No voltage at pressure switch

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Old 03-03-10, 07:58 PM
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Question No voltage at pressure switch

I have an SJC ( became Tappan ) furnace circa 1980.

It is an intermittent spark ignition unit.

When the thermostat calls for heat, the inducer runs, but there is no ignition to the pilot. In fact, there is not 24V to either the gas valve or the spark unit.

This furnace does not have any control boards, just a transformer and what appears to be a relay of some type.

I bypassed the pressure switch with a jumper from the transformer to the pressure switch, and then the pilot lit, and the main burners came on.

I can not locate any schematics for this unit.

The transformer is mounted in a 4" box with a rats' nest of wires.

There appears to be a relay of some sort which is powered by the transformer. There is 24V going in to it, but I have no idea what this relay controls.

Any ideas ?

Thanks !
 
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Old 03-04-10, 07:34 AM
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You need a new pressure switch and thats all.
 
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Old 03-04-10, 09:05 AM
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Ender - not necessarily.

The way the furnace drafts/vents could be obstructed somehow!

Could be something as simple as condensate water interfering. (Cure: Blow out condensate water lines both directions.)

Or, the vacuum tube that goes between the pressure switch and inducer is plugged inside the inducer fan housing nipple.( Cure: Remove that vacuum tube there and use something like a thin nail or whatever, to clean out that nipple.)

And make sure nothing is obstructing the exhaust or intakie vents outside the house. (Cure: You can determine if that is it by TEMPORARILY removing them at the furance, for testing prurposes only! Danger of carbon monoxide!!!! And run furnace that way only a couple minutes to see if then you get ignition/fire. If so, then you have obstruction in vent. Clean/rod out/shopvac, etc. Rehook up vent(s)!!!

.................................................................................

Note: By jumpering the pressure switch, all you have done is bypass the FUNCTION of the pressure switch. Notice I did not say you only bypassed a possible defective pressure switch itself!

It WOULD be the pressure switch though if you could actually see the plunger get sucked in all the way, when furnace runs, and yet current did not pass through it. But - if the draw of the furnace is NOT sucking in the plunger of the pressure switch - that alone does not prove the pressure switch diaphram is leaking, since it could also mean any of the things I posted about above, which are actually more common to occur than the pressure switch from being bad. This is why there is a testing device called a "manometer" - to find out if the draw the furnace is the way it should be, or not. They can also be made, and you can research how, on the web. Then with tool in hand, you also have to know what the pressure switch is rated by it's water column number (i.e, say .9 inches w.c.).

..............................................................................

BTW, JPP - your thread title is a misnomer in a way, because you DO have power at the pressure switch, proven by the fact you could jumper it. What you have is incoming power to it, just not power able to continue on past it.
 

Last edited by ecman51; 03-04-10 at 09:19 AM. Reason: added last 2 paragraphs
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Old 03-04-10, 10:20 AM
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About 5% of the time the pressure switch is the problem when it wont close. 95% of the time, the problem is in the furnace or vent lines.

Bypassing safety switches for other than test purposes is a dangerous practice. Death or serious injury by carbon monoxide poisoning is the most likely hazard to human safety ---- but getting the furnace plugged up by sooting which will result in a destroyed furnace is probably more likely. Which option do you prefer?

If you can't identify the problem, call a repair service.
 
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Old 03-04-10, 09:16 PM
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Thanks for the replies.

However, there is really no voltage present at the pressure switch, according to my meter. I jumped 24V directly from the transformer to get the spark, pilot & main burners to come on.

I am trying to include some pics to make the layout more clear.

[IMG]e:\pressure_sw_030410.jpg[/IMG]
 
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Old 03-04-10, 11:12 PM
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When the thermostat calls for heat, the inducer motor should be switched on and 24 VAC should be applied to one contact of the pressure switch, When the pressure switch closes, the voltage is passed back to the circuit board which continues the ignition sequence.

Does the inducer motor start up? If so and you have no 24 VAC at the pressure switch as you say, it suggests a bad circuit board.
 
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Old 03-04-10, 11:35 PM
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The inducer motor does run. However, the only way I have been able to get a spark, pilot and main burner is to run a jumper from the transformer to the pressure switch. There do not appear to be any circuit boards at all, since this is an old ( 1980 or so ) furnace.

there IS what appears tobe a relay, which is wired to the transformer.
I have no idea what it does, but since it takes 24V, it MAY power the pressure switch.
It has 4 wires, at least one of which is 24V.
 
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Old 03-05-10, 07:01 AM
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if you go between one point of the pressure switch and ground do you get 24v? if not try the other point
 
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Old 03-05-10, 04:38 PM
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Originally Posted by ender2272 View Post
if you go between one point of the pressure switch and ground do you get 24v? if not try the other point
There is not 24V present at either terminal on the pressure switch.

