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Old 02-13-2018, 02:05 PM   #1
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Lubricating bushings on Suburban Furnace

2018 Cardinal 3350RL
Suburban SF-42Q

Did a voltage drop test across the 15 Amp fuse on the furnace. Occationally, I get 71 mV (15 amps) when the furnace kicks on, and the fan starts & stops 5 times quickly, and gives up on the start sequence. My assumption is over-amping or insuffienct amps to start.
When I do get it running, the voltage drop across the fuse is 55 mV (12 amps). The manual states that it should only draw 9 amps.

Ducts and cage are clear of debris, bugs, etc.

Should I lubricator the cage and motor bushings? And if so, with what (WD40, Strike Hold)?

Thanks
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Old 02-13-2018, 02:53 PM   #2
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WD-40 will probably work for a short time, but if you want it to last use machine oil.

Oiling the bushings won't hurt anything, but I'd be surprised if it helps a great deal unless the rotor is pretty hard to turn.
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Old 02-13-2018, 03:35 PM   #3
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I will go get some machine oil today. I’ve read that Strike Hold is amazing for applications like this. But it would take a week to get here if I ordered it today.

I will report back with the improvement if any once I get machine oil on it.
I considered replacing the fuse with a 20 AMP fuse, but read of the concerns with over-heating the wiring.
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Old 02-15-2018, 08:22 AM   #4
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I would be very careful replacing fuses with larger ones. You'll probably void the warranty should anything go wrong, and as you said overheating wires, which could be catastrophic.
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Old 02-15-2018, 02:19 PM   #5
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No oil.

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Originally Posted by Daniellekristopher View Post
Should I lubricator the cage and motor bushings? And if so, with what (WD40, Strike Hold)?
The Suburban repair manual does not recommend lubricating the motor. I think I've memorized that book.

WD-40 is not really a lubricant. It was designed as a rust-preventative and the vehicle is a pretty good penetrant. It will get between two surfaces and quiet them for a time. But it washes out the original lubricant and dries quickly, meaning you will see the original problem return soon.

There are a lot of folks who don't believe this, so I'm expecting some flack.

Larry
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Old 02-15-2018, 02:37 PM   #6
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Quote:
Originally Posted by Daniellekristopher View Post
2018 Cardinal 3350RL
Suburban SF-42Q

Did a voltage drop test across the 15 Amp fuse on the furnace. Occationally, I get 71 mV (15 amps) when the furnace kicks on, and the fan starts & stops 5 times quickly, and gives up on the start sequence. My assumption is over-amping or insuffienct amps to start.
When I do get it running, the voltage drop across the fuse is 55 mV (12 amps). The manual states that it should only draw 9 amps.

Ducts and cage are clear of debris, bugs, etc.

Should I lubricator the cage and motor bushings? And if so, with what (WD40, Strike Hold)?

Thanks
Could you help me understand how you are making your calculations? What sort of meter are you using and how much resistance is that fuse giving you?
I have had very good support from Kris Spengler at Suburban on my furnace. FR made the furnace plenum to small and it constantly tripped the over temperature switch on the furnace.
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Old 02-15-2018, 11:19 PM   #7
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Is the rv plugged in to 110 volts to charge the battery? The furnace needs a good 12+ volts to work right. The furnace is a big power hog and will drain a good battery down to 10 volts very quickly if not plugged in to recharge the battery. below 10 volts the furnace will not run or will not fire up.

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Old 02-16-2018, 11:04 AM   #8
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Measuring voltage drop across fuse is misleading

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Originally Posted by TimVWulp View Post
Is the rv plugged in to 110 volts to charge the battery? The furnace needs a good 12+ volts to work right. The furnace is a big power hog and will drain a good battery down to 10 volts very quickly if not plugged in to recharge the battery. below 10 volts the furnace will not run or will not fire up.

tim
Yes, and when the supply voltage drops, measuring voltage drop across the fuse to calculate current is misleading. The right way to measure current is to do it directly. Set your multimeter to the 10 amp range, remove the fuse and plug the multimeter probes into the fuse socket, thus running all the current to the furnace directly through the meter.

Most of the inexpensive multimeters have this function. Harbor Freight sells this one for $5.99, but gives it away for free on promotions. I've gotten at least six of them for free, given them to sons and fellow RVers.

Larry
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Old 02-16-2018, 11:21 AM   #9
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Furnace motors are sealed bearings. WD 40 is a waste of time.
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Old 02-16-2018, 11:33 AM   #10
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Quote:
Originally Posted by Daniellekristopher View Post
2018 Cardinal 3350RL
Suburban SF-42Q

Did a voltage drop test across the 15 Amp fuse on the furnace. Occationally, I get 71 mV (15 amps) when the furnace kicks on, and the fan starts & stops 5 times quickly, and gives up on the start sequence. My assumption is over-amping or insuffienct amps to start.
When I do get it running, the voltage drop across the fuse is 55 mV (12 amps). The manual states that it should only draw 9 amps.

