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I need to replace my AC unit's contactor (stuck open so have to flip breaker to turn it off).

My understanding is that key variables are number of poles, amperage, and control voltage. Replacement matches up on those, but has slightly different layout.

Old: old contactor

New (pictured without the right-side screw/spade terminals): new contactor

Questions:

Are there any other variables to worry about when sourcing a replacement than number of poles, amperage, and control voltage?

The 24v leads on the old are both at the bottom, on the new one they are at top and bottom (confirmed by checking resistance). Am I correct in believing it doesn't matter which 24v lead goes on top or bottom?

The old one has a lead in the middle going from the capacitor to the middle of the switched side. There's continuity between that lead and the screw terminal to the left of it, so it seems like that lead should go to the left end on the new one. Is that correct, and would it matter if it went on the right end instead (since slightly closer)?

It's a Rheem RAKA-042JAZ, the Rheem replacement is: https://parts.rheem.com/product/RPD-42-25101-01

The parts that come up as replacements do not have the top middle lead that goes to the cap on the original, so figuring that wire on the old contactor should go to the top left terminal group on the new.

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  • where the 24 Volt comes from
    – Traveler
    Commented Jul 5 at 6:39
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    @Traveler They come from inside at the furnace, one leg is on the fused output from the stepdown transformer and the other looks like it is tied into the compressor control on the thermostat (yellow wire).
    – Hart CO
    Commented Jul 5 at 17:50
  • the one with yellow is the ac on/off, the other one is heat pump (heating) mode
    – Traveler
    Commented Jul 5 at 18:08
  • @Traveler I don't have a heat pump, the 24v to the contactor just turns the compressor and fan on. I'm just trying to make sure order doesn't matter on those two.
    – Hart CO
    Commented Jul 5 at 18:56
  • Phone Rheem, maybe they can help. Can you get the model of the old contactor?
    – jay613
    Commented Jul 14 at 11:00

2 Answers 2

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+50

Yes ...

The layout doesn't matter, so long as the new contactor fits, and can be mounted physically, so that the wires can reach their respective contacts. You must follow the documentation of the new contactor. Your black/white is live, the wires on the left are load and the bottom two are control. Find the respective functions in the docs for your new one. We'll come to the top one later.

The polarity doesn't matter, on any side of the contactor. The source (black/white) can be reversed, the control wires can be reversed and the load wires can too. Just make sure the load wires that are grouped, currently through the top and bottom holes, remain grouped the same way.

If the new contactor is two-pole with the right voltage and current rating, those are the specs that matter.

... But

You need to understand what that extra wire is at the top. You can't use an ohm meter to assume it's just part of the top-left terminal group. There is an unused terminal there on the left, so it's reasonable to believe the center-positioned terminal has a different function.

There is a kind of contactor that pre-charges a capacitor briefly through a special contact before the main contacts are energized. It's a rudimentary form of soft-start.

I don't know much more, only that they exist, and your little orange wire may serve that purpose. It may be hard to find a replacement because these days, soft-start is done through more sophisticated separate devices.

You need more info. If you can remove and clean the old contactor and find a model number on it, that would help. The model number of your compressor (or a photo of its nameplate) may also help.

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  • Thanks! If there's continuity and negligible resistance between the top middle terminal and the top left terminal group then I don't understand how it might have a different use, but I will take a poke for more information to confirm. Added some info to question about the part number.
    – Hart CO
    Commented Jul 13 at 18:25
  • If I'm right (and this is no certainty but if...) the two become connected and disconnected in a timed sequence. Your point in time measurement isn't a useful test.
    – jay613
    Commented Jul 14 at 10:58
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I would replace the old one and connect the wires to the new one in exactly the same pattern. Make sure your wire from the capacitor has the same continuity to the left side terminal you described in your post. Keep it on the left not the right. The brown and the yellow at the bottom should be the coil wires and the others are connected from left to right when the coils pulls in. Orange to white, and brown to black. The reason you have to use the breaker to turn it off is that the contacts are welded together and they won't open when power is removed from the coil. Edit: looking at your photo again the contacts are quite obviously burnt.

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  • Main question on the 24v coil wires: Am I correct in believing it doesn't matter which 24v lead goes on top or bottom? One of the output legs on the transformer is fused, but I'm pretty confident it doesn't matter, just wanted confirmation.
    – Hart CO
    Commented Jul 12 at 12:45
  • "top or bottom" ?? The coil wires are on the bottom of the old relay in your photo they are brown and yellow. The new relay should have come with a diagram or instructions. If you do a continuity test on the new relay coil stabs on the bottom in your photo you should get continuity between them. The left hand stab on the top should get continuity to the left top relay contact screw. Otherwise, the relay coil does not care which lead goes to which side of the 24V transformer. The coil will operate either way. Presuming this is an AC transformer and an AC coil.
    – ArchonOSX
    Commented Jul 13 at 9:12
  • Thanks, yes the new one has the 24v leads at top and bottom, just wanted to confirm it didn't matter which went where from the transformer.
    – Hart CO
    Commented Jul 13 at 18:10

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