964 Head Temp Sensor -- Adding a Gauge

Jack Olsen · 03-29-2005, 09:50 PM

The 964 motor comes with a head temp sensor for the motronic unit, right?

I'd like to get a look at my cylinder head temps while I test out some different aero items that might be heating up my motor.

Can anyone think of a way to utilize the stock sender with a cheap gauge as a temporary solution?

ChrisBennet · ChrisBennet's Garage

I don't know how precise your measurements would be. The factory manuals give a broad range for temp vs resistance. If all you care about is the relative temps then read on.

On these sensors resistance goes down as temp goes up. With a fixed voltage source (supplied by the DME) the current will go up as the temp goes up.

You can measure the current flowing through the sensor without much trouble. To do this you could make a harness with connectors on both ends so you could plug the sensor into your harness and then plug your harness into the factory (car side) harness. Place a small value resistor (<10 ohms) in series with the sensor and measure the voltage across it with a high impedance voltmeter (digital not needle) to determine the current. (Voltage = Current * Resistance).

(My cat told me this. I don't know nuthin' 'bout e-lec-tricity.)
-Chris

Joeaksa · 03-30-2005, 06:27 AM

Jack,

I have an extra CHT gauge off of the plane that am not using right now.

If you can give me an idea how to wire it or what terminal its off of would also like to have this.

JoeA

ChrisBennet · ChrisBennet's Garage

Joea,
The Porsche temp sensor "goes the other way" Negative Temperature Coefficient (NTC). It decreases in resistance as the temp goes up.
A thermocouple (do airplanes use them?) temp sensor actually produces a very small current when heated.
(Again, according to my cat.)
-Chris

buster73 · buster73's Garage

You can get CHT sensors which just slip under the spark plug and replace the spark plug washer. Easy to install and you can move them around from cylinder to cylinder. Costs ca. $ 15.

BlueSideUp · 03-30-2005, 10:38 AM

Airplanes do use thermocouples to measure CHTs. You can even get six probe sensors so you can check each cylinder but that's getting into some cash.

The spark plug idea seems to be the best for a temporary installation.

Joeaksa · 03-30-2005, 10:49 AM

Quote:

Originally posted by ChrisBennet
Joea,
The Porsche temp sensor "goes the other way" Negative Temperature Coefficient (NTC). It decreases in resistance as the temp goes up.
A thermocouple (do airplanes use them?) temp sensor actually produces a very small current when heated.
(Again, according to my cat.)
-Chris

Chris,

Many of the aircraft use a CHT sensor (as Blue above mentions) that goes under the spark plug or bayonet plug into the head.

What type of gauge do we need to find and what are the values to be looking for?

JoeA

Jack Olsen · 03-30-2005, 11:28 AM

Okay, so I've learned I can get readings through the DME, but I need to turn the car off and disconnect the cable.

"Valid CHT sensor readings across pins 30, 45 of the DME plug per the Porsche shop manual (page D 24/28 - 12):

0 degrees Centigrade (C), 6.8 kilohms - 4.4 kilohms
15C - 30C, 3.6 kilohms - 1.4 kilohms
40C, 1.3 kilohms - 1.0 kilohms
80C, 390 ohms - 250 ohms
100C, 210 ohms - 160 ohms"

Does anyone know if it's possible to piggyback a separate gauge off of the output from the stock sender? Would this mess up the DME's voltage as it comes in? I have an oil temp gauge wired to my tranny. Since this is a temporary thing, it would be great to simply bridge the factory sender and my existing temp gauge. Is this at all a viable idea?

Tyson Schmidt · 03-30-2005, 11:57 AM

I think you should just get a head temp gauge and sender and keep it. It's a nice feature.

You should place it under the #5 spark plug.

Bill Verburg · Bill Verburg's Garage

I think you need will needto get your diagnostic port hooked up.

Here is the relevant section of the shop manual

Cylinder head temp

***Is auto resizing kaput again?***

Rob 930 · 03-30-2005, 12:32 PM

Tyson,

I'm about to install a factory Carrera head temp sensor in one of my 930 heads. I have no current plans to monitor the temp or use the temp as an input for my injection system, but I thought that while I was at it, it would be good to install one. You said cylinder No. 5 is where to put it. I've heard cylinder No. 3 is the best place. What is your reason for saying cylinder No. 5?

jfpound · 03-30-2005, 12:40 PM

Is #5 the hottest?

jfpound · 03-30-2005, 12:43 PM

Best location might be different for a 930 motor vs 964?

randywebb · 03-30-2005, 12:46 PM

These thermistor circuits are non-linear. I like a good 'ol thermocouple. The under the spark plug gizmo is one - probably a type K. You could also get a fine wire one (Type K) and put it down where the resistive sensor is located. Omega is the WalMart [I mean that in a good way] of thermal sensors, process control, etc. They are on the web.

Jack Olsen · 03-30-2005, 01:09 PM

What's the typical head temp, and high end for a 964?

I'm thinking of biting the bullet and getting a VDO gauge to install permanently. At $110, it's going to cost more than the whole skirts-and-diffuser project it's monitoring the effects of.

KTL · 03-30-2005, 01:47 PM

I'd not bother hacking into the car wiring. Seems like more trouble than it's worth. If planes get by with the thermocouple under the plugs............

I have the same gauge you posted Jack.

Mine hovers around 300 all the time. Have yet to see it higher. But I haven't tracked it yet. We'll see what happens @ Blackhawk May 4. I like how it needs no power to operate. That's pretty cool. Electricity is like magic to me!

egauges.com has good prices on the VDO gauge. That's where I got mine. Your $110 is what it'll cost from them. AutoMeter makes a cheaper gauge, but you don't get the luxury of the spark plug thermocouple. AutoMeter uses a threaded fitting you'd have to tap into the cyl. head. Or use the setup posted by buster with an AutoMeter gauge for a few less bucks total?

Whatever the case, it's an easy install. The hardest part is mounting the gauge. If I can do it? Anybody can.

Bill Verburg · Bill Verburg's Garage

Quote:

If I can do it? Anybody can

Maybe i'll try it

buster73 · buster73's Garage

From the data posted above, it looks to me that the DME sensor just reads up to 100 C. This might be enough for the cold start enrichment function, but will not help you much with your challenge. Typically the CHT will be between 300-375 F (150-190 C) and the max. limit should be 400 F (205 C). The spark plug location typically reads 50-100 F higher than other locations such as head studs. If the spark plug CHT sensors are a resistor type (?) you might indeed use them with an oil temp gauge. You would need either the specs of both sensors or better one of our electrical gurus to figure out how to connect or adjust for the range (sorry, I'm a mechanical engineer and try to stay away from life wires if possible).

Jack Olsen · 03-30-2005, 02:20 PM

I just ordered the 0-600 degree VDO unit. I'm going to make it a permanent install in my center console, which currently has a Voltometer and a Trans Temp gauge.

Tyson Schmidt · 03-30-2005, 04:38 PM

Quote:

Originally posted by Rob 930
Tyson,

I'm about to install a factory Carrera head temp sensor in one of my 930 heads. I have no current plans to monitor the temp or use the temp as an input for my injection system, but I thought that while I was at it, it would be good to install one. You said cylinder No. 5 is where to put it. I've heard cylinder No. 3 is the best place. What is your reason for saying cylinder No. 5?

#5 always seems to be the first to detonate.

The Factory uses the back side of #3 to install theirs, but I suspect that's more for ease of packaging