Trust your meter?


So, how many of you “trust” your meters?  You’re probably asking yourself, “what is he talking about?”  So I’ll try to explain my question with some examples.

The other day, I had a contractor call who was looking to confirm his diagnostic on a roof top unit. Now, granted it is very cold on roofs this winter and we all try to get off as soon as possible so contractors take readings and go get in their truck, try to get warm, call for help or confirmation before they have to go back on the roof.  Now, it is always a pleasure to deal with some one who has proper information and just has a mental block because it is cold.

Well, here we go — this is an older unit that has both 24 VAC and 110 VAC controls in the unit. The problem is the blower would not come on so neither would the heat.  The contractor had traced out the problem and determined it was a bad blower contactor.  So he goes to his truck, finds a contactor (old one was 20 amp rated, new one is 40 amp rated but both have 110 VAC coils so should not be a problem).  He puts the new contactor into the unit, wires it up and it does the same thing — it won’t pull in on its own.  If he pushes it is, it will stay in but it won’t engage itself.  Well, here is where he does all his diagnostics again; gets all of his readings; gets out of the -30 degree wind chill factor on the roof; gets in his truck and calls me.

I get the call, pull up the wiring diagram for the unit and we start going over everything.  He tells me what is happening and now thinks it is a “bad board” (when all else fails — change the board attitude).  I tell him it is not the board.  So we walk through everything.  As I tell people, when tracing out an electrical circuit you need to start either at the power source or at the light bulb (load) —  everything else in between is just switches. Since the contactor is line voltage and “new”, we find the transformer from 208/230 to 110 volts and it is putting out 121 VAC — no problem here.  That power then goes through the contacts of a normally open relay that is in a board and powered by 24 VAC coil.  The board has 24 volts  — the coil is energized — the contact as are closing — and the closed contacts show no voltage across the switch meaning it is closed and passing power. We finally get to the new contactor. The contractor measures voltage at the coil and he has 121 VAC to ground on one leg and “0” VAC to ground on the other leg.  When he reads across the coil he has 121 volts. — So — what’s the problem here?

The problem is the contractor can’t believe the new contactor is bad out of the box so therefore, it has to be somewhere else (the board) –  Even though his meter is telling him he has voltage at the coil and the contactor should be pulling in, and it will pull in if he “helps it” — then it still has to be something else causing it. After a lot of discussion, I finally convinced him to get a new contactor.  He put it in and, lo and behold, the unit worked.

How many times have you “questioned” your meter or diagnostics because the component or unit or whatever isn’t doing what it should be doing yet your meter says it should be working? Think about it. ( And don’t be afraid to ask for help — even if it is just to get off the roof and get warm in your truck. Just make sure the information you have is not “fabricated”.  As they say with computers — garbage in — garbage out.  It is the same with Tech support person like me — I can only help if you give me good information.)

This is just one simple example — I’m sure we all have stories we can tell when WE didn’t trust our own meters.

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About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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4 Responses to Trust your meter?

  1. There are cases, especially when working with solid state ac switches, where a digital meter will read 120 vac when in reality the triac switch is off and there is not enough current to operate the load. This can happen because a solid state switch can leak microamps when off while the digital meter’s high impedance reads it. Won’t happen with relay contacts and won’t happen with analog meters.

  2. Christian Chan says:

    i always have this problem with my vacuum meter, sometimes i can get 500 microns, sometimes no matter if my pum pulls for hours it never drops 1000 microns (even that i’am sure that there is no leak in the system because i’ve already tested with nitrogen)

    • Christian — you may not have a leak but you could still have moisture in the system. The true test is when you shut off your vacuum pump -see if you hold at that pressure, or if the pressure rises and then stops (indication of moisture), or just keep rising(indication of leak) Just because it held nitrogen does not mean there is no leak or no moisture in the system — especially today with the hygroscopic POE oils

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