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possible short in rheem furnace ??

i have a rheem rgph-10eamer, that wasn't cooling. power led was off. checked primary power, had 120v, to board and primary xmer, but no low voltage output on secondary. ohm'd out primary...it was open. replaced with new xmer and it started smoking. replaced both control board and new xmer...fuse blew on secondary side upon turning on main power. suspected a short. pulled out blower asmby out of furnace. checked each speed for continuity and each lead to case for short, everything looked good. checked each speed for operation and amps. motor is rated for 115v 60 hz 7.1 amps 1,000rpm, 1/2 hp...3 speeds were 4.3, 5.5, 6.9, and 9.2 amps and while running blower case to ground there is 35 volts. the xmers that where mounted to blower case as manufactured, where the ones that the primary and secondary were blown. the one that smoked wasn't touching the blower.

 i believe this could be my problem, since i have a > 10% over amp situation on one speed, and voltage on the case when running??? could 35 volts on case, going to xmer cause three xmers to go bad..one primary, one secondary, and one too smoke???

why would i only get a short through the case when running, but not have any continuity between any leads and the case when off??

do you have to have primary power and low voltage power on the control board in order for the power led light to be on??

on the xmer change out that smoked, the power led was on, but not the ok led and no board operation. upon removing the xmer. i still had continuity on the primary and secondary, so could the extra 2.1 amps i had on the blower, when it was tested out of the furnace, prevent the ok led to not light, the power led to light up and have no blower operation??

thanks in advance mel

Comments

  • Techman
    Techman Member Posts: 2,144
    edited July 2012
    Problem

    Why do you have 4 amp readings on a 3 speed motor? With the blower assy out and testing for amps ,you will have higher than normal amps reading because the blower has no "back pressure"( duct work) to make it work properly as it does when in the unit. The xfmr problem ( primary/secondary) should not be affected by a problem w/ the motor.I think you should be looking for a problem somewhere on the 24v side of the xfmr. If there was a secondary side fuse on the 1st xfmr you would have been replacing fuses ,not xfmr's.A shorted,grounded, or over amps on the 24v side will blow the xfmr or the fuse.Was the AC calling when you energized the AHU? Remove the low volt wirers (BOTH WIRERS) to the cond unit and retry.Did you have an amp meter on the xfmr secondary side when testing? An instant blowing of the fuse would indicate a short or ground, a high amp reading for a second or two or three, and then blowing the fuse, would indicate something like a stuck contactor
  • Empire_2
    Empire_2 Member Posts: 2,343
    OK Try this

    Remove all wires from Lo vac terminal strip.  Use your jumper to jump out each circuit to check for lo vac over current draw.  You will need a clamp on ammeter.  Check R-w, R,g etc...  If that is ok, then move on to checking the wires from t-stat and out to condenser.  I'd bet something is grounded.  Also, make sure that furnace is grounded back to the elect. box.



    Mike T.
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