where is the voltage going ?

[bellerose]

Trane 10 ton roof top unit. Blower fan comes on but the compressor will not.
Manually pushed in contactor and compressors run fine.
Checked transformers and getting 24 volts.
Removed the wires from the compressor contactor and read 24 volts.
Ohmed out the coil on the contactor and got 8.5 ohms.

Put the wires back on the contactor and read 0.22 volts.

Put the wires on a different contactor and got same result. 0.22 volts.

Checked all connections. Nothing loose.

Any ideas ?

[Healey Nut]

Sounds like you need a new stroll compressor ,but if you wait long enough Batledo will advise you. That guy can fix anything ,he's awesome ??????????????

[Tech Rob]

Is this a Voyager with a UCP?

Are all of the safeties closed? Do you have any cool fail diagnostics at the thermostat or low voltage terminal block?

If the above checks out, you have a bad UCP.

Check out the Voyager Microcontrols book from Trane and read the section about "Snubber Circuits". It can be pretty confusing. I think the engineers at Trane could have accomplished the same thing with a varistor.

[hvacker]

Removing the wires doesn't count as there are a number of ways 24 volts will be there with no load. The fact that the voltage disappears indicates a problem downstream.

[Tech Rob]

Removing the wires doesn't count as there are a number of ways 24 volts will be there with no load. The fact that the voltage disappears indicates a problem downstream.

If the coil common was shorted to ground, wouldn't the contactor close? If there was a loose connection on the common side, 24 volts would be present up to the loose connection.

[dandyme]

[$QUOTE=bellerose;1929792]where is the voltage going ?

Any ideas ?[/QUOTE]



If I tell you, I'll have to kill you.

[-459FZPE]

My thought would be that your off on a safety switch of some sort. HPS, LPS, OPS. I would work your way from where Y comes into the unit towards the compressor and see where you lose your 24 at.

[berg2666]

My vote,

Lost common or bad contactor coil......

[Tiger93rsl]

It sounds to me like you have an open safety even though you are getting your 24V. Or as previously stated a bad common connection to the controll transformer. Those UPC's can be a little tricky

[desertdweller]

Have you cycled it through to cooling on the test pins? Does it have an economizer? What controls it Tstat, Tracer,etc?

[-80 chuck]

If this is a carrier unit check the CLO board had problems with these before they will let trace voltage through but not enough to power the contactor coils.

[superandy]

There is something in series with the contactor circuit dropping the voltage. Most likely a safety or control of some sort. When you break the circuit by removing the contactor there is no voltage drop through the circuit because there is no current.

[crab master]

I've had this happen with undersized wires on a remote condensing unit a few times. The wires without a load would read 24VAC, but when I pushed in the contactor manually the contactor would hold and stay in when there was a call if I pushed it in manually.
The first system worked fine for a few years and then when I got there on the initial call the compressor contactor would not pull in all the way. The coil overheated and bulged out and the coil could not pull in due to the swelling of the coil. Replaced the contactor and then I had the problem of having to manually push in the contactor to get it to pull in. Tried a different contactor, same thing. Called out one of our senior techs and he installed a pilot relay who's coil operated off of the existing 22 ga control wiring and then switched the compressor contactor with the condensing unit's voltage to power the coil. This worked just fine. The pilot coil did not need near the inrush current as the compressor contactor so the 22 ga wire at about 200 ft could pull in that coil.
In your instance I do think you may have a common issue as others are saying but try pushing in the contactor when you have a call and see if it stays pulled in.
This is a prime example of why I hate it when people/designers switch the common wire, as it is harder to troubleshoot.

[DAllen]

If its a voyager, the first thing I would do is power it down, disconnect the economizer harness, and power it back up. the board should then recognize that no economizer is installed. (you do not need a harness jumper) If it starts I would check the enthalpy sensor and control.
I had this same problem, turned out to be the control.
Incidently the low voltage between the thermostat and unit is D.C., follow the directions on control panel front for troubleshooting. (I'm sure you knew that but there it is anyway's)

[JRINJAX]

The advice to disconnect the econo and run a cool test is where I would go.

There are a lot of cheaper circuit boards that use Triacs [solid state AC switch] instead of relay outputs that will show 24 VAC but will not pull in a coil. Usually this phantom voltage will disappear with a load is applied to it [unless the relay/contactor coil is open]. Just putting a Fluke meter across it does not generate a load and the phantom voltage will appear on your meter even though it is not useful to pull in a load. The Triacs are also used on damper motors too.

Clear as Mud?

[3rdgenhvac]

Just a lil concerned w/ just pushing in a contactor, they do blow apart sometimes especially if the circuit is off on a safety for a reason.

[timebuilder]

The advice to disconnect the econo and run a cool test is where I would go.

There are a lot of cheaper circuit boards that use Triacs [solid state AC switch] instead of relay outputs that will show 24 VAC but will not pull in a coil. Usually this phantom voltage will disappear with a load is applied to it [unless the relay/contactor coil is open]. Just putting a Fluke meter across it does not generate a load and the phantom voltage will appear on your meter even though it is not useful to pull in a load. The Triacs are also used on damper motors too.

