CHT Gauge, WTFis wrong with this thing!? Options

[Chris H.]

I bought a new-in-box CHT gauge from e-bay and installed it per the instructions. It does not appear to work. Does anyone know how I might go about troubleshooting it? I have the thermocoupler thing under plug #3 and the wires running to the gauge correctly. The gauge does not "wake up" and move to the 100 C mark when I turn the key on like the other ones, but I have no idea whether it should or not.

I know it is hooked up right and have triple-checked all of the connections etc. This thing is pissing me off!

The only thing I can think of that is out of the ordinary is that when I was pulling the wires through the firewall the ground wire must have caught the lip of the hole and the insulation was peeled back on a very small section. I wrapped it in electrical tape, so it should be fine.

Thanks in advance for any advice. Sure does look purty in the console though! (IMG:style_emoticons/default/mad.gif)

[PorscheTom]

ChrisH, give it a bit of time. It does not pop up when power is applied because technically, there is no 12vt power to it - other than the light bulb. Think of it as a mechanical gauge. Now, start your engine and drive it around the block a couple times and the needle should start to move. If it doesn't move, double check your connection at the gauge and that area you mention scraping off some insulation. Do not cut any of the sender wires, that will throw off the reading of the gauge. There really isn't much that can go wrong with the CHT gauge.

I've hooked up probably over 8 of these gauges and never had one not work.

Good luck!

[Chris H.]

Thanks Tom.

I actually did hook it up and drive around a bit. If it moved it was just barely, but I don't think it moved at all. What should it read under normal conditions? I think I heard 250-300? I'm afraid it's the gauge itself since I tested the sender wires and they were OK.

[SirAndy]

I heard 250-300?

wouldn't that be F and not C ???

you mentioned 100C in your first post, that's way beyond BOILING HOT, if your car is 100C with just the key turned on, you're on venus (IMG:style_emoticons/default/biggrin.gif)

seriously, is the gauge Celsius or Fahrenheit?
Andy

[Elliot_Cannon]

When I installed mine it didn't work initially iether. I discovered the wires were hooked up backwards. I swapped the wires around and it worked fine. It sounds pretty simple and you probably already tried this but that or a broken wire is about the only thing I can think of that would prevent the gauge from working.
Cheers, Elliot

[Chris H.]

I heard 250-300?

wouldn't that be F and not C ???

you mentioned 100C in your first post, that's way beyond BOILING HOT, if your car is 100C with just the key turned on, you're on venus (IMG:style_emoticons/default/biggrin.gif)

seriously, is the gauge Celsius or Fahrenheit?
Andy

Yes, Fahrenheit. See how pissed I was? My blood was 100 C. (IMG:style_emoticons/default/biggrin.gif) I have been messing with this thing for 2 weeks!

Anyway thanks all. Elliott I did check the wiring, but I think maybe I just need to start with a new set of sender wires.

Note to self... when everything is working fine, DON'T MESS WITH IT BY ADDING MORE STUFF!!!!

[lapuwali]

Assuming the gauge is working at all, it's pretty hard to screw this up. The wires do, indeed, need to go on one way, and if they're on the wrong way, the needle will never move. The gauge is powered by the weak current the thermocouple itself generates. Any external power is only for the light. Ignition on or off should have no effect at all on what the gauge reads. Engine not running and dead cold, and my gauge usually reads around 150dF, so I think below about 200dF or so, the gauge is simply not reading at all (current too weak). Within 30-40 seconds of startup, it should read about 200-250dF.

Normal readings vary on the state of tune of the engine, but figure cruise at surface street speeds will be 250-300dF, cruise at freeway speeds 300-350dF, and heavy loads (pulling up a hill at freeway speeds) will be 350-400dF. Readings will be higher on cold days, and lower on hot days, as the gauge is actually displaying the difference in temperature between the thermocouple ring and the connector, adjusted to assume the connector is at 70dF. If it's hotter than 70dF, the gauge will see a smaller difference, and read low.

[STL914]

Does anyone have any idea what the normal readings should be on a Celsius graduated gauge?? My 72 has one and I'm not sure what I should be looking for.

[lapuwali]

120dC is about 250dF, 175dC is about 350dF, 230dC is about 450dF

For more detail, I'd look up one of the zillions of temperature conversion tools available online.

[Chris H.]

