be cut as it states that you should coil the excess and store it.

Thanks for the info, it helps clear up a puzzle.

Dave

[philinjax]

Jamcleod (above) has it exactly right and very simply explained.

Phil