Ducts and cage are clear of debris, bugs, etc.

Should I lubricator the cage and motor bushings? And if so, with what (WD40, Strike Hold)?

Thanks
Voltage drop is measured in volts not amps. Voltage drop is not measured across a fuse. It is measured from the beginning of a circuit to the end based on the size of the wire and the load. Motor bushings are sealed. No way to lubricate.
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Old 02-16-2018, 11:41 AM   #11
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Furnace motors are sealed bearings. WD 40 is a waste of time.
Interesting. The last one I took apart had bronze sleeve bearings. It was a suburban.

Now the HVAC blower in the sticks and bricks has roller bearings, but even they aren't sealed. They are shielded, but not sealed.
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Old 02-16-2018, 11:49 AM   #12
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Interesting. The last one I took apart had bronze sleeve bearings. It was a suburban.

Now the HVAC blower in the sticks and bricks has roller bearings, but even they aren't sealed. They are shielded, but not sealed.
Gotcha. My point is that they are not designed for routine maintenance. If they are stuck or making noise it's probably time to change. One time you might get some more life out of it.
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Old 02-16-2018, 10:39 PM   #13
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Voltage drop is measured in volts not amps. Voltage drop is not measured across a fuse. It is measured from the beginning of a circuit to the end based on the size of the wire and the load. Motor bushings are sealed. No way to lubricate.
If you ever opened up one of the inexpensive multimeters, you'd see that the 10A current neasuring function consists of a short length of wire that's soldered between the 10A input and ground. The meter actually measures the voltage drop across the wire and reports it as amps. As the wire heats up from current flow, it's resistance increases, leading to a false reading. Fortunately, this jumper wire is much thicker than the test lead probe wire which will overheat long before heating effects of the jumper effect the current reading.

Current in a circuit can easily be measured by checking the voltage drop across a known resistance. Fuses have published specifications, including their resistance. (It's actually heating caused by the fuse's resistance that causes it to "blow".) If you don't have the specifications for a fuse, you can remove it from the circuit and measure it with a calibrated ohmmeter. Ohm's law relating voltage, current and resistance makes it easy to calculate any of the factors if you know the other two.

I've had situations where I needed to measure current in the 20-40A range, higher than the capability of any of the meters I owned at the time. My solution was to insert a small value, high wattage, resistor in the circuit, usually around 0.01 ohm and measure the voltage drop across it. Thirty amps flowing through that resistor will generate a 0.3V drop, something that's easily measured with a two or three decimal digital multimeter.

Phil
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Old 02-17-2018, 01:53 PM   #14
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It's tricky

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Originally Posted by pmsherman View Post
If you ever opened up one of the inexpensive multimeters, you'd see that the 10A current measuring function consists of a short length of wire that's soldered between the 10A input and ground. The meter actually measures the voltage drop across the wire and reports it as amps. As the wire heats up from current flow, it's resistance increases, leading to a false reading. Fortunately, this jumper wire is much thicker than the test lead probe wire which will overheat long before heating effects of the jumper effect the current reading.

Current in a circuit can easily be measured by checking the voltage drop across a known resistance. Fuses have published specifications, including their resistance. (It's actually heating caused by the fuse's resistance that causes it to "blow".) If you don't have the specifications for a fuse, you can remove it from the circuit and measure it with a calibrated ohmmeter. Ohm's law relating voltage, current and resistance makes it easy to calculate any of the factors if you know the other two.

I've had situations where I needed to measure current in the 20-40A range, higher than the capability of any of the meters I owned at the time. My solution was to insert a small value, high wattage, resistor in the circuit, usually around 0.01 ohm and measure the voltage drop across it. Thirty amps flowing through that resistor will generate a 0.3V drop, something that's easily measured with a two or three decimal digital multimeter.

Phil
Yes, hence my concern about determining circuit current by measuring the voltage drop across the fuse. Like any other metallic conductor, the resistance increases with temperature, hence current. If you divide the actual measured voltage by the published fuse resistance at 20C, you haven't accounted for actual ambient temperature or for the actual temperature hence resistance of the fuse under current conditions. (Fuses, of course, are warm at rated load.)

It is not hard to design a circuit that can accomodate for both of these conditions at the meter's shunt resistor and I expect the IC in the cheap multimeters does this.

Back around 1963, cars switched from meters to idiot lights and I bought a Stewart-Warner gauge set, oil pressure plus ammeter, for my high school ride. I have used the oil pressure gauge from time to time since then, mostly on my son's VW beetle. I've used the ammeter, a -50-0-50, center zero, analog magnetic movement meter a great deal more often. I use it today for currents more than 10 amps. I've got a pair of three-foot 14-AWG stranded leads on it with heavy clip leads. Needless to say, it's not subject to the effects mentioned above. The design compensates for most temperature effects. This is a very similar unit for about $14 and a good thing to have in your toolkit.

Larry
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