Mexstan

I have installed a pre and post probe on my turbo. (Why? Just because I wanted to) I am using one guage with a DPST switch on one wire. For the other wire I intend to join them just before the guage and turn it into a common. Anybody see any problems by doing this? To put it in another way, each red feed wire will go to the switch and the two yellow leads will become common at the gauge.

Geico266

The only problem I see is a guy with way too much time on his hands! Why not monitor all the EGT's at each cylinder? That would be a better thing to do.

Mexstan

Hey, great idea!
Actually to do this only takes at the max about another 20 minutes. I did them both together and ran the two leads at the same time. The only extra time was drilling the exhaust for the second probe and that went fast as I did not have to worry where the chips went. Then a few extra minutes soldering the wires to the switch.

BRayls

the switch is made to go with the egt wiring correct? I have talked with my instrument repairman at work and he said it may throw the reading off by as much as 100*(not more and probably only 50). he said it would work as long as the switch is made to be compatible. just the differance in reading may be there. the gauge reads by sending a signal voltage through the thermocouple, the return is differant from the resistance that changes with temp. if the switch is of similar resistance than it should be good. only the differance from the switch being in the loop.

BigBlue

I ran a dual EGT setup on my truck and the only time it read different was at WOT and it was almost 500* different. Cool down and normal driving they were within 100* of each other. Now that was back when all I had was a BHAF and a 4" exhaust.

BRayls

I am not refering to the temp differance from pre to post. just the differance the gauge will see from the extra resistance. he is talking about 1 gauge, 2 thermocouples.

BigBlue

I know. I was just stating my observations from running both gauges.

BRayls

I have seen the same as you on all the 3 gens with the stock turbo, no longer is the 300 degree differance a good rule of thumb.

BigBlue

Nope. I'd love to see the difference now that I'm putting a bunch more fuel through my truck.

Mexstan

BRayls, I don't understand what you mean by a switch that is made to go with the wiring. What kind of a switch is that? I know this system operates on millivolts so is ultra sensitive, but if I cut the wire and do not alter it's length and insert a switch with zero resistance ie a common Radio Shack mini switch, then how would this affect the reading? How would your instrument guy know how much the reading would be affected if he does not know if the lead length was changed and what kind of switch was used?
What extra resistance would there be by installing a switch that makes or breaks a circuit? If I use an ohmmeter set to the highest reading I doubt if I could tell the difference in resistance between a short, a half inch length of egt lead wire and the closed switch. My installation does not change the length of either wire any more than about .25". Even if I shortened the wire by about 2", it should not make a discernable difference in the reading. I can see a possible change in the reading if only one leg was shortend, but not both equally.

BRayls

the switch should be made of the same material the wire is. I am told omega has the switch we/you want. he says the change would be a max of 100 degrees ,but you are correct that he would not know for sure without all the info. he says the change is very small but enough to change the readings.he said the gauge was made to work with that wire and length specifically ,any change will throw off the reading . its above my head you might want to call tech support of your gauge maker for specifics. BigBlue I seen around 500 on mine with both tst and ez stacked , under wot.

Jim O

The switch should not add or take away any resistance this can be measured with an Ohm meter. Caterpillar have used this in marine applications to measure left bank and right bank temps on vee type engines I know that some people feel that this is a waste of time but I have seen Caterpillar engines in boats with rotory switches so that you could check individual cylander temps . There is not a better way to find a bad injector in a engine room with four other engines running and no one is allowed to shut them down for you.
On my Cummins I installed my thermocouple in the exhaust brake elbow I know the popular method is to go pre turbo but every class 8 truck I have seen with a Cat or Cummins or Sixty series Detroit has the thermocouple installed post turbo. Those guys at Peterbilt, KW, Western Star etc. must know something so thats why I went that way.
Jim O

BRayls

off topic, but Jim O have you tried to check each with just a infrared lazer gun at the manifold port? we have pretty good luck checking the sprint cars engine at each header tube.

Mexstan

Jim, you just reminded me about something. Many general avaiation aircraft especially the six cylinder ones (Lycoming & Continental) use an aftermarket EGT gauge and many of them have a switch to select individual circuits. It's been a while so I forget the circuit, but from memory I don't remember when selecting individual cylinders that both legs of the circuit were switched. Seems to me that only one leg was switched. I also do not remember that the rotary switch was anything special, maybe it was, but memory fails me. I do remember that the system was trouble free.

BRayls

that rotary switch is what omega has. the material the contacts are made of is the same, as the wire used in the type k or yellow/red wire. but I know this its above me .I only know not to cross the 2 types we use on the furnaces. if my memory is right you are correct only one leg at a time.