Furnace Woes

Hope someone can point me in the right direction.
I have a Johnson Gas Furnace HAS1308 and I am having the problem of the blower not switching on automatically. The blower works fine when you set the fan limit control switch (Honeywell) to manual or by setting the thermosat to run the fan constantly.
I am thinking it could be the Fan Control Center Relay ( (White RodgersModel #90-113) but I am not sure as when I put my voltmeter across the R and C connections I get a reading of 28 V. Should I be checking the relay for continuity? If so, how so I go about it?
Any ideas?
Cheers David
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David wrote:

If relay is bad the blower won't come on in any mode. When flame is ignited, temp rise will trigger a sensor which in return turns on the blower thru it's relay. I'd suspect that heat sensor is culprit. Tony
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Is the fan limit switch working?
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Christopher A. Young
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