EIS probe wires

I'm finally diving into the FWF wiring for my EIS and boy is my head spinning. I think I've figured out where to place the EGT probes but am confused by the install manual on the type of wire required. The wires that come attached to the probes are obviously special, but are not long enough to reach my panel. Meanwhile, the wires that come off of the EIS are plenty long enough to reach the motor. Finally, the manual says that to extend the probe leads requires special yada yada.... So, what is SOP here? Do I penetrate the firewall with the wires from the EIS and then connect near the engine to the probe leads? I'm trying hard here not to sound stupid, but I'm not sure it's working. Any expereince would be appreciated and pictures are the best.

szicree said:

I'm finally diving into the FWF wiring for my EIS and boy is my head spinning. I think I've figured out where to place the EGT probes but am confused by the install manual on the type of wire required. The wires that come attached to the probes are obviously special, but are not long enough to reach my panel. Meanwhile, the wires that come off of the EIS are plenty long enough to reach the motor. Finally, the manual says that to extend the probe leads requires special yada yada.... So, what is SOP here? Do I penetrate the firewall with the wires from the EIS and then connect near the engine to the probe leads? I'm trying hard here not to sound stupid, but I'm not sure it's working. Any expereince would be appreciated and pictures are the best.

Click to expand...

You've got it figured out Steve - route the wires from the EIS through the firewall, and make the connections to the EGT Lead wires up there. The lead wire lengths are important, the wires between them and the EIS are not. I ran mine in fire-resistant snakeskin to make the FWF look a bit neater.

Paul

Connections to EIS?

So what do most folks use to connect the 8 EGT/CHT probe wires to the EIS wires? I mean, I've got the regular wires all coming forward through the firewall, and 8 special probe leads coming off the engine, so how to I join em? knife splices? Molex, terminal block?

The connectors came with my unit-----------I think you should contact the manufacture of your equipment, and see if they have any specific recommendations.

Are there any connectors already on the probe wires??? If so, I would think just using the mating connector should be enough.

Be sure to support these connections/wiring well, lots of vibration there.

Spade Plugs

I used spade plugs that I "deoxit'ed" then heat shrinked so they cannot come apart. all the engine wiring then goes into a firewall pass through with quick release connectors on either side so that you can remove the engine or panel easily without having to cut any wires.

Nucleus

Steve, are you talking about GRT EIS4000? You should have received additional connectors. The engine-side wires were pre-terminated.. and all you have to do is trim the EIS-side wires and put connectors that came with the EIS..

Still trying to understand the theory here.

The probe wires are clearly special and the manual says that to extend them requires additional special wire. However, it says that shortening them is ok. Can somebody explain exactly what the science is behind this?

Thermocouples work because dissimilar metals in contact with each other generate a voltage that is proportional to temperature. With a type K thermocouple commonly used in EGT probes, you have one Chromel wire, and one Alumel wire. Everything is just fine until you join the Chromel and Alumel wires to copper. By doing so you form two new junctions. When the temperature of those junctions change, small voltages will be induced. When all is said and done, your thermocouple is essentially detecting the difference in temperature between the main thermocouple junction, and where it first joins normal copper This usually happens at the EFIS, but it could also occur if you use ordinary wire to extend the leads. Most modern EFISs compensate for the effect by measuring the temperature somewhere near the EFIS terminals. Older self powered round dials do not.

The issue of lead length is a round dial issue. The meter movement needs to draw a small amount of current to do its job. That small current across the thermocouple leads, will lead to a small voltage drop. This wouldn't be such a bad thing if we weren't trying to measure voltages in the millivolt range. Adjusting the length of wires in a self powered gage will introduce error. With a modern EFIS, the impedances are usually high enough that lead length will not have a measurable effect (as long as you use the proper thermocouple wire)

Make sure you have the proper extension wire. Type J wire will not work with a type K thermocouple.

Clear as mud?

Paige
RV-8A

I'd love to say I get it now, but I still don't. The wires off the efis are ordinary copper wire. The wires from the probes are special. I can connect the special wires to the plain wires and all is good, but to extend the probe wires I must use special wire. Well, couldn't I simply extend the plain wires from the efis longer instead? I know I'm misunderstanding something.

Sorry, for the late reply, I've been out of town.

The EIS is measuring the temperature difference between the Copper-to-Thermocouple wire junction, and the Thermocouple probe. I'm really surprised they used ordinary copper wire. You might be able to get away with it in the cabin, but I would think a copper-to-thermocouple junction in the hot engine compartment would give you erroneously low readings on all of your thermocouples.

Are you sure it's copper? Thermocouple extension wire usually has ordinary PVC insulation. The only way to tell is to strip the wire.

http://en.wikipedia.org/wiki/Thermocouple

Paige

My understanding of this is that at every dis-similar metal junction a voltage will be generated due to temperature changes. As long as you make exactly the same type of connection to each wire in the pair from the thermocouple, a voltage will be generated at each junction, which constitutes what is called a common-mode voltage. Since the same voltage is generated in each wire, they effectively cancel since the instrument is looking for a voltage DIFFERENCE. Do not gang the returns together! Keep the pair separate all the way to the instrument connector. I have a Molex connector in my four EGT and CHT wires, and a four-pole, four throw rotary switch which allows me to look at any one cylinder's EGT and CHT on my RMI engine monitor. As long as all of the interconnects are the same, there is no problem. Don't crimp - solder all connections!

PaigeHoffart said:

Are you sure it's copper? Thermocouple extension wire usually has ordinary PVC insulation. The only way to tell is to strip the wire.

http://en.wikipedia.org/wiki/Thermocouple

Paige

Click to expand...

Good idea. You're right and I was crazy. Once stripped it is clear that this is special wire. Thanks for the help.