Carrier RTU contactors buzzing and chattering

[airproguyrick]

48TCD008A1 RTU 2 years old with HH63AW001 Economizer control. Both compressor contactors badly burned and 2nd stage one was welded together. Another company supposedly replaced the economizer control last July. Today we replaced both contactors due to damage and both still chatter. Disconnected economizer with bypass plug and the unit ran normally. Ordering new economizer box, but any idea where to look for shorts on this unit if control problems persist? Just wonder if there are any favorite spots to look for wire rubs or wires chewed by damper blades and gears etc on this unit.

[airproguyrick]

just wondering if any one else has had similar problems with these units...

[socotech]

Have you checked that the transformer is tapped correctly? I have seen 208v supply hooked to 240v tap a cause a large voltage drop on secondary. Maybe unplugging econ unloaded transformer enough to stop chatter.

[VTP99]

Have you checked that the transformer is tapped correctly? I have seen 208v supply hooked to 240v tap a cause a large voltage drop on secondary. Maybe unplugging econ unloaded transformer enough to stop chatter.

seen it many of times myself.

[airproguyrick]

no, but I will. forgot to check the simple things first. that number one contactor buzzes really loud and arcing and frying the contacts. also the small relay they use for the condenser motors was having trouble pulling in also. hope it is just a transformer tap.

[tuba]

I have seen what looks like a bad run of high pressure switches on these carriers. The wires short out inside the switch and any movement of the wires cause the chattering. I found it by accident when I was investigating a unit that would knock out the store power when it was switched on. Even though the contactor was about 2 months old, the carbon buildup from the chattering caused the 480 volts to jump from the line side of the contactor to the frame of the unit. BOOM!
I have also read that sometimes that economizer controller cannot decide what to do when the weather is cooler, and causes the contctors to chatter. I usually put on that dry bulb sensor and dial 55 degrees in, so there is no more shananigans about enthalpy.

[B1978]

I have also replaced a lot of high pressure switches. I get a lot of rub outs going from the controls to the safeties. Had a short in the economizer, due to the access panel pinching the wires. Some people replace a lot of CLO boards. Wire rubs by the fan against the blower compartment.
Check end play on the condenser fan also.

[Armament]

Poke around and follow the 24 volts to your contactors. Check for voltage drop throughout the lv circuit. If that doesn't turn up anything test components individually.

[gravity]

if you put the bypass plug back on and it worked then you most likely have no wires rubbing or shorts going from the LVTB thru the unit to the economizer and back.

could also be that the transformer tap isn't correct.

[Six]

Bypassing the economizer doesn't re-tap the X-former so thats not the issue.

Sounds like you've isolated it down to something in the economizer or even something in the economizer plug and /or harness.

If you think its a High/Low pres switch by pass them one by one until you find the issue.

[vzenuh]

Have you checked that the transformer is tapped correctly? I have seen 208v supply hooked to 240v tap a cause a large voltage drop on secondary. Maybe unplugging econ unloaded transformer enough to stop chatter.

ditto

[dunkman]

I've seen this happen when the wires going to the economizer plug were squished between the blower housing and the top of the unit, rubbing through the common 24v wire. That'll make all the contactors chatter. That doesn't really explain "the bypass plug fixes it" though.

[VTP99]

[QUOTE=Six;14408321]Bypassing the economizer doesn't re-tap the X-former so thats not the issue.

Unless the additional load from the economizer overloads the transformer on a wrong tap. I would look close at the plug though as stated.

[jaysipes]

I've seen this happen when the wires going to the economizer plug were squished between the blower housing and the top of the unit, rubbing through the common 24v wire. That'll make all the contactors chatter. That doesn't really explain "the bypass plug fixes it" though.

Been behind a many a tech who missed this. I've seen many of these with a short in 24v red leaving lvtb going to economizer, sometimes in blower compartment, sometimes just in side filter section where wire passes thru. Also, does it have the high static motor with internal overload? Seen that cause intermittent problem too