I have a problem with a Coleman 7900 series furnace. The blower motor came on for no reason. I believe it is because the fan switch is bad. I tested it with my fluke meter. The schematic shows normally open. The switch is shorted, or measures 0 ohms. I had a problem last summer with the outside A/C unit blowing fuses. Is there some connection. I'm an electronics tech, but not a heating and air guy. What do you think?
Type of HVAC: Heater & Furnace
I replaced the thermostat last year. The blower wouldn't run while at the same time the outside A/C unit was running. The A coil froze up. After that is when the outside A/C unit started blowing fuses.
All work to be done by licensed professional. WE KEEP GOING UNTIL YOU HAVE THE INFORMATION YOU NEED, I come and go. This is step ONE.
If the fan switch is bad and shorting one contact to the other.. it will signal the control card to close the contacts on the fan relay. and would not cause a fuse to blow elsewhere.
IF... the control card is defective, it could produce all sorts of negative symptoms including causing its own fuse to blow. and possibly causing the fuse to blow on the control card in the outside unit..
Tell me in detail which fuse is blowing in the outside unit, is it a fuse in the control circuit or is it a fuse in the main power disconnect...
It is the fuse to the main power disconnect.
Those should be dual element fuses, called time delay fuses. If those blow *instantly* when the outside unit contactor comes in, then there is a dead short in the system.
IF the fuses blow after the compressor hummms a bit, then the problem could be a bad compressor run capacitor.
I can coach you on trouble shooting that.
The fuses blow instantly. Some of the connections were wire nutted together.
Those fuses blowing would have an effect on the Furnace due to the nasty electrical spike produced with dead shorts on any electronics in the house... especially any mounted to the same hardware.
Either one of the wires has over heated, melted and is shorting to the conduit or a ground....or the compressor or fan motor is burned out and shorted to ground.
First look around for burnt wires.
l checked them one at a time to make sure they were ok when I was having the thermostat problem. The furnace worked OK all winter but I haven't got around to finding out where the short is.
Next be sure the power is off... then triple check for power at the unit... make sure its all devoid of power
Next, locate the spot on the compressor where the wires attach, remove that cover.
Next make a detailed wiring diagram, mark all wires, then remove them.
Next, set your ohm meter to read 0-10 ohms or the lowest range it has.
and check the resistance between all 3 combinations of the 3 terminals, those are marked C,S and R.
C to S should read 3 ohms typically (varies with horsepower a bit)
c to R should read 2 ohms
s to R should read the total of the two, 5 ohms in this case.
it is this total that counts,
None of the terminals should give any reading to ground or the unit cabinet or piping
If you get a reading to ground the compressor is burned out...
Let me know how that checks out, we keep going as necessary. Phil
C - S =228 ohm
C -R = 228 ohm S - R =4.7 ohm That was with a digital meter. Should I get out my good analog meter?
I guess I need a new compressor.
Try your analog meter
S - R was close to a very good read
you would not have gotten that read if the windings actually read 228 ohms
visualize a V
Common is at the bottom of the V
S is one leg, its a finer copper wire winding that the R winding
when you read S - R you are reading the total of both, through the C connection.
C connection can fail but its not likely to in that fashion
S is the start winding
R is the run winding
Tell me if you any reading from any of the terminals to ground.
Find the run capacitor.... the wire connected to the S terminal goes directly to the run capacitor.
tell me if its bulged, or has any oil on it.
Be careful, those can explode if they are shorted.
If they are good, they hold a 370 or 440 charge
370v to 440v charge I meant to type
S-R= 7 ohm, C-R= 25 ohm, C-S= 23 ohm That was with my Triplett analog meter.
Checking for grounds now with analog meter. Didn't get any with the digital meter. I did find the schematic on a sticker inside the cover.
No shorts on the compressor.
Those are bad ohm reads, they make no sense, and are not dead shorts of course, so should hot blow a dual element type fuse instantly, but would cause it blow in a few seconds..
I've been in this business for 51 years... when i get reads like that its usually because i am not making good contact with my probes against the terminals. You have to rub the probe tips hard against the terminal posts.
Tell me if the V diagram makes any sense to you.... from that you should be able to see why those reads don't add up.
Is the C terminal showing signs of over heating?
It is more than slightly odd that you got the same reads with your digital meter.
The only thing that would account for that is burnt contacts in the internal compressor overload device... breaking power at the common when it trips. so neither the run nor start windings get power.
and leaving the R and S still connected at overload fit between the Common post and a base of the V
Ive never seen that before however.
The contacts on those overloads do fail, but not with resistance... they are either open or closed.
and would not cause the disconnect fuse(s) to blow instantly.
It is possible there is an intermittent short that only occurs under high tension... thats not common however.
We use a megohm meter to check that...
I understand the concept. The coil is 5 ohms end to end. Common connects almost in the middle. If it was in the middle it would be 2.5 ohms to the other posts. I did try to make good contact . I can't see the C terminal very well because of the cover. I did get the plastic cover off but there is a partial metal cover attached to the compressor over the terminals. I may have to disassemble more to get a good look.
The readings on the digital meter were 228 ohms. 20 ohms on the analog, instead of 2 or 3 ohms The 5 ohm was the same. I suspect the leads on the digital .
I have used a megometer meter at work. I don't have one at home.
I may have to try again in the morning. If the compressor does end up checking good, what next?
There are two coils, the run winding coil and the start winding coil....these are connected at one end only, there is no middle connection.
visualize it as a V with one coil on each end. if you check between two uprights you read both coils in series
because they are connected at the V
You get 5 ohms
thats typically a good read depending on the horsepower, it varies with HP
so that the start winding should read 3 ohms roughly thats C to S
and the run winding should read 2 ohms roughly thats C to R
the only way you can get higher reads is if there are bad contacts in the over load shown in the picture I posted, thats in the Common feeding the base of the V
or if there is burnt wire connection around the C terminal or the overload.
Thats possible, but not common at all
What I do in such cases is double and triple check to be sure,
I was just looking at the picture you sent. That makes sense. A few ohms higher from c - s , or c-r, shouldn't blow the fuse.
If you are not getting the reads you need, next step is to hook it back up and check the amperage draw on each leg using a clamp on type amp meter
You should get amperage on all three legs, amps on the S leg are through the run capacitor. if you get zero amps there, then the capacitor is bad
The fuse blows too fast.
Regarding C to S and C to R ohms reads
I should say both blow too fast.
higher ohms will result in *lower amperage draw,
are the fuses marked
One Time Buzz fuse
or Dual element fuse
what amp rating are they
the compressor is not shorted
it could be the fan motor that is shorted
Yes 30 amp... FRN-R30
Fuzes are correct
Check the fan motor for a short to ground
you can also check at the contactor to ground, terminal 1 to ground and terminal 2 to ground.
the fan motor and the compressor are both connected to T 1 and T 2
if no short there, then the short can be on the Line side of the contactor, possibly in the control transformer, or wires shorting to the unit frame
I checked T 1 and T 2. They seem ok. Thanks for the help. It's getting late. I should let you go and try again in the morning.
Have a good night.. see you tomorrow