Dumb question...is it possible to use a single thermocouple and splice the wires to go to two seperate units? I have a banks six-gun and speed loader on the way, which has a thermocouple with it so it knows when to back off the fuel. I also want to install a gauge, and i was curious as if it is possible to simply splice the wires from the probe and go to the gauge too. I'd like to avoid drilling the manifold twice, escpecially condsidering the fact i may screw something up on the first attempt. Any input on this subject will be extremely appreciated.

 

I don't know how that system works, but I can say in general terms that when you want a thermocouple to provide a signal to multiple units you usually use a transducer that converts the thermocouple voltage to a 4 to 20 mA signal and send that to the inputs. I know it is easy enough to find a display that will handle the 4 to 20 mA signal but I have no idea if the Banks stuff uses any kind of transducer or you connect the thermocouple wires straight to it.

Regards,

Morris

 

Hmmm.... In reading my reply today I realize that perhaps I didn't actually answer your question. While I have to say that I don't absolutely know that what you want to do is impossible, I don't believe it can work. Another possibility however is a dual element thermocouple. It is simply two junctions in a single probe. That may be the simplest solution of all.

Regards,

Morris

 

Somewhere I read that the length of the wire was critical to the gage calibration. I doubt that one gage could read input from two probes unless routed through a switch that would give you the ability to select the probe.

 

Bob, you are right on. If you change the impedance (resistance) of the lead in wire the gauge will NOT be calibrated. The guage can be re-calibrated but most don't have the equipment to handle this. The link from the probe to where the lead in wires connects has certain amount of impedance and this coupled with a specific length and wire size provides a set amount of impedance thus voltage/current is determined by this. It's called Ohm's law. It might be possible to put a switch right where the probe lead and the lead in wire attach through a single pole/double throw (SPDT) switch "if" you had the exact same size and length lead in wire for both gauges. This however will also make the gauge a little out of calibration due the to the impedance of the switch. Now the switch will not add much impedance but it will add some.

 

Just a minor grammatical correction.

At DC the capacitive and inductive reactance of the wire and switch are zero and what we are talking about really is the resistance of the items. Impedance is the complex term that contains both the reactance and the resistance.

 

I knew I could count on you guys to simplify my highly technical solution

 

The problem here is it is NOT truely DC. It is a varying analog signal being created from the probe. Therefore the correct term is indeed impedance.

 

The rate of change of the DC voltage is so low that the capacitive and inductive reactance associated with the wire and switch discussed are negligible. If the signal from the probe was AC in the MHz range I might agree with you but the signal is more likely varying at a rate that is effectively still DC.

The temperature signal from the transmission for example is a fairly low level DC that varies between 0 to 5 VDC in most vehicles. Each model is different and according to the guy at BD-Power that I talked to many models vary between as little as 1.5 to 3.0 VDC over the entire temperature range.

The voltage swing is non linear over temperature and requires some fairly complex polynomials to implement an accurate temperature display over the range.

So to give you an idea of the rate of change, let's assume that the voltage change across the range is linear and that 1.5 VDC represents 32 deg F and 3.0 VDC represents 250 deg F.

It is freezing outside when you start the truck and it takes 10 minutes for the temperature of the tranny to reach 100 deg F. The voltage would have climbed from 1.5 VDC to 1.968 VDC (1.5 + (1.5/218) * 68) in 10 minutes or 7.798 x 10^-4 V/s.

So I stand by my earlier statement

Daryl

 

Oh, and to provide some useful input as to the original question, I wouldn't splice or tap off the thermocouple wires coming back from the pyro probe without really good instructions to do so from the manufacturer of the 2nd unit.

The instructions with my BD-Power X-Monitor were very clear about not splicing or trying to extend the cable coming from the pyro probe.

In the case of the tranny temp lead where they are splicing in to it to feed my X-Monitor, I believe that the signal is being fed in to an Op amp with very high input resistance so as not to load and disturb the factory temp signal.

 

Daryl, we are supposed to be having fun not using our brains here.
Impedance = is an expression of the opposition that an electronic component, circuit, or system offers to alternating and/or direct electric current.
Reactance = is an expression of the extent to which an electronic component, circuit, or system stores and releases energy as the current and voltage fluctuate with each AC cycle. Reactance is expressed in imaginary number ohms. It is observed for AC, but not for DC.
I'm ready for the weekend...

 

I'm glad my Auto Meter instructions were written so that even I could understand them...duh...my head hurts....

 

That is correct Purplewg, there is no reactance in a DC circuit.

Impedance is the complex sum of the resistance and the reactance, both expressed in ohms. The resistance value is in real ohms and the reactance is in imaginary ohms, i.e., Z = 50 + i75 for an inductive impedance comprised of 50 ohms of resistance and 75 ohms of inductive reactance. Z in this case is 90.1 ohms. In the case of a capacitive impedance it would written as Z = 50 - i75 if one were to substitute a capacitive reactance of the same value.

Impedance is appropriate for talking about speakers, coaxial cable feeding antennas, and in any application involving AC. While you can use impedance when talking about DC circuits, it is rather pointless because the impedance is purely resistive and therefore resistance is the more correct term to use.

 

Yes, I will agree and that is why I stated "impedance (resistance)" for the purist.

 

I think I'm getting a migrane...