Voltage Drop Under Load
Voltage Drop Under Load I'm running a trunk-mount battery in my 71 Nova, which has a stock 105a LQ4 alternator, Vintage Air and Taurus fan. I'm trying to troubleshoot a voltage issue. Here are voltages I've measured with the engine off and at idle (with little/no change if the engine is revved). These were verified using an 8ga test wire connected directly to the battery ground wire at the rear subframe:
Car off
Alternator 12.35v
Battery 12.35v
Relays 12.22v
Idle/no load
Alternator 14.02v
Battery 13.23v
Relays 13.28v
Idle with Taurus fan and Vintage Air on
Alternator 14.10v
Battery 12.82v
Relays 12.54v
My wiring consists of:
I also measured voltage under load at the alternator fuse, and found 13.53v in and 13.06v out. I plan to remove the fuse and retest.
What else should I be looking at?
Car off
Alternator 12.35v
Battery 12.35v
Relays 12.22v
Idle/no load
Alternator 14.02v
Battery 13.23v
Relays 13.28v
Idle with Taurus fan and Vintage Air on
Alternator 14.10v
Battery 12.82v
Relays 12.54v
My wiring consists of:
- 1ga ground from battery to rear subframe
- 1ga from battery to kill switch, with a 200a ANL fuse
- 1ga from kill switch to relay center in the glovebox
- 1ga from relay center to starter (relay center is a mid-90s Ford that has separate terminals for battery in/starter out)
- 4ga from alternator to battery positive terminal (I'm using a Scosche multi-wire terminal with one 0ga, one 4ga, and two 8ga outputs), with 150a ANL fuse near the alternator
- one 8ga ground strap from the engine to the front subframe
- one more 8ga ground strap from the engine to the firewall
I also measured voltage under load at the alternator fuse, and found 13.53v in and 13.06v out. I plan to remove the fuse and retest.
What else should I be looking at?
First thing I notice is you only seem to have 1 ground from the battery to the car. Usually I like at least three. If you look at any OEM battery ground, it usually ties both the chassis and the engine block together somehow, at the very least.
Next, voltage drop from the alt to the battery can be helped by using a larger wire there. Or a shorter wire.
Some computers are programmed to turn off the alternator at high rpm under load. Seeing a voltage drop or duty cycle drop at high rpm is not necessarily something to worry about unless it causes an obvious problem, like a misfire or electronics shutting off kind of things. Also remember that the more 'fully charged' the battery is, the less voltage the alt will likely produce. It should be highest when you first start since the battery took a beating cranking the engine over, usually.
Next, voltage drop from the alt to the battery can be helped by using a larger wire there. Or a shorter wire.
Some computers are programmed to turn off the alternator at high rpm under load. Seeing a voltage drop or duty cycle drop at high rpm is not necessarily something to worry about unless it causes an obvious problem, like a misfire or electronics shutting off kind of things. Also remember that the more 'fully charged' the battery is, the less voltage the alt will likely produce. It should be highest when you first start since the battery took a beating cranking the engine over, usually.
Thanks, I'll add a few more grounds and consider running the battery ground to the engine block.
I found one of the studs loose at the alternator fuse, which alleviated some of my issue. It's now reading 13.5v at the battery, 13.0v at the relay box under full load (AC, taurus fan on high, lights). There's still a full volt being lost between the front-rear-front, but it's much better.
I found one of the studs loose at the alternator fuse, which alleviated some of my issue. It's now reading 13.5v at the battery, 13.0v at the relay box under full load (AC, taurus fan on high, lights). There's still a full volt being lost between the front-rear-front, but it's much better.
