head temp guage question Options

[Steve Thacker]

Hey all,
I have a easy one for some of you who have BTDT with CHT units. I have two VDO head temp guages I want to hook up. However I don't know what type of sensor to get. I found some on evilbay see below.

http://search.stores.ebay.com/ws/search/In...t+sender&sofp=0

Will either one of the sensors work work me. And if so, what size ring do I need?

Appreciate the help !

[ArtechnikA]

914's use 14mm plugs.

CHT's are actually microvoltmeters and measure the very tiny voltages produced by the thermocouple under the spark plug. as these are very small signals, the gauge and sensor must be very carefully matched. the typical VDO CHT comes with a long harness that the instructions warn that you should not cut, lest it affect the calibration.

i think if you're going to the bother of fitting CHT's, you should get matched gauges and sensors to have any confidence in the data you see. Aircraft Spruce and Wicks have CHT's, as do all the race-car supply places. consumer-grade CHT's are not designed to be field calibrated.

[morph]

get a matched set (IMG:http://www.914world.com/bbs2/html/emoticons/agree.gif)
james

[dmenche914]

Although I have not tried this, i bet the gage has some calibration in it, hopefully an adjustable. So you cold remove the age back and see. At any rate, even a new matched set would be wise to be tested in boiling water.
i did add a heavey gage wire to increase the length of my gage to sender, and with the boiling water test found no difference. The key is using a heavy gage wire to minimize voltage drop. Use a thin wire that is too long, and the gage will read low.

As pointed out, there are two spark plug size senders, get the fat one that fits our 914 plugs.

I also found it helpful to grind away a bit of the metal around the spark plug hole to allow space for the sender terminal, else it gets real tight,a dn the sender can get buggered up after repeated spark plug changes.
Be sure to remove teh original spark plug metal gasket, as the sensor will now serve as the gasket, if you do not remove the stock gasket, the spark plug won't be in the head as deep.

[Steve Thacker]

Thanks guys I think I will check the guages for internal calibration. If they don't have it, I'm off to purchase a matched set as suggested. Appreciate the help as always.

[Steve Thacker]

Which size ring would fit? The 14mm one below right?

[bernbomb914]

you can get a switch to change the leads so you need only one guage. Westach make a set that I have installed on my car. got it at Aircraft spruce in corona. They have a web site

Bernie

[lapuwali]

The VDO senders for the VDO gauges are available from several places, and are only $15-20 each. Aircooled.net has 'em, as does CIP, and lots of the other Bug places.

The warning about not cutting the sensor wires has NOTHING to do with the main part of the wiring harness. If you look at the VDO or Westach sensors, you'll note that it's a braided bit of wire (actually two wires) with a ring crimped on at one end, and a plastic connector at the other. This is the bit of wire you should not cut. The reason you don't want to cut the thermocouple wires is they're two different oddball metals, which can't be soldered back together again, nor can they even be spliced with butt connectors. The way a thermocouple works is by generating a voltage between the two ends of a pair of different metals, which rises as the temperature difference between the two ends gets wider. Cutting the wires destroys this effect, and using a butt splicer or solder of another material also destroys this effect. So, if you cut them, you can't join them back together again, so don't cut them.

The rest of the "harness", which has a plastic connector on one end and is regular 18g copper wire, can be altered in length to suit your setup. The difference in resistance between 1ft of 18g cooper wire and 12ft of 18g copper wire is in the milliohm range, and isn't going to throw off the measurements by enough to matter at the scale we're operating on. Switching to heavier gauge wire is also going to make too small a difference to notice on these gauges. You'd have a hard time measuring the difference with a DMM that reads down to 0.01 ohm. If you wanted a lab-grade instrument to measure temperature down to tenths of a degree, then worry about it. Here, knowing it to within 10dF is more than enough. The VDO gauges aren't adjustable for this very reason.

[Steve Thacker]

lapuwali and others
I certainly appreciate your help. I placed an order for the 14 mm ring size leads and 6' extension wires I thought I would need to give this a go. If it fails with the VDO route, then I will get a matched set of guages.

I remember reading somewhere on what cylinders to place the sensors on.
Do you remember which ones give an overall temp layout of the engine, i.e cyl 1 and 3?

Thanks!!!!

[SirAndy]

QUOTE (Steve Thacker @ Feb 20 2005, 12:42 PM)

Do you remember which ones give an overall temp layout of the engine, i.e cyl 1 and 3?

#3 is the hottest (rear, pass. side) ...

[Dr Evil]

Anyone know what size to use and which plug to put it on for a /6?

[ArtechnikA]

QUOTE (Dr Evil @ Feb 20 2005, 06:44 PM)

Anyone know what size to use and which plug to put it on for a /6?

911's also use 14mm plugs.

if i were going to pick one, it'd be #6. that's almost as far from the fan as #3, and it sits right next to the oil cooler. in a 914 it's actually pretty easy to get at; in a 911 it's a bit of a bitch. but in general, 911 cooling distribution is a bit more even than the 4-cyl cars, so pick one you like.

[Steve Thacker]

Thanks Andy

My stuff will be shipped out tomorrow. From there it will be a wait and see. I'm glad I started this discussion. Maybe a few others who have thought of protecting their investment will gain something from this.