Foretravel Owners' Forum For owners and Fans of Foretravel Motorcoaches September 11, 2025 ...

Fluctuating Volt meter

June 27, 2012, 03:38:19 pm

My Volt meter and VMSpc readout at highway speed fluctuates from 12.4-6 to 13.9-14.0. This happens only when the AC is operating and not constantly. Any help from the forum will be greatly appreciated! Is this normal!

                    Thanks all,
   
                        Josh

Quote Selected

Re: Fluctuating Volt meter

Reply #1 – June 27, 2012, 04:59:42 pm

Are you referring to the dash AC? If so, can you turn the dash AC on/off and see if it correlates to changes in the voltage measurements?

The measuring points for the voltmeter and VmsPC may be sensitive to a load introduced by the AC. If so, you might consider finding a different point to take measurements.

Quote Selected

Re: Fluctuating Volt meter

Reply #2 – June 27, 2012, 05:07:24 pm

Yes, it seems to happen only when the dash AC is on but not constantly.

                      Thanks J D,
                          Josh



   

Quote Selected

Re: Fluctuating Volt meter

Reply #3 – June 27, 2012, 05:24:13 pm

Josh,

The AC compressor normally cycles on and off while the AC is turned on. That would account for the voltage fluctuation.

Try this: Set the dash fan speed on high. Leave the AC off. Observe the voltage. Turn the AC on and observe the voltage. Turn the AC off and observe the voltage.

If you see the voltage drop when you turn the AC on, then rise when you turn it off, it indicates that the voltmeter and VmsPC are sensing voltage from a spot that is significantly affected by the load imposed by the AC clutch and/or condenser fan. That may be "normal," but not satisfactory. You might want to move the voltmeter and VmsPC sense points to an unloaded connection to the battery you wish to monitor.

We don't have VmsPC (C8.3 mechanical engine). I have noticed that the voltage indicated by the voltmeter at the dash drops significantly when the headlights and marker lamps are on. I adjust my interpretation of the reading based on whether the lights are on.

Quote Selected

Re: Fluctuating Volt meter

Reply #4 – June 27, 2012, 05:28:12 pm

Since it only happens with the A/C on, let's focus on that.  What happens when you put the dash HVAC to FAN not A/C? That will tell you if the issue is with a fan motor (does RPM change/fan starting to seize up) or perhaps the A/C clutch on the compressor.

I would also rig up alligator clips and voltmeter from the ignition solenoid to see if voltage is fluctuating at that location.  If you can duplicate this with engine running in park, better yet, check for voltage fluctuation at the chassis battery.

Quote Selected

Re: Fluctuating Volt meter

Reply #5 – June 27, 2012, 05:47:16 pm

Doesn't seem to fluctuate with fan on all settings and no AC. Only sometimes with the AC on.

                Thanks again,
                  Josh

Quote Selected

Re: Fluctuating Volt meter

Reply #6 – June 27, 2012, 05:54:45 pm

Josh,

Hard to imagine the A/C clutch pulling enough amps to drop voltage and NOT blow it's fuse.

Brett

Quote Selected

Re: Fluctuating Volt meter

Reply #7 – June 27, 2012, 06:14:42 pm

My take on it more toward a corroded, poor electrical connection somewhere in the circuit, OR the meter or VMSpc is picking up some stray emf radiating near by.
But agree on the fuse if any real current draw was involved to drop voltage that much.
Dave M

Quote Selected

Re: Fluctuating Volt meter

Reply #8 – June 28, 2012, 12:57:21 pm

Depending on AC types, a valve/switch will cycle the AC compressor on and off to prevent ice buildup in the evaporator. (If ice builds up, the air can't flow through and no cold air comes out. Also, operation in high humidity sees less time between cycles as ice builds faster) The increased amount of electricity needed to engage the AC clutch causes the voltage to drop a bit but only as long as the clutch is engaged. This is why the voltage fluctuates. The size of the wire that feeds the accessories including the AC, is the reason for the amount of voltage drop. The bigger the wire/cable, the smaller the drop, the smaller the wire... If you measure the voltage at the battery, the drop will be very small as the AC clutch cycles.

If all cars had voltmeters, drivers could watch the voltage fluctuate with the AC compressor operation so this is a normal function of the automotive electrical system, not just your Foretravel. Headlight operation does the same thing. And yes, if the compressor clutch were drawing too much juice, a fuse would blow. Corroded terminals/connections can add resistance and cause the voltage drop to be greater. This was a big problem when aluminum fuses were available as replacements in automotive electrical systems (and home wiring).

Pierce

Quote Selected