I will pull the relay I mentioned earlier, since I suspect it MAY power the pressure switch. Wires from the relay go into the 4" box with a rat's nest of wires.
 
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Old 03-05-10, 05:08 PM
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So---


Do you have 24 VAC present at the W terminal when the thermostat is calling for heat?
 
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Old 03-05-10, 07:03 PM
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Originally Posted by JPP388 View Post
The inducer motor does run. However, the only way I have been able to get a spark, pilot and main burner is to run a jumper from the transformer to the pressure switch. There do not appear to be any circuit boards at all, since this is an old ( 1980 or so ) furnace.

there IS what appears tobe a relay, which is wired to the transformer.
I have no idea what it does, but since it takes 24V, it MAY power the pressure switch.
It has 4 wires, at least one of which is 24V.

SP, you'd THINK that if the inducer is running the way JPP said, that the stat would have to be working - so I'd think anyway.

But to be sure - JPP, is the inducer off?, but then comes on properly when you turn up the stat? Or is the inducer continously running even without the call for heat, in a locked out state. Although, I'm not sure how it could even "lock out" without some sort of motherboard. You sure their isn't one in either of the two furnace compartments? Some furnaces have it inside a metal box inside the blower compartment.

JPP, due to lack of electrical componnets in your furnace, can't you easily trace to where the wires start and wind up?
 
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Old 03-05-10, 08:58 PM
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The inducer runs properly, when the thermostat calls for heat. It will continue to run, since the furnace does not fire.

I will remove and try to test the one remaining suspect component, which is what I think may be a relay which POWERS the pressure switch.
 
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Old 03-06-10, 09:33 AM
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Let us know what you find. You'd think that since your furnace sounds less electronic, that tracing down the problem, should be fairly easy. Sort of like trying to find a wiring problem under the hood of an old Studebaker pickup (landlord has one -plain engine w/ only a few wires you can see where they all go).
 
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Old 03-11-10, 05:34 PM
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Since I could not find a schematic, and I did not want to poke around in a rat's nest of wires, I fixed the problem with a jumper from the transformer to the switched side of the pressure switch.

It works completely normally now.

If I had to guess, there's a broken wire in there somewhere that used to power the pressure switch.

I will probably replace the furnace this summer. I can't complain, since I checked to see that I installed this furnace in 1979, and the only other problem I've ever had was a bad blower motor two years ago.

Thanks for the replies !
 
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Old 03-11-10, 06:28 PM
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The switched side? You do not want to be bypassing that safety switch. What happens if you jumper instead to the power side of the pressure switch? If it does not run then, then you have a problem with the the inability oif it to draw air through it correctly.

Let us know if you indeed bypassed that pressure switch.

But even if you didn't, you may have bypassed the limit switch. It is not a good thing to be circumventing any safety feature on any furnace, as you could possibly ruin the furnace or endanger your life with carbon monoxide.

We should be able to help you find the exact problem at hand.

I am also wondering that if you lack electronics in your furnace, why then you have so many wires? You sure there isn't a control down in a box inside the blower compartment?
 
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Old 03-12-10, 11:12 AM
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I agree with Ecman --- your bypass/fix may create a hazard.


As Ecman suggests, you may have a limit switch or something else that isn't closing properly --- but bypassing it creates a bigger problem.

Sounds like you need to call a pro to sort this issue out.
 
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Old 03-12-10, 08:09 PM
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The jumper powers the switched side of the pressure switch, and requires that the pressure switch close the contact. It is "switched" and not hardwired "on".

The limit switch appears to be powered normally, unaffected by the "jumper to pressure switch" wire.
 
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Old 03-13-10, 06:24 AM
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that mystery relay is for the ID fan on the heat side the stat heat call closes that relay and the Dwyer is looking for proof of air on the combustion side to start the sequence..pull one of the relay wires either it will click on coil side or spark line side with the ID running...any action with the pull that lets you know what it is......sounds like a loose/broken wire on that pressure switch(dwyer) not passing 24V to your burner...the ID motor is 115V needs a relay with a 24V coil for the W1 call
 
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Old 03-13-10, 03:08 PM
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Something does not add up here -IMO.

I have never heard of a relay inline between the limit switch that he says works, and the pressure switch that does not have power on the incoming side of it.

Sminker,

Is the relay, being mentioned, the "sequencer", that takes 24 volt W stat power and closes that relay to allow 120 volts to progress to the ID? If so, JPP's ID comes on. I'd assume the sequencer is good. His problem is what happens after the inducer comes on.

Once an inducer is running, the only thing that can stop the ignition process on any furnace I have worked on(lots of brands) is where some switch(limit, roll out(s), pressure switch, exhaust gas temp switch......... in the 24 volt safety circuit is open.....if jumpering something to the pressure switch makes the furnace then fire up.
 
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