Clear as Mud?

Yep.

A high impedance meter will show you voltages that have a negligible current potential. If you switch to "lowZ," you will usually drop these voltages across the meter circuitry.

The Trane UCP board (or the Reliatel) is not the only control situation where you you can have a voltage at the coil, but have insufficient current to pull the coil in. Any high resistance in the circuit can limit the current to a value that is less than pull-in current. This could be a high resistance in a safety switch, for example.

Just a couple of weeks ago, I found the same situation on an old York heatpump outdoor unit. The board was replaced, and all was well. The solid state triac on the board could pass the potential voltage, but not flow enough current to pull in the coil.

Jayguy may correct me if I'm wrong, but I would run the test mode and see if the contactor will pull in, after checking for open safeties. If it won't pull in during test, I'd look for another board.

[jrbenny]

Voltage is present due to the snubber circuit (capacitor and resistor wired in parallel with relay). It lowers the voltage potential across the relay to minimize arcing and pitting.

With that said, economizer or safety is at fault.

If you don't have a copy of the Voyager Microcontrols Manual, right click and save as.

http://www.hvac.amickracing.com/RTU%20Training/Trane%20RTU%20info/VoyagerMicrocontrols.pdf (http://www.hvac.amickracing.com/RTU%20Training/Trane%20RTU%20info/VoyagerMicrocontrols.pdf)

Snubber circuits are explained on page 170.

[JRINJAX]

Voltage is present due to the snubber circuit (capacitor and resistor wired in parallel with relay). It lowers the voltage potential across the relay to minimize arcing and pitting.

With that said, economizer or safety is at fault.

If you don't have a copy of the Voyager Microcontrols Manual, right click and save as.

http://www.hvac.amickracing.com/RTU%20Training/Trane%20RTU%20info/VoyagerMicrocontrols.pdf

Snubber circuits are explained on page 170.

Sorry about giving wrong advice, I did not realize they were snubbed relay outputs. I ASSUMED they were TRIAC and gave bad advice.

Will try to either do better or not speak....

[crab master]

The advice on the triac is good future advice - I didn't think about that as an option. Anyway it sounds like he's got the proper advice now for this unit but hopefully he'll be a bit more enlightened on other units.

Also I've pushed in a lot of compressor and fan contactors and never blew one up. Knock on wood.

[jrbenny]

Sorry about giving wrong advice, I did not realize they were snubbed relay outputs. I ASSUMED they were TRIAC and gave bad advice.

Will try to either do better or not speak....
Always speak.

Just don't get mad when someone gives you the correct answer.

You'll do just fine here.

[timebuilder]

Thanks to jrbenny for the micrcontrols pdf file. Maybe we should start a library section here with resources like that, with a section for each brand, and maybe a list of what each manufacturer has available for the asking.

The snubber circuit reminds me of the capacitor across the Kettering breaker point ignition we used to see on automotive engines. It absorbed the short duration transients when the points opened to initiate the field collapse in the ignition coil.

I'm still curious to hear about what happens on the Trane unit in question when the machine is placed in test mode.

[jrbenny]

That's a great idea......

;)

The Education Forum is coming soon.

[timebuilder]

Are you saying that this was already slated to be a part of the new education forum? :D

By the way, when is new section this going up on the site? :confused:

[shifferbrains]

Just a lil concerned w/ just pushing in a contactor, they do blow apart sometimes especially if the circuit is off on a safety for a reason.

This is a very valid concern.....

Most tech’s will eventually end up manually closing a contactor that doesn’t pull in. For some reason we just gotta hear that compressor kick in. And when it does.... Instantly we know..... well the compressor is good. Is it a good idea? NO! Is it the safest thing in the world to be doing? NO! Do we all do it? Yes. I kind of move to one side and squint my eyes when I do it.... That way I won’t have to see my fingers leave my hands if the contactor blows up.

Actually in the case of the problem in this string I wouldn’t have manually exercised the contactor. The proper procedure is to skip backwards from the contactor coil and check all safeties and controls looking for an open circuit. When you find the open device you can make a more informed decision on whether or not to close the circuit for test purposes.

[jrbenny]

Are you saying that this was already slated to be a part of the new education forum? :D

By the way, when is new section this going up on the site? :confused:
September '08 or sooner.

[joey791]

Always keep an open mind, my first guess is open safety circuit,second that the coil is shorted(get another and ohm and compare), , or third(this is where it gets real fun, never forget to keep an open mind)

Did you check the multitaps on your transformer to make sure its tapped for the right voltage?

We had a Voyager once that was dropping voltage on the compressor contactor and every once and awhile would blow the control fuse, sat there and watched the unit run, would always straighten out when you changed the contactor, yet it kept on eating contactors. Finally in an effort to find our voltage loss, we threw everything out the window and started going through everything from scratch(I wasnt there but my dad was), he found that whoever wired in the smoke detector/fire alarm used and left a whole roll of thermostat wire to hook it up and they had put it on top of the duct.
Hopefully its something simple though