Dead Pyro?

Pyrometer, actually. Yesterday morning on my way to work I noticed that the pyro was stuck. This stuff is all brand new, so maybe I got a lemon? Anyway. ISSPRO R605-P gauge and R658 thermocouple with a 10 foot extension harness (I think is was). So this morning I get it to move, but still doesn't seem right. Reads low until it hits about 600* and shoots up for a second then seems to lag and read too cool.

Everything I've read says that the gauges rarely die and it's usually the thermocouple. I just pulled the gauge out of the truck and tested it with a spare AAA battery and the needle jumped, so the gauge is good. I checked ohm'd out the thermocouple leads at the harness side in the gauge pod right after shutting down and got about .8 ohm and then it started searching. I started the truck back up and tested voltage and played with the pedal. It raised and lowered in millivolts with the throttle, but only between 22 and 30mV. So is my thermocouple trash? (Installed new in March).

My other question is, the R658 thermocouple I have is rated for 1600*. Can I use the R658-HT (rated to 2000*) on my R605-P gauge and still be accurate? Diesel Manor lists that it can be used with any ISSPRO gauge up to the rated temperature, but I just want to be sure that it will still be accurate.

R658 and R658HT are both K-type and will have the same output at any given temp. The HT has more insulation on the wire and a longer probe, thus can read a higher temp before cooking the wires from heat transfer.

A table of K-type output says 900-1330* F for the mV range you specified... I'd guess your thermocouple is DOA or there are wiring issues. Check it again when the truck is cool. 77* should give you 1 mV, so expect a value around that.

I'll try checking it Saturday and pulling the sheaths off the connectors between the extension harness to see if something came loose. I have the harness running through a grommet I installed on the firewall (plenty of vizio) but I'll try to check it out as well.

i liked pirate better

I did too, lol. The wife calls it the pirate-meter. Tells me how many are nearby.

Thermocouples rarely go bad. When they do go bad you usually get an open circuit because the junction is broken. I would check the wiring carefully. It is possible for them to short out internally but that usually results in a lower than expected reading. The pyrometer gauge itself is actually just a millivolt meter calibrated to the "type" of thermocouple. Bouncing around would lead me to thing there is either a break in the wire or something is grounded out. BD seems to be spot on with the temps. You used standard thermocouple wire all the way to the gauge right? Is there a plug anywhere between the gauge and the probe?

It's all ISSPRO's ring terminal connectors and bolts with fiberglass sheathing zip tied around the connections. When it works it's reading lower than normal and doesn't pickup like it did when I pull 3rd gear. The top part that I can see of the probe looks like it has a little line in it where is been heated. Going to check the worrying on the extension harness connection to the thermocouple and the grommet in the firewall first. It would be nice if it was just the harness. That's a lot cheaper to fix.

How is the extension harness connected? It the two wires touch anywhere you get a false junction and if I recall, the reading you get is essentially an average of the two spots.

Bolts and ring terminals. Both connections have separate fiberglass sleeves that keep them from touching. They shouldn't have slipped or rubbed through as I have a zip toe on either side of each one and another securing those and the extension harness to the bundle of hoses/wires on the cowl.

Best pic I have at the moment.

http://thesensorconnection.com/category/egt-probes/all-egt-probes/diesel
check these out, seem to be a heavy duty TC, better construction then most gage companys supplied TCs, US made also, you can get either normal response time, commercial duty, or motorsports fast response, commercial ones just have thicker probe,more insulation, makes them slower to read to your gage.

Just got to work and checked to make sure the fiberglass sleeves haven't slipped or worn or anything. Also made sure the extension harness hadn't slipped the firewall grommet. Haven't taken those sleeves off yet, but from the looks of it, it's either a problem with the thermocouple or the harness has rubbed under dash (I'll look for that tomorrow or Saturday).

Pic of the probe.

Tracked the complete length on the extension harness and pulled the fiberglass sleeves off the connections and everything looks good. Since the truck hadn't been started today I checked the leads with the multimeter again (engine cold).

Ohm 0

Voltage -0.05mV

So I pulled the thermocouple out of the exhaust manifold and cleaned the carbon off it with a dry paper towel and had little grey/white chunks falling out of where the wires enter the probe. Reinstalled and started the truck, but can't get a reading sitting still, so I guess I'll see how it acts Monday on the way to work before I go ordering another thermocouple.

-0.05 corresponds to about 35*

It was high 70's to mid-80's ambient outside and the truck was sitting in the sun.

I'd say you have a dead probe. It's probably internally shorted and that's why it reads continuity. It worked correctly before right? Lead wires are yellow and red? I know you probably said this somewhere but what was the probe rated to for temp? While type K is type K, there are differences in construction for a barbecue meat thermometer and a pyrometer probe.