Yes, the title of this post is exactly what a contractor had done on a unit — replaced 4 boards in 4 days. I finally got the call from the owner of the building, who was very frustrated and upset with our product because the unit was only in it’s second cooling season and they had changed the unit out to avoid a lot of problems. Now they can’t get cooling and the contractor is telling them we have a “bad bunch of boards”! After calming the building owner down and assuring him that we are not having a “run of bad boards”, I have him call his contractor and tell him that I will meet them on the site and we set a date. The building owner agrees.
Now, how many times have I posted that, in most cases, there is nothing wrong with the board? Well, this jobs is going to prove that point again — it usually is not the board! This morning, I meet the contractor at the site, we go up on the roof, and he is telling me all that he has done and that it has to be bad boards.
We start our diagnostic. First we check the voltage into the unit and it is reading 212 VAC (3 phase system). We look at the transformer and it is still on the 230 VAC tap. We have our first problem — this causes low secondary voltage — 23.1 VAC with no load — to the control and safety circuit. We move the power wire to the 208 VAC tap on the transformer and we now have 26.3 VAC secondary voltage. This should help solve some possible issues.
We then go to the board, the thermostat wires have been removed, and we want to turn on the cooling. Since the thermostat wires were still off the board, we leave them off to see what is happening “locally” in the unit. I get out my trusty jumpers and place them between R, Y, and G to turn on the cooling. I get a flash code 2, which is good, meaning the board is in a 5 minute anti short cycle mode. I bypass that with the reset button and I get the flash code 7 and then back to the flash 2. Flash 7 is the board saying that the suction line freeze safety is open. We ohm out the freeze stat and t is closed. We disconnect the freeze stat from the board and jump it out on the board, recycle the power to reset it and still get the flash 2 followed by the flash 7. It is just this scenario that the contractor has been saying occurs and why they have been replacing the boards.
So, did they have 4 bad boards? What else should they have checked? The board currently in the unit has the correct voltage to it now that we reset the power side of the transformer. We have eliminated a possible thermostat problem by disconnecting it off the board — so what else could it be?
Here is where the old analog meter becomes a valuable diagnostic tool. With the meter attached to the 24 VAC power into the board, as soon as the time delay passed or was bypassed, the 24 VAC dropped down below 15 VAC, the board went into a lockout and started the sequence all over again. We did this a couple of time to convince the contractor that something else was causing the lockout and the board was doing what it was designed to do.
The first thing we disconnected was the compressor contactor since that probably has the largest current draw of any controls attached to the board. WE recycle the power again, watch the meter, the time delay passes and the voltage stayed constant and the needle on the old analog meter never bounced.
We were pretty sure the problem was a bad 3-pole contactor for the compressor at this point. The blower motor came on fine so it wasn’t that circuit. there were no flash codes on the board other than a normal heart beat meaning everything was fine. To verify, we reattached the compressor contactor to the board and as soon as the delay passed, it went into the lockout. We put in a new contactor and everything now worked.
A few points to be taken from this hopefully — (1) it usually is not the board — (2) an analog meter is still an excellent diagnostic tool — (Most digital meters do not react quick enough to a voltage drop like this or to a “bouncing” switch). When we were done, I further explained to the contractor that the reason the flash code 7 showed up is because it was the “shortest” run of wire for the safeties. The other safeties were in the condenser section (high & low pressure switches), so the board saw this open (probably only momentarily) before the other switches and flashed it’s code. That is why, even when he jumped it at the board, he still got that flash code.
Hopefully, you readers have learned something valuable today? Us “old timers” will never get rid of our Simpson 260’s or even our cheap pocket analog meters. They are too valuable in making this type of diagnostic. Digital meter are great but for certain things you can’t rplace the analog meter. Also, hopefully I have shown again, it usually is not the board!. By the way, the building owner was happy that we solve his problem and he had cooling again!