blower only works on high January 22, 2010, 09:20:58 PM My heater fan only works on high. I have the manual heat / ac system. According to everything I have read the resistor needs replaced. Is there a way to check the resistor yo verify that it is bad? I have power to all the wiring at the resistor except the black wire which I believe is ground. With the resistor plug in, I have power to all the wires at the switch except the black wire which I believe is ground. Quote Selected
blower only works on high Reply #1 – January 22, 2010, 10:53:59 PM lift one leg of the "thermal fuse" and read across it with your ohm meter ,,it should read open.just cut it out and insert a very very small strand of wire to act as the new fuse. any strand of 22 or 24 awg should do such like what you might find in a cat5 or cat5e cable. 26awg would be even better though.the fan may have build up on the squirrel cage causing it to be more heavy for the motor to spin. When this happens, the motor will draw more current to spin at the rpm demand.just replace the thermal fuse. Thermal limitter is what its called i am pretty sure.obviously you have found the location of the resistor back so yank it out and do the above.some people complain that the brushes in the blower motor are getting worn and the repairs made eventually do not work. the motor may be going bad as well, brushes dont last forever. Quote Selected
blower only works on high Reply #2 – January 23, 2010, 01:53:53 AM Quote from: jcassity;307517lift one leg of the "thermal fuse" and read across it with your ohm meter ,,it should read open.just cut it out and insert a very very small strand of wire to act as the new fuse. any strand of 22 or 24 awg should do such like what you might find in a cat5 or cat5e cable. 26awg would be even better though.the fan may have build up on the squirrel cage causing it to be more heavy for the motor to spin. When this happens, the motor will draw more current to spin at the rpm demand.just replace the thermal fuse. Thermal limitter is what its called i am pretty sure.obviously you have found the location of the resistor back so yank it out and do the above.some people complain that the brushes in the blower motor are getting worn and the repairs made eventually do not work. the motor may be going bad as well, brushes dont last forever.I hate to be buttstuff or a worry wort about this, but I am concerned that replacing the thermal fuse with a wire is not a good thing to do.The purpose of the thermal fuse is to open (blow) if the heat generated by the resisters is not carried away by the moving air.Replacing it with a piece of wire defeats this purpose.The reason they put the resistors in the air plenum is because they get hot enough to be of concern. They used the open coil type resistors so the air can move through them for cooling.Do they get hot enough to cause a fire? I don't know, but that is what I am concerned about. Hopefully the engineers on here can tell us how hot they may get. Quote Selected
blower only works on high Reply #3 – January 23, 2010, 11:45:30 AM Thinking some more about this.I think using the thin wire is more of a fire hazard than using heavy gauge wire. With heavy gauge wire you only have to worry about temperature of the resisters.For the thin wire to "blow" it must reach the melting temperature of copper, 1083° C, 1981° F. So it could get hot enough to ignite leaves or other debris in the air plenum well before it is hot enough to melt the copper wire.Bottom line is, if you want to try to repair this thing you should install another thermal fuse.The 84 shop manual says it is a 121°C (250°F) thermal limiter. Quote Selected
blower only works on high Reply #4 – January 23, 2010, 12:40:29 PM if he has "high" then it's not a fuse. one or all the colis are probably broken. the high setting bypasses the resistor entirely. Quote Selected
blower only works on high Reply #5 – January 23, 2010, 01:39:44 PM Quote from: 32VFoxBird;307575if he has "high" then it's not a fuse. one or all the colis are probably broken. the high setting bypasses the resistor entirely.look again,, there is a thermal limit fuse on the assembly that allows speeds lower than high.softouch- your right.Most people can make the repair like stated but they have to understand that the jumper must continue to be the weakest link , thats why i say the smaller wire the better.I have been running a 30awg strand with no insulation for a long time. to determine the watts / btu, we would have to have more details on the blower motor resistance and from there it would be easier to determine.how much heat does it take to blister a well calloused hand? dono myself but it happened to me. Quote Selected
