Fluctuating output voltage


Old 05-15-14, 05:35 PM
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Fluctuating output voltage

I have 1999 Generac PP5000T Mdl: 1140-2, which has served me well over the years but has recently developed a problem with the output voltage.

I use this unit as back-up power for my home to run a few essential, furnace, well pump, fridge and a few lights. During a recent power outage I noticed that when the furnace powered up it didn’t sound like it was running correctly the fan speed appeared to be much slower then usual and the lights would dim a little which they never did before.

I measured the output voltage of L1 and L2 at the transfer switch with a DVM, without a load applied it read around 127 – 130 volts for L1 and L2, when I applied the furnace load to L1 the voltage dropped and varied between 99-104 volts and fluctuated quite a bit which it has never done before. The same applied to the L2 side when I applied a load on that side. The RPM’s on the Tecumseh was holding fairly steady at around 3600 RPM with the load applied and 3750 RMP with no load.

When I looked at the manual I could not find any brush assembly thinking it would use a simple bridge rectifier and brush combo assembly as this genny doesn’t have any type of AVR.

What I did see was a 35µF cap behind the control panel and 2 diode/varistor assemblies located in the powerhead.

Anyone have any suggestions as to why the sudden voltage variance? As I said it never did this in prior usage and it always held a fairly steady voltage until this last use. Is it time to say farewell to this genny?
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Old 05-15-14, 06:46 PM
Tolyn Ironhand's Avatar
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Voltage will drop as load is applied, however a drop of about 30 volts does seam excessive. How many watts/amps is your furnace? Is this the only load you had on it? Does it seam to run OK? What kind of meter are you using to test? Does the meter have fresh batteries? (I have seen meters give odd readings when the batteries are low)

Last edited by ray2047; 05-15-14 at 06:55 PM. Reason: fun>run
Old 05-15-14, 09:24 PM
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Fluke DVM batteries are OK. The furnace runs fine on line voltage starting watts for the furnace is approximately 1380 watts or 11.5 amps, however even after initial start-up while on the generator the furnace blower still runs much slower then normal which it never did in the past.
Old 05-15-14, 11:02 PM
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I really don't know or have any expertise with this problem but it seems to me that I have read similar posts, maybe on a different forum, and the problem was traced to a defective capacitor. I think I would start by removing the capacitor-varistor-diode networks and testing the parts individually.

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