Have to remove cartridge fuse in order to test?

[destrekor]

I'm having issues with my thermostat (Ecobee won't pull power).

It worked fine previously for a few months since installing, so original wiring checks out. I can manually turn the blower to continuous run, so I'm not sure if that confirms 24v is working or not.

It has 6 cartridge fuses on the board. I used a multimeter to test them and found there was resistance connecting the two ends, but I left them installed. I did flip the breaker though.

Do I have to actually remove them? They seemed to not want to budge, they're likely ancient (paper wrap is all worn and peeling) and I haven't a clue how fragile they are so I haven't desired to pull them unless I have to.

 

[RLGL]

I used a multimeter to test them and found there was resistance connecting the two ends, but I left them installed. I did flip the breaker though.

You do not want resistance when checking a fuse. The meter should show a reading of close to zero ohms, allowing for the resistance of the meter. It is best to isolate them from the circuit when testing.

 

[destrekor]

You do not want resistance when checking a fuse. The meter should show a reading of close to zero ohms, allowing for the resistance of the meter. It is best to isolate them from the circuit when testing.

Well I mean, basically registering something as opposed to 0L.

 

[Red Squirrel]

I would remove it, as it rules out the reading being caused by rest of circuit. If there is a real fault and the fuse is blown, by jumpering it you may even complete the circuit and have voltage going through the multimeter while it's on ohms mode, which is probably a bad idea.

I would test the terminals and see if you get 24v, also worth jumpering them to see if you can activate the heat or A/C just to rule out the thermostat. If it works, the it's the thermostat, if not, it may be another problem.

I forget the color codes, I think yellow + red is A/C, and red + white is heat.

 

[crashtech]

You have to get at least one end of the fuse free to test it, otherwise the meter will be reading other elements of the circuit. In my basic electronics training, this was covered in part by Thevenin's theorem.

 

[destrekor]

I was beginning to assume that would be the case, glad I asked. Thanks. Now, to test, if I can get them out. Sounds like there are typically puller tools for this? Here's to hoping previous homeowner left it with the other odds and ends (cabinet full of some tools and lots of fasteners and nails). There are spare fuses too, so there's a decent change.

 

[destrekor]

Well, no puller that I can find. But I tested with breakers on and off at the thermostat end again, this time making sure to have the right mode set, I found no voltage bridging Rh to C, and from what I reckon, it should. But no combo of wires would pull any voltage.

 

[Red Squirrel]

Check right at furnace mainboard too (need to get into the blower compartment, typically) , just to rule out a furnace issue, maybe the transformer blew or something. Though I don't think you'd be able to turn the blower on either... Maybe some of the wires just got loose.

 

[Rubycon]

Set your meter to volts (be sure to use the correct setting, i.e. AC or DC).
Connect a lead to each side of the fuse. A good fuse will read zero volts. If you read voltage ACROSS a fuse that's in a live circuit that means the fuse is open (blown).
If the fuse has been removed, set your meter to read resistance (OHMS) OR if it has a continuity (beep confirmation when leads touch or complete circuit) use that. Infinity OHMS or silence in continuity mode indicates open (blown) fuse, whereas beeping or low/zero ohms indicates a GOOD fuse.

 

[destrekor]

Set your meter to volts (be sure to use the correct setting, i.e. AC or DC).
Connect a lead to each side of the fuse. A good fuse will read zero volts. If you read voltage ACROSS a fuse that's in a live circuit that means the fuse is open (blown).
If the fuse has been removed, set your meter to read resistance (OHMS) OR if it has a continuity (beep confirmation when leads touch or complete circuit) use that. Infinity OHMS or silence in continuity mode indicates open (blown) fuse, whereas beeping or low/zero ohms indicates a GOOD fuse.

So I have a cheaper multimeter, not a fancy fluke.

Okay measuring DC across the fuses while in circuit and hot, I got 0 volt for each.

I also get 0 volt when touching the Rh and C wires at the thermostat mount. Is that a normal reading?

 

[destrekor]

Also, see this imgur album http://imgur.com/a/ruLgF for photos of what I'm looking at. These are from when first trying to install the Ecobee, you can ignore the text on the last photos.

 

[Rubycon]

That meter will do fine. You can use a Simpson 260 for this kind of thing if so inclined!

However, those pics you linked to show AC line fuses. Make sure your meter is set to read AC volts!
If you're reading voltage across a fuse, it's open.
The transformer reduces the line voltage to 24VAC which is typical control voltage for residential controls.
If your transformer is working, jumping the R terminal to G will run the air handler blower.
If you're missing 24VAC either your transformer has an open winding or its primary isn't receiving line voltage.
Be careful when working around line voltage. The terminals on those fuses, if touched and you're touching chassis or otherwise grounded, will shock you well!

 

[destrekor]

Well I did buy a fuse puller and tested them with the continuity option, and they got the meter beeping.

But testing one of those yellow wires coming from the transformer into a wire nut, I was not seeing 24V... I saw maybe 2V on the meter?

 

[destrekor]

All good! Had a pro come out, he determined it was indeed the transformer (much to his surprise, when after I suggested that as my guess he basically said maybe, but highly doubtful). $243 was the final total bill for the trip, new transformer, and labor.

I feel I could have replaced that myself but did not want to test that theory just yet, and couldn't guarantee that was even the issue.

My house is getting cool! It was over 90F and muggy today, and it's set that to be the norm until Monday. So glad I'm not sweating when I walk in the door lol

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