Trane RTU intermittent electrical problem

[hvactrekkie]

Occupants turn heat on then later turn ac back on and ac doesn't run. I open the Trane RTU and see 3 contactors. The first one is engaged. The other two are 3-phase contactors, one for each 3-phase compressor. The control board is getting 24V call for compressor but first stage contactor not engaging. I shut breaker off, turn back on. Same thing. Control board not sending 24V to first stage contactor. There are two fuse cover knobs by the toggle switch. I twist off and on the top one and suddenly the control board sends 24V to said first stage contactor and the unit runs. Turned breaker off, started over. Same thing happens. What is going on? Why would turning on the heat or turning off-on the breaker stop the unit but playing with the fuse cover knob wake the control board up? Checked fuses. All good. Metal cylinder fuses for line voltage just like the breaker.

[yg1box]

Maybe low refrigerant charge.

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[BBeerme]

I've had to replace those fuse holders before. For me, the tell tale sign was a bit of a burn spot on the fuse where the connection was poor.

[timebuilder]

Not only could the fuse holders have a problem, but the fuses themselves could be intermittent... because typically these are dual-element fuses and one of the elements can get sort of wonky, and not really be good even though you can measure continuity on your ohm meter.

So, the first and easiest attempt at a solution is to replace the offending fuse with an exact equivalent fuse.

You could bring along a new fuse holder in case you need it....they are not very expensive.

[timebuilder]

By the way... those fuses are probably powering the transformer which powers the combustion blower motor.

[buford]

Maybe in lockout and ur just resetting it?

[Kevin Orye]

Is it an RTRM board? Does it have cool fail alarm when you first come to it? They don't flash a long series to indicate specific problem just flash twice, you measure DC between pins 6-7 6-8 on the J6 strip as I recall. You can also check the econo for fault code, sounds like a low ambient situation. If they are using heat.

[icy78]

Is it an RTRM board? Does it have cool fail alarm when you first come to it? They don't flash a long series to indicate specific problem just flash twice, you measure DC between pins 6-7 6-8 on the J6 strip as I recall. You can also check the econo for fault code, sounds like a low ambient situation. If they are using heat.

^^^^
X2, sounds like you're doing a board reset.
Run the cooling and look for issues.

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[timebuilder]

How about the model and serial number....that would be very helpful, because you could be looking at a Voyager or you could be looking at a Precedent unit.