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Topic: Voltage via VMSPC readout questionable (Read 508 times) previous topic - next topic

Voltage via VMSPC readout questionable

While driving noticed DC voltage readout from VMSPC display ranges as follows:
Headlights on-Dash AC on high= 12.8-12.9 VDC
Headlights only on = 12.9-13 VDC
All off = 13.1 VDC
Is this normal?  or should I be testing some things?  what for example?
Chassis batteries less than 3 months old.
Fiddler
1959 Parris Island grad
40' 2001 U320 Build 5875
1999 Suburban K1500 toad
1986 Corvette Pacecar
ssoftail@comcast.net

Re: Voltage via VMSPC readout questionable

Reply #1 –
Fiddler, Assuming that dash gauge is accurate you should be seeing 14.1  / 14.3  with nothing drawing voltage.  I suggest that you use a VOM and check voltage at batteries with engine running.  And first things first, clean all battery connections while you are checking voltage.
Gary B

Re: Voltage via VMSPC readout questionable

Reply #2 –
That all looks normal to me. Those appear to be voltages with the engine running. The voltage at the batteries will be higher.

There will always be voltage losses with a load on a battery. Longer wires cause more voltage drop. Smaller wire means more voltage drop. More load means more voltage drop. The will be voltage drop internal to the batteries as load increases. That is a separate issue from voltage drop due to discharge.
J D Stevens
1997 U295 CAI 36' Build #5085
2002 Subaru Outback
Motorcade 16869
Bellville, TX

Re: Voltage via VMSPC readout questionable

Reply #3 –
JD, You are right, my thinking was to verify that the voltages at the batteries were indicating that the alternator was working properly.
Gary B

Re: Voltage via VMSPC readout questionable

Reply #4 –
Correct.  The more amps drawn by "front of coach" accessories, the lower the voltage as read at the front of the coach.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Voltage via VMSPC readout questionable

Reply #5 –
Quote
While driving noticed DC voltage readout from VMSPC display

Where is the VMSPC getting the voltage reading? At the ECM? My VMSPC voltage reading is almost always between 12.9 to 13.4 no matter what lights, etc are on.

Re: Voltage via VMSPC readout questionable

Reply #6 –
Mine are like Bob's but mostly 13.4 volts.  I have seen 13.1 from time to time but it usually pops back up the 13.4 and I have seen 13.8 but it does not stay there long.
2025 Wanderbox Outpost 32 on F600 Expedition Motorhome
2015 Born Free Royal Splendor on Ford 550 nonslide version  for sale
Former Coaches  covering. 360,000 miles
1999 34 U270
2000 36 U320
2001 42' double slide U320
2018 Jeep Rubicon

Re: Voltage via VMSPC readout questionable

Reply #7 –
Clearly, the question is WHERE does the VMSPC pick up its voltage signal.  If at dash, the readings would be considered normal.

If at engine or battery, there should NOT be that kind of voltage drop/fluctuation, as the alternator is sized to well-handle all the electrical loads mentioned.  Only exception would be right after starting if batteries are deeply discharged.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Voltage via VMSPC readout questionable

Reply #8 –
Quote from: wolfe10 –
...Clearly, the question is WHERE does the VMSPC pick up its voltage signal...

Hey folks, Art here

The VMSpc is attached to your Diagnostics port. As such, it's being told your Chassis Battery Volts by the Engines' ECU.  It's not reporting or reflecting the actual drop of the batteries--but a slight line drop which is to be expected. The JIB doesn't do any measuring.  Not to worry.  The drop has always been there though the old steam gauges didn't/ don't have resolution in the 10ths of a volts to relate it.

cheers

--
Art Renda
Documentation Central
Silverleaf Electronics
(888) 741-0259
Knowledge is Horsepower
 

Re: Voltage via VMSPC readout questionable

Reply #9 –
I have recently downloaded and implemented 4.0.2. I have not driven with it yet as we are parked for a while, but it seems to have much better functionality. The Retarder gauges look much different with temp, status, and percent displayed. Other gauges, e.g. message box, tank minder, cruise status all seem better. So far I am very happy with the results.
Royce & Denise, MC #17410
'01 U320 4220 ISM450 Build #5895 SOLD
Toad: '10 Honda CR-V

Travel makes one modest. You see what a tiny place you occupy in the world. - Gustave Flaubert