CHT wiring confusion

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roachdogg28
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CHT wiring confusion

Postby roachdogg28 » Fri Jun 03, 2011 5:34 pm

So i got a dual aircraft gauge, its a falcon tach and cht gauge. I have it installed, and i cant get either one to work at all. I have gathered accurate temp readings from the multimeter i have, but with the same wiring i get nothing on the gauge, and i have tried reversing connections, with no success. I have the instructions that came with the gauge at the bottom. I dont quite understand them nor what this loop thing is on the back of the gauge. I have standard points, so i didnt snip it. With everything wired i get absolutely nothing. Also im confused because there are a total of 4 connections... I figured there to be one for the tach signal, power, ground, and two for the thermocouple. I emailed the seller to see if i could get further instruction/clarification, but im sure he wont be able to tell much about it.
Plz help, i have bee trying to get an accurate CHT for about 2 months now, and i refuse to drive without one.

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mondshine
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Re: CHT wiring confusion

Postby mondshine » Mon Jun 06, 2011 9:53 pm

I have one of those gauges mounted in my '74 Thing. It is actually a Westach gauge.
http://www.thesamba.com/vw/forum/album_ ... _id=679730
It's probably 10 years old. I just went out to the garage to take a look at the back of it and here's what I found:
First, I was surprised to see that the tach doesn't require 12V power. Maybe I'm just blind. Your diagram seems to show +12V with ignition switched on connected to the terminal labeled #1.
I couldn't see a terminal labeled #1 on my gauge (that doesn't make sense to me) and I can't see it in your photo either.
Anyway, the terminal next to the light socket (labeled #5) is connected to ground.
The first terminal opposite the mounting bracket (labeled #4) is connected to the thermocouple black lead.
The next terminal (labeled #3) is connected to the thermocouple white lead.
The last terminal, almost opposite the light socket (labeled #2) is connected to terminal 1 of the ignition coil (with the green condenser wire).
The yellow loop is cut.

If the CHT gauge does not function, try reversing the black and white leads.
The thermocouple ring must be installed on a spark plug and no-place else. #3 is best.


I hope that was helpful, Mondshine
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roachdogg28
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Re: CHT wiring confusion

Postby roachdogg28 » Tue Jun 07, 2011 2:19 am

That helps alot. I have gotten into contact with the manufacturer support staff, and they gave me similar advice. But the only thing is you mention a total 5 connections, i only have 4 connections. Im waiting for a response from the manufacturer about the tach wiring, because i get the 12v switch, ground, red CHT, white CHT. Only 4 connections, im wondering how i get a signal for the tach without connecting anything to the coil.
You say cut the loop, my manual says to cut only if i use cdi, i still have standard points.
Also the manufacturer said it required a j type TC, i never thought it mattered, but idk.
Its good to hear someone has one similar thats operational. How do you like your westach? Do you feel its accurate? Thanx for the help!
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mondshine
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Re: CHT wiring confusion

Postby mondshine » Tue Jun 07, 2011 6:54 am

The Westach gauge is absolutely accurate.

As I mentioned earlier, the gauge terminal labeled #2 is connected to terminal 1 of the ignition coil (with the green condenser wire).
The terminal next to the light socket (labeled #5) is connected to ground.

If the back of your gauge matches this description, try connecting just terminals 1 and 5 of the gauge to your engine and check whether the tach will function.
This will be easier if you remove the gauge from the dash and just use short wires for this test.
You can also check the CHT function by connecting the thermocouple directly to the back of the gauge (terminals 3 & 4 on the back of my Westach) and immersing the ring into a pot of boiling water.

Let me know what you hear from Falcon.

Good luck, Mondshine
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roachdogg28
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Re: CHT wiring confusion

Postby roachdogg28 » Tue Jun 07, 2011 11:14 am

Ok, sounds good, i will try the tach as you mentioned. The manufacturer says the same thing, but he now tells me it will not function with a k type thermocouple, i need to get a j type now... great. TC wire doesnt come cheap, and now i have about 13ft of useless k type TC wire.

Thanks for your help! I finally know how to wire this SOB.
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roachdogg28
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Re: CHT wiring confusion

Postby roachdogg28 » Tue Jun 07, 2011 6:33 pm

Hey mondshine, i wired up the tach and it works, just 4k rpms inaccurate. I was wondering, you said your little loop is cut, do you have cdi ignition? My manual says only to cut if i have cdi ignition. I have standard points, and im thinking maybe i should cut it, but i dont want to damage it, just in case i need to return it. Im pretty sure the cht side should work once i get the j type TC. Its interesting to see im at 3k rpms at idle...
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mondshine
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Re: CHT wiring confusion

Postby mondshine » Tue Jun 07, 2011 7:01 pm

You can buy a Westach Model712-4WK thermocouple (J) with a ring for 14mm spark plugs for about $20.00 Shop around online.
The leads are 48" long. It is perfectly OK to extend the leads to reach the dash; use 18 gauge wire. It is never OK to shorten the thermocouple leads.

