Low voltage

K-rover

Well-known member
Jan 15, 2010
1,873
1
Raleigh, NC
Havent driven the truck in a while. had to jump start it last week. It fired up tonight a bit hesitant. My old OB2 bluetooth device finally died. I got the new one in today. Drove the truck to go get gas and noticed the voltage was reading low. 12.7-13.1 while driving and idling. I put my volt meter on the battery while it was running at got 13.4 steady, while the OBD2 was fluctuating between 12.7 and 13.1. I then put the meter on the alternator terminal and got 13.9 which seems normal.



So what gives? Is there an issue I need to worry about? The Alternator is fairly new (less than a year) and is a 150amp.



Battery is old 5+yrs.


Im going wheeling for the day tomorrow. Should I worry?
 

Icannap1

Well-known member
Mar 3, 2015
74
0
OC, CA
Same here my OB2 also reads the same although the voltage at both battery and alternator are higher just like you experienced. I guess it's normal that it reads like that.
 

llamasayswhat

Member
Nov 30, 2017
16
0
Kailua-Kona, HI
I notice the same inconsistency with my OBD dongle and using Torque app. It will say ECU is reading 12.6-maaaaaybe 13.1, but measured at battery and alternator, and the actual voltages are 13.9-14.1 at RPM. Although with A/C on full, I only output 13.6 or so
 

p m

Administrator
Callsign: AK6PM
Apr 19, 2004
14,112
3
53
La Jolla, CA
www.3rj.org
K-rover - a 0.5V drop between alternator output and battery terminal seems a little high. Otherwise, I think I see the same behavior.
I'll check what the voltage drop is at idle on my truck (with nearly dead battery) and report.
 

K-rover

Well-known member
Jan 15, 2010
1,873
1
Raleigh, NC
K-rover - a 0.5V drop between alternator output and battery terminal seems a little high. Otherwise, I think I see the same behavior.
I'll check what the voltage drop is at idle on my truck (with nearly dead battery) and report.

Yeah Im thinking of just replacing that wire from the alt to the battery. Im sure its internally corroded, causing voltage drop.
 

LR Max

Well-known member
May 1, 2004
1,176
0
Hotlanta, GA
Perform a voltage drop test. At any wire or connection, if it shows 0.2v or more, its a bad wire or connection.

I'd say you've got a bad connection somewhere. Do a voltage drop test and see if you can isolate the bad connection.