blower only works on high Reply #6 – January 23, 2010, 01:49:14 PM at a 100% fuse, thats 420watts @ 14vdc420w x 3.413 (1 watt equals 3.413 btu need of cooling)= 1433btu of cooling1433 / 12000 = .119 ton of cooling required. (ie 1/10th of a ton.12000 btu is equal to 1 ton of coolingin short the formula is watts x 3.413/12000 = btu of cooling in tons.an example of a facility powered by 3phase 120/208 400A serviceL1 - 128A , L2 - 140A , L3 - 114A (not very balanced as typical)average current is 127A127 A x 208 x 1.73 = @ 47.7kw site load..........(1.73 is the square root of 3 phases)hvac will consume about 40% of this load so lets see .......60% of the load would represent the non linear loads (ie all electronic gear running)Now we are at 28.6kw of actual equipment load producing heat.28600kw x 3.413 / 12000 = 8 ton of cooling required.....................site should have 3) 5ton units . (2 for primary and one redundant unit along with AC4 controls)i dont have an engineering degree. Quote Selected
blower only works on high Reply #7 – January 23, 2010, 02:09:23 PM maybe you can convert the btu calc to temp in degrees,, i love math. see if my btu is right Quote Selected
blower only works on high Reply #8 – January 23, 2010, 07:17:33 PM Quote from: jcassity;307586Most people can make the repair like stated but they have to understand that the jumper must continue to be the weakest link , thats why i say the smaller wire the better.I have been running a 30awg strand with no insulation for a long time. I think using any wire to replace the thermal fuse is a bad idea. I also think using small gauge wire is even a worse idea.Reread post #4 Quote Selected
blower only works on high Reply #9 – January 24, 2010, 12:33:25 AM i already said you were right, i agree totally.Lets also look at the wires used as fuses on the solenoid where the awg is stepped down a couple sizes in order to create that heat and fire that could happen.it would be better if a section of solder could be bonded to replace the thermal fuse but durability might be the trade off.the open coil resistors have a lower resistivity than the small conductor i have installed, however, again, the smaller wire is the weakest link in series with the blower motor on high.since the resistor wires are much larger than 30awg, they would have less resistance. example, 18awg uncoated has about 7 to 8 ohms resistance per 1000ft. larger wires have less resistance for the same 1000ft.this would mean that a 30awg wire would have greater resistance per 1000ft.throw any resistance value up against ohms law and we have the current drop across the 30awg wire.lets assume 30awg is 15ohms per 1000ft.,, thats a current drop of .8A. much more likely to blow more easily than a larger wire.i agree using a solid wire isnt the engineered idea but neither are fuse links a good idea imho. there is a reason why fuse links are the smaller / weakest link.maybe im confused on why a larger conductor would be better. it would take longer to blow and collect much more heat and may not be the weakest link in the circuit.where can we easily find thermal limiters off the shelf?I remember saying that it might be better to use a circuit breaker type fuse or a plain fuse in place of the thermal limiter on another thread a long time ago and you disagreed wth that also.iether way, you are right. fix it right would be to use the thermal limiter. Quote Selected
blower only works on high Reply #10 – January 24, 2010, 02:35:01 PM The thermal limiter (fuse) has contacts that are spring loaded to be open. The contacts are held closed by a substance that will melt at the rated temp and allow the contacts to spring open.The heat that causes them to open is not generated by the current flowing through them but by an external source. The resisters in this case.The 30 AWG wire adds another potential heat source to the resister block. This is the condition the thermal limiter was there to guard against.The thermal limiter opens at 250°F. Copper wire melts at 1981°FThe "weakest link" is the 30 amp fuse in the fuse panel. Quote Selected
blower only works on high Reply #11 – January 24, 2010, 03:12:31 PM Quote from: jcassity;307682where can we easily find thermal limiters off the shelf?hosfelt.com has 15a 121°C thermal fuses for $1.25.Anyone who a has fan resister assemble out, please look at the thermal fuse and tell us what markings it has on it. Quote Selected
blower only works on high Reply #12 – January 26, 2010, 09:59:08 PM I got this fixed. The problem was this resistor assy. One of the circuits did not have continuity. I got a used one from a local u pull it yard out of a taurus (Ford part number is slightly different). For whatever reason, I didn't receive any email notification for this thread.softtouch, I'll see if I can read what is written on the thermal fuse tomorrow. Quote Selected
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