According to Westach, if their tachometer indicates double the RPM, cut the loop.
Check with Falcon before you cut anything.

Good luck, Mondshine
Steve Arndt
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Re: CHT wiring confusion

Postby Steve Arndt » Tue Jun 07, 2011 7:51 pm

ebay the TC wire. It is about 1 dollar per foot if you buy it in partial lengths. I got 100 feet for 30 bucks!
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mondshine
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Re: CHT wiring confusion

Postby mondshine » Tue Jun 07, 2011 8:13 pm

If you choose to make your own thermocouple wire, be sure to use the length that the gauge is calibrated for. (Roach-check with Falcon)
The thermocouple will supply a tiny current, based on the temperature of the joined end and on the length of the wire.
Read up on how these things work; check the Westach website, you can't just make it any length willy-nilly and expect the gauge to read accurately.
Also be sure to locate the ring under a spark plug (preferably #3), otherwise, the gauge is just a nice decoration on your dash.
Good luck, Mondshine
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roachdogg28
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Re: CHT wiring confusion

Postby roachdogg28 » Tue Jun 07, 2011 9:36 pm

Yeah i have a listing in mind thats 20$ for 50ft. The wiring will be done right, it was never the issue. The issue was my dumbass not researching to see what sender this gauge took, so im paying for it.
Anyway the tach is up and running accurately, but i will get back to you guys when the cht is working nicely in a week or so.
Thanks guys!
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Re: CHT wiring confusion

Postby Turbinepowered » Tue Jul 19, 2011 7:09 pm

How did you fix your tach issue? I'm curious, as I'd like to run one of those as well. What were your final wiring positions for the tach side?
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Re: CHT wiring confusion

Postby Turbinepowered » Tue Jul 19, 2011 7:28 pm

mondshine wrote:If you choose to make your own thermocouple wire, be sure to use the length that the gauge is calibrated for. (Roach-check with Falcon)
The thermocouple will supply a tiny current, based on the temperature of the joined end and on the length of the wire.
Read up on how these things work; check the Westach website, you can't just make it any length willy-nilly and expect the gauge to read accurately.


This is slightly confusing, as Westach also sells "extension leads" for their CHT and EGT gauges and probes, to lengthen the wires for other applications.
Steve Arndt
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Re: CHT wiring confusion

Postby Steve Arndt » Tue Jul 19, 2011 7:42 pm

The TC wire generates a voltage potential based on the temperature difference between the hot junction and cold junction. The length doesn't matter within reason.

You can run one TC lead 10 feet, and another one 100 ft. Same reading on the gauge, meter, etc. This is the job of the TC amplifier circuit and a function of the physics. These wires are run from 5000 ft spools in industrial applications. Do you think they measure the lead length and calibrate it based on that?

The confusion comes where VDO cheaps out and sells plain copper wires as TC extensions. This isn't how it is supposed to be done.
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mondshine
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Re: CHT wiring confusion

Postby mondshine » Tue Jul 19, 2011 7:52 pm

The thermocouple wire is made of two special dissimilar metals which will create a tiny electrical current based on their length, and the temperature of the end where they are joined.
Gauges that use a thermocouple are calibrated to a fixed length of thermocouple wire between the thermocouple itself (a ring or probe) and the "cold junction" at the other end of the thermocouple wire.
The cold junction can be at the gauge, or an extension can be added if the gauge is far from the engine.
As an example, let's say that a 48" Type J thermocouple wire puts out 4 millivolts when the ring is at 225* That is what the gauge is "expecting" to make the needle indicate 225*.
If you were to shorten the length of the thermocouple wire, the millivolt output would be altered, and the gauge would be rendered inaccurate.
If you were to extend the length with jumper made of standard copper wires between the cold junction and the gauge, provided the wires were adequately sized (20 gauge wire would be plenty), the gauge would be accurate.
So in short, you can extend the thermocouple wire (with standard copper wire) but you can't make the thermocouple wire shorter.
Hope that helps, Mondshine
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Re: CHT wiring confusion

Postby Piledriver » Tue Jul 19, 2011 8:02 pm

The length means nothing, sorry.
Never use copper wire to extend a TC, it adds 2 more dissimilar metal connections and throws the accuracy of the gauges in the dirt.
Bad. Idea.

The voltage is created by the dissimilar metals at the joints.
The voltage/current is very small, so the length is not significant unless it gets silly, like >200 feet.

Extend only with the same TC wire type, and only with the wires crimped together on one side of a crimp or using matching TC connectors.
(an insulated crimp (with the wires both on one end) or even wire nuts are not "in the circuit" electrically, so they don't throw the accuracy off)
Last edited by Piledriver on Tue Jul 19, 2011 8:13 pm, edited 2 times in total.
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