Pre/Post Pyro Relay

[IRoss]

I was thinking of just buying another thermocouple and using a relay to switch the signal from post to pre turbo while still using the one sending unit and gauge... has anyone done this? If so, what kind of relay am I looking for?

Thanx... .

 

[PToombs]

Why would you need a relay? You don't have a high amps situation. Just use the switch to change from one to the other, you need it anyway. How many wires on the thermocouple? If only 1, get a 2 position, 3 terminal switch. If 2 get a 6 terminal switch. Wire the gauge to the middle terminal(s) and a thermocouple to each end.

 

[RGramlow]

I would use a relay then you only have to run one pair of wires into the cab, and a wire to activate the relay. either get two relays single pole single throw, or one double pole double throw. hook the wires to the gauge up to the 30 terminals and the wires from one sending unit to the 87 terminals and the wires from the other to 87a terminals. 85 terminal gets power to activate the relay, and 86 gets ground to activate teh relay. which ever sending unit you hook up to the 87a terminals will be the one you are seeing on the gauge with the switch off.

 

[Moab4x4]

Thermocouple Tech question

For those that have installed a switch or relay in their thermocouple line, have you noticed a change in temperature readings?

The reason I ask is that I did a bunch of scientific work with type K thermocouples (the same as my EGT gauge uses) in one of my prior lives.



The type K thermocouple uses two different alloys of wire, Chromel and Alumel. The junction of the two causing a voltage differential based on temperature. The gauge then compares that voltage to a known junction in the control circuitry to calculate the temperature. Whenever you introduce a different alloy of wire into the circuitry there exists the likelyhood of a different voltage created at that junction, confusing your electronics.

All the connectors and wiring we used had to be made of the same alloys to minimize voltage errors.



IF all your junctions are the same on both wires, both in alloy and temperature then your measurement would probably be close enough. But I am curious as to that point. I could see a problem if you used two different relays for example of different internal construction.



or is this one of those things that mattered in a lab but just isn't an issue in our environment?

Thx

Ken

 

[dpuckett]

I see the point, but the wire in the lead wire is copper. If one uses copper wire and a cwitch big enough for the load, wouldnt it make for the same readings? i. e. if there is no change in voltage along the way, it should read the same whether it is wired directly, or goes through a switch.



Daniel

 

[Moab4x4]

If the lead wire is copper, there must be something else compensating somewhere else and I agree, a switch would not change anything. Current capacity is really a non issue here as we are talking about millivolts. I would see good quality contacts as a more important issue.



The Type K lead wire extension wires that I have dealt with were also of Chromel and Alumel. That is why the color code matching is so important on the leads between the TC and the electronics. I assumed that the leads I got from Isspro were matched alloys as well but I will admit that I didn't check them to see if they were copper.

If I remember correctly, to see which is Chromel and which is Alumel, one is magnetic and the other isn't. If my wires weren't buried in loom or under the dash I would whip out a strong magnet and check it to see if one of those wires was magnetic.

Ken