Need help- 12v EGT guage in 24v truck

So I installed my EGT gauge in my truck today. My original plan was to use a 24v to 12v converter to do the install and then I would have the converter to use for other stuff anyways. Well, my buddy convinced me that all I would have to do is hook up the light for the gauge in-line before the 12v supply for the gauge. In other words, hooking the light up in series with the gauge would bring the voltage down to around 12v for the gauge to operate on. I have seen this work with two 12v bulbs hooked up in series to 24v. Each bulb uses 12v and viola.

For some reason, it didn't work with the gauge. I hooked the light up in series with the gauge, and when I turn the ignition on the gauge "zeroes" like it's supposed to but the bulb doesn't come on. I'm not quite sure what is happening here and if I can't find an easy solution I may just hook up my converter. The bulb is good and the gauge is getting voltage because like I said it does "zero" whenever the ignition is switched. I didn't try to warm the truck up and see if the gauge works because I don't want to fry the gauge.

Any suggestions?

Putting the loads in series is a voltage devider and will work with 2 like resistive loads. You do not have like loads and they may not be resistive.
Either use you converter or connect it to one of the truck batteries, 12V.

Never seen an EGT gauge that has a dc voltage requirement.
The thermocouple supplies the voltage to operate the EGT gauge on all the equipment we have. The only thing the electrical system voltage needs to drive is the gauge illumination. If that's your situation, then just swap out the bulb.

Try hooking up the temp probe (thermocouple) leads to the gauge (pay attention to polarity) and heating the probe end with a match or lighter-- it should give you a reading on the gauge. If it doesn't, try hooking up the probe leads to a Volt/OhmMeter set to the lowest dc voltage scale. If it gives you a voltage reading, then the gauge may be bad.
If the gauge or thermocouple tests bad then get a replacement from the same manufacturer as the thermocouples are matched to the gauges.

By the way; the temp probe leads are a specific length and shortening them will throw the reading off. If you have excess, just roll it up and secure it with a zip-tie.

They do make digital EGT gauges.

Do a search on pyrometers, on one of the sites this was covered the solution involved an inline resistor. The non-voltage type of pyrometer uses a Type K thermocoupling.

Jones said:

Never seen an EGT gauge that has a dc voltage requirement.
The thermocouple supplies the voltage to operate the EGT gauge on all the equipment we have. The only thing the electrical system voltage needs to drive is the gauge illumination. If that's your situation, then just swap out the bulb.


By the way; the temp probe leads are a specific length and shortening them will throw the reading off. If you have excess, just roll it up and secure it with a zip-tie.

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This gauge does require 12 volts for the gauge itself, not just for the bulb.

That's a good heads up on shortening the leads. I cut the original eyelets off the ends of the wires because they were too small for my gauge. I put bigger ones on but didn't lose much wire so I should be OK.

drgreg said:

Putting the loads in series is a voltage devider and will work with 2 like resistive loads. You do not have like loads and they may not be resistive.
Either use you converter or connect it to one of the truck batteries, 12V.

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That makes alot of sense.

Yeah the only reason for 12V is for immulination. But the thermocouple does not supply a "voltage" nor does it make any. "resistance/oham's" is what makes the little needle move! Just like a fuel gauge.

That reminds me, I need to install a "pyro" on my rig.... Thanks

Just went to the ISSPRO site; went to pyrometers, then to installation, selected R605.
The wiring diagram shows two leads (positive and negative) from the thermocouple to the gauge and no other wires... and no 12vdc feed. www.isspro.com/installation/IS119revC.pdf

Might need 12 volts for a digital indicator or a specialty app. like drgreg says. I'll be the first to admit I don't know it all-- just saying I've never come up against an EGT gauge that didn't use the thermocouple's output to drive the gauge needle.
And yeah, thermocouples do put out voltage; it's in millivolts, but it's there.

Thanks for the help guys but I'm going to say this for the third and hopefully final time-

THIS GAUGE REQUIRES 12 VOLTS TO OPERATE.

If you scroll down to the pictures you can clearly see two wires for the light, two for the probe, and two for power and ground.

http://www.autometer.com/productPDF/1129A.pdf

Have you called (815) 899-0801, that's the first place I would ask .

Jones said:

... just saying I've never come up against an EGT gauge that didn't use the thermocouple's output to drive the gauge needle....

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and i've never come across one that did. newer guages are all electronic.

doghead said:

Have you called (815) 899-0801, that's the first place I would ask .

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Thanks, DH. I got it all figured out. I hooked the lights up in series for my EGT and my boost gauge and use my 24v to 12v converter for the gauge power. Works great now. I'm actually pretty surprised at how high my EGT's are and how much boost I have. I've got an LDS 465 1A in my deuce and with the truck empty and my foot into it I got up around 800F and about 15-18 PSI boost with a relatively cold engine. I'm not positive exactly how much boost I'm getting because my gauge only goes up to 10 PSI, but the needle goes way farther than that. I figured the truck would make about 10 PSI max when I bought the gauge. Oops.

I have an Isspro pyrometer. It does require 12v to operate. I changed the light to 24v but the guage has an electronic gizmo box that uses 12v. I called them after I received it and they had another gizmo, similar to a resistor that they sold me to put in series with it. It was not the best solution as it generates heat and can't be covered up.

The guage works good though and I too was surprised at my exhaust gas temperature.

I think I have maxed out the safe amount to turn up the pump with one turn as it does get pretty hot with full throttle on moderate longer grades. I do have to keep an eye on it. Seems like I get around 15lbs of boost.

12 from 24

If your truck has two 12vdc Batteries find the battery that has the system ground attached to it and run a fused positive from the battery to the cab of the truck this should provide you a source 12vdc power. it is like center taping a transformer. or you could build a voltage divider but you will need to take in to account the current draw and internal resistance of the meter because it will effect the design of the circuit as it will be in parallel with your number 2 resister and the total circuit's resistance needs to be chosen based on the current flow requirements. Sorry for the long answer but I hope this helps

no voltage from a thermocouple?? how do you suppose it works?? magic? two dissimilar metals, when in contact and heated make a small voltage with almost no current. the older type gauges were just meter movements and read this voltage directly. problem is they load the couple. all the modern good gauges use a dc to dc converter to amplify the voltage while putting no load on the junction. these gauges therefore require a power supply. this dc to dc converter is very stable as to current draw so it is very easy to just use a resistor in series with the meter to divide the 24 volts, 12 for the resistor and 12 for the meter. you simply measure the milliamp draw of the meter on 12 volts and using the ohms law formula calculate the resistance and wattage of the resistor. the one i used does not even get warm, so they are probably just saying that to be on the safe side.

_E_
I x R

E= Voltage
I= Current
R= Resistance

you will need a resistor near the same resistance of the meter to keep the voltage drop equal on the resistor and the meter. The total resistance must also be low enough to allow sufficent current to flow.

Holy crap. I dunno where you enisteins are going with this, but the gauge is hooked up with 12 volts like it was designed to be and it is working fine.