When turned on it just humms and blows the 15 amp breaker in the panel

FUSE PANEL LOCATION
The fuse panel is located in the engine compartment. A label on the inside of the cover lists the fuse amperage and circuits protected.
volvo 850 fuses fuse panel layout service repair manual
FUSE PANEL IDENTIFICATION
Fig. 1: Fuse Panel Identification
Courtesy of Volvo Cars of North America.
WARNING: Always disconnect battery ground cable before servicing "high-current fuses. It is recommended that "high-current" fuses be replaced by a qualified technician.
* 1 - 15 Amp
Fuel Injection, Distributor Ignition, Automatic Gearbox
* 2 - 15 Amp
Fuel Pump, Alarm System
* 3 - 10 Amp
Courtesy Lights, Oil Level Sensor
* 4 - Not Used (1993-95)
* 4 - 10 Amp (1996)
Electronic Start Inhibitor (Immobilizer)
* 5 - 30 Amp
ECC Blower (Speed 4)
* 6 - 25 Amp
Central Locking System, Alarm
* 7 - 15 Amp
Audio System (Amplifier)
* 8 - 25 Amp (1993-95)
Power Antenna, Trailer Connector
* 8 - 40 Amp (1996)
Electronic Start Inhibitor (Immobilizer)
* 9 - 30 Amp
ABS, Traction Control System
* 10 - Not Used (1993-95)
* 10 - 15 Amp (1996)
Heated Rear Seat (If Equipped)
* 11 - 30 Amp
Heated Rear Window & Side Mirrors
* 12 - 10 Amp
Brake Lights
* 13 - 15 Amp
Hazard Flashers, Headlight Flashers, Alarm
* 14 - 30 Amp
ABS, Traction Control System
* 15 - 10 Amp (1993-95)
Audio System, Courtesy Lights, Door Open Light, Trunk Light, Glove Box Light, Remote Control Central Locking System,
Oil Level Sensor, Seat Belt Reminder
* 15 - 10 Amp (1996)
Audio System, Courtesy Lights, Door Open Light, Trunk Light, Glove Box Light, Remote Control Central Locking System,
Oil Level Sensor, Seat Belt Reminder, Data Link Connector
* 16 - 30 Amp (1993-95) Accessories
* 16 - 30 Amp (1996)
Power Antenna, Trailer Connector, Accessories
* 17 - 10 Amp
Key Reminder
* 18 - 25 Amp (1993-95)
Heated Front Seats, Power Mirrors
* 18 - 15 Amp (1996) Front Fog Lamps
* 19 - 15 Amp
Left High Beam Headlight
* 20 - 15 Amp
Right High Beam Headlight, High Beam Indicator
* 21 - 15 Amp
Left Low Beam Headlight
* 22 - 15 Amp
Right Low Beam Headlight
* 23 - 10 Amp
Left Parking Light, License Light
* 24 - 10 Amp
Right Parking Light, Bulb Failure Warning Sensor
* 25 - 10 Amp
Rear Fog Lamps, Fog Lamp Indicator
* 26 - Not Used (1993-95)
* 26 - 25 Amp (1996)
Heated Front Seats, Power Mirrors
* 27 - 15 Amp
Back-Up Lights, Turn Signals
* 28 - 10 Amp
Cruise Control, Heated Rear Window, Seat Belt Reminder Light, Shiftlock, Bulb Failure Warning Sensor, Heated Rear Seat
* 29 - 15 Amp
ABS, Traction Control System
* 30 - 10 Amp
Cigarette Lighter
* 31 - 25 Amp
ECC, ECC Blower (Speeds 1-3)
* 32 - 10 Amp
Audio System, Remote Controlled Central Locking System
* 33 - 15 Amp (1993-95)
Power Seats, OBD Test Socket, Tailgate Washer/Wipe (Wagon)
* 33 - 15 Amp (1996)
Power Seats, Tailgate Washer/Wipe (Wagon)
* 34 - 25 Amp
Windshield Wiper/Washer, Horn, Headlight Washer/Wiper
* 35 - 10 Amp
Instrument Lights, Sun Roof, Traction Control System
* 36 - Not Used
* 37 - Circuit Breaker
Power Windows, Power Sun Roof
* 38 - Not Used
* 39 - Circuit Breaker
Drivers Power Seat
* 40 - Circuit Breaker
Passengers Power Seat
RELAY PANEL LOCATION
The relay panel is located at the left corner of the instrument panel.
i copied this off a volvo site Hope this helps

First question what happened to blow the breaker they rarely just go bad...and If you do not understand how the panel works it is best to let a professional do it...saying that I will offer my no responsibility advice,,.first open the door panel and at the top you will see the LARGER master shut off switch that shuts all the power coming through the panel...make sure have something like a fan or lamp on where you can see it when turning the power off ONCE THE POWER IS KNOWN TO BE OFF,,,REMOVE THE INNER BREAKER PANEL COVER AND PULL OUT THE BAD ONE AND REPLACE WITH THE SAME AMP ONE replace the inner panel and push the new breaker to ON and turn the master switch to on...you should have power to the lamp or fan etc. JUST MAJE SURE ALL POWER IS OFF ABD BE VERY CAREFUL..

That's a complicated system that needs a qualified technician to service it. Lots of people can install them and they run fine, for a few years. Your indoor unit, operating on 2 heat elements, will feel like its blowing cold air even though its heating, just not so well. And not like you are accustomed to. Each element (5kw)=22.7amps, on average. So 3 elements operating should trip a breaker. Its probably been operating on 2 heat element plus the heat pump. And now you have trouble with the heat pump. If you can't TRUST the guys working on your equipment, try someone else. A second opinion. Normally a 3 or 4 ton unit will have 15kw heat installed, heatpump or not. 5 ton, 20kw. If yours is a 3-4ton, cooling, the 100amp(or normally two 60 amp) breaker should have been installed upon equipment change out. If yours is blowing cold all the time, something is wrong with your heatpump. The electric heat inside operates only when the heatpump can't keep up(real cold outside), the heatpump is in defrost(then it's actually in cooling mode), or if you have the tstat in aux/em. heat. Try a local trane dealer? Of a technician with 15-20 yrs under their belt. A trane dealer should have trane tech support also. BIG HELP!!!

Probably. Consider also there may be a problem with your impeller ... the motor will humm (because it is running) but it will not pump anything because the impeller may have the center spun out of it.

Hi dtsorrell...

If your somewhat of a mechanic/electrician then you can by-pass the pressure switch by hooking the color coded motor wires together with the incoming color coded wires from your power cord in the proper order.
Plug it in and then check to see if it blows the breaker again, if it does then it is your motor that is faulty.
Please take time to rate me.

This may just be the start celonide. Look's like a "D" battery or bigger in the back of the motor or on the top in a half round case. It is ment to boost the power to start the motor turning. If it bad the motor just humms till the breaker blows. Sounds like just what you have.

Must be your breaker amp must be below requirement. Do not use an extension cable, make it short as possible to the source. It would also be better if you have a separate breaker switch just for your compressor in the panel box.

nope, need a new motor

it may be the fuse that has blown out.

but not sure.

if fuse is out the whole unit wont start i think