Assuming the gauge is working at all, it's pretty hard to screw this up. The wires do, indeed, need to go on one way, and if they're on the wrong way, the needle will never move. The gauge is powered by the weak current the thermocouple itself generates. Any external power is only for the light. Ignition on or off should have no effect at all on what the gauge reads. Engine not running and dead cold, and my gauge usually reads around 150dF, so I think below about 200dF or so, the gauge is simply not reading at all (current too weak). Within 30-40 seconds of startup, it should read about 200-250dF.

Normal readings vary on the state of tune of the engine, but figure cruise at surface street speeds will be 250-300dF, cruise at freeway speeds 300-350dF, and heavy loads (pulling up a hill at freeway speeds) will be 350-400dF. Readings will be higher on cold days, and lower on hot days, as the gauge is actually displaying the difference in temperature between the thermocouple ring and the connector, adjusted to assume the connector is at 70dF. If it's hotter than 70dF, the gauge will see a smaller difference, and read low.

Thanks James, and yes I agree this is pretty easy to do. The oil pressure gauge I did at the same time was a lot more complicated. Something must be defective and I'm afraid it is the gauge. There are only 2 wires to the thing, "+" and "ground". Pretty hard to mess that up. Either the sender wire has a short or the gauge is no good.

[bperry]

You may want to verify that your wires are good from
your sensor to your meter, Eespecially since you mentioned
you had some wire coating damage when they were
pulled through the firewall.
It could be that one of the wires is actually broken inside
the coating. I've actually had this happen to me before
and it is not always obvious that this has happened.
I'd test this by disconnecting
the CHT meter and the sensor then using an ohm meter measure
the resistance betwen the 2 CHT wires and then between each
wire and chassis ground with nothing attached to the other end
of either wire.
This should be infinity for all 3 tests. If not, you have a short.
Again with meter and sensor disconnected,
then jumper the far end CHT wires together at the meter
and measure the resistance between the wires where the
sensor plugs in and it should approach zero. If not, you
have a broken wire somewhere.

If that passes, then I'm not sure, but here is a link to some
interesting information with some test procedures that you might
find interesting and useful.

http://www.rst-engr.com/rst/articles/egtcht.pdf

--- bill

[Chris H.]

Thaks Bill! (IMG:style_emoticons/default/beerchug.gif)

[Chris H.]

Got it! Hey Bill, thanks again. Geez that PDF was exactly what I needed. Below is the post-mortem in case anyone else has trouble.

So anyway I took the whole f*(&(ing mess out of the car completely and brought it inside. I checked the sending wires, thermo, gauge, it all seemed OK. So per the document Bill provided I tested it. I heated the thermocoupler and at first I got nothing. It just sat there even though the heat source I used should have made it register almost immediately. After about 20 seconds the gauge woke up. It was sort of like it suddenly remembered its job. Works OK now, although it reads a tad lower than I expected. I know, it's hot so it will read low...

The funny thing is that I have discovered that my oil temp sender must either be defective or the wrong one. Since I installed the bigger combo-type gauge it seems to read high, like 3/4 of the way up. The gauge is not defective, and the engine isn't running hot. When I turn the key the needle moves about 1/8 inch, even before I start it. Easy fix, right? I hope so. I will order a new sender monday.

So one gauge reads high, one reads low. Ah... 914s... ya gotta love 'em!

[Joe Bob]

Oil temp gauges need to be matched with the correct sender. They are all not the same....."I" always check with a hand held pyrometer....I have found four or five that have varied by as much as 50 degrees. They also come in handy for water cooled cars and tire temps....good tool to have.

[Chris H.]

That's good info Mike. Thanks. I thought there might be more than one, my car had a '73 console gauge in it which read normal, and I have tried a later-style console gauge and the big combo gauge and they read high at exactly the same point. Must be a different one for the later years, or mine is toast.

[type47]

i'll second the post about the connections. i bought a gauge at a swap meet and had to buy the sender and the wiring harness online. ok, says me, how hard can this be? so i install all the stuff and hook up the gauge. so i figure that red must be + and black must be - right? go on a test drive and gauge doesn't work. it's a simple thermocouple, no power to the gauge to make it work.... hmmmmm, i wonder what would happen if i reverse the connections..... like i said, i bought the gauge at a swap meet so no instructions were included. now i have an operating CHT gauge (IMG:style_emoticons/default/jester.gif)