Engine Cylinder Heat Sensor, Modification? Options

[martinef1963]

Hello to all. I have never read on the site, not that it hasn't been discussed - the use of the FI engine cylinder heat sensor being used on VDO gauge for a car that has been converted to carbs.

The install of the ring of the VDO sensor is held in by the #3 cylinder sparkplug to engine case - while R&Ring the sparkplug that ring gets beat up and access to it with the tin is a real pain in the....

If one splices the wire to positive on the FI sensor, does the sensor not ground itself to the engine and send signal to VDO gauge? Or do you still need the negative feed back to the gauge. Just asking.

thanks,

Fern

[ThePaintedMan]

I don't know enough about electronics to know if the signal that the original CHT sender puts out would even be close to the very, very tiny electrical signal the VDO guage needs, but I doubt it.

On top of that, many more experienced engine gurus here have said the most accurate place to measure the hottest engine temps is under the spark plug.

The solution is here:
http://www.germansupply.com/xcart/customer...t_install_2.tpl

I just did it on my spare engine while it was out, and it worked like a charm. Never have to worry about mangling the sender again, and I was able to run it back through the original CHT hole in the tin.

[timothy_nd28]

Initial resistances are all wrong, but if you are witty enough you could make anything work

[McMark]

Completely different setup. vDO is volt based. FI sensor is resistance based.

And it's a terrible location for a CHT gauge. The stock sensor just tells the ecu when the engine is warmed up in a broad sense.

[stugray]

Most gauges we use on the older cars are spring loaded torque motor known as a d'Arsonval movement.
It uses current through two coils (and sometimes a magnet) to deflect the needle.
Most sensors like fuel level, temperature, or pressure are of the variable resistance type.
They change the resistance between two terminals based on changes in whatever they are sensing.
The gauge provides 12V to the sensor and based on how much current flows moves the gauge needle.

The CHT temp sensor is a Thermistor. It changes from ~2250 Ohms at 77F to about 70Ohms at 266F.
It is almost a dead short above that and the accuracy is poor.
The stock use with the FI is just to tell the ECU when the engine is "Warm enough".
It didnt need to be accurate at running temps.

So while you could modify another gauge and "calibrate" it to read the CHT, it would not be terribly useful.

I bought the Innovate 4 channel Thermocouple reader.
With it I get to monitor 2 EGTs & 2 CHTs but I dont have a realtime display when driving unless I want to lug around a laptop.

Stu

[McMark]

If the spark plug ring connection bothers you enough, just pull the motor, pull off the engine tin and affix a custom sensor setup. I'm not being snarky, either. Really, that's what I would do. (IMG:style_emoticons/default/beerchug.gif)

[martinef1963]

I wanted to thank all that commented. The link (German Supply) is very useful, don't know if it bothers me that much to pull the engine at this moment, heck I got to get it running first. If I had to in the future will definitely use the method on the link. Thanks again.

Fern

[r_towle]

You could just use an old cht sensor to hold down the ring to the head.

It will tell you a temp once you understand where the gauge should never go, you should be fine...