This is not a problem. Or it is just a little one. ;-)

I would be grateful if someone could explain why voltage (while running) on my 1987 740 (looking at the analog gauge in the dash) is LOWER if I start my car with the headlight switch already on, and higher by 1/2-1 volt if I start the car with headlights switched off -- and then switch them on.

I mean...
1. headlights switch left on, start car, get shy of 14 volts (more or less)
2. switch off while starting, start car, turn on headlights, get maybe 14-1/2 volts

This is consistent, across seasons.
The regulated voltage does, of course, decline as I add additional loads, such as a HVAC fan, or stereo, and wipers.

The regulator is one of Dave Barton's adjustable units. Internal. (I mounted an external [remote] one on our 240, for easier access.)

I noticed this phenomenon because I tried to get religion, and always run with headlights on. So I eventually started leaving the headlight switch on... and noticed the reduced voltage; which especially mattered as cooler and damp weather came, and fan and wiper loads increased, and voltage fell to punky mid-range (and below mid-point) on the gauge.

Now I guess I am back to doing the lights manually, because I would really like to see 13-1/2 volts to 14+ anyway.

Thank you!

P.S. Is the word "excitation" somewhere in the answer? Dunno about any Good Vibrations in the car, but a little excitation sounds good!

Hi Gregg,

You dont want the voltage at 14.5V, then you'll be frying light bulb
filaments . 13.8-14.0 is better.

Bill

Bill reminded me that I need a list to check when getting a new car.

It took me WAY too long to realize that a 1992 245 I got some time ago was charging at something like 17 volts.

AFTER I fried headlight bulbs, dash bulbs -- these were the obvious things, and obvious CLUES -- and maybe some of the relays and such that were dodgy or suspect. ouch.

AND it had mottled paint around the battery. Doh! Spurting the acid with overcharge, weren't it?! :-(

Thanks, Bill. I will try to remember that.

Something like battery voltage (12.7) + 1 or 1-1/2 volts?

I'd like to remember this, but have been said to have "porous memory."
Even when I write things down I misplace the notes.
Some day they will do a TV show about my house (and the notes), and it won't be pretty. ;-)

Time to cut a thin wedge of tape and put it on the voltmeter in the appropriate place!

This old graph will remind you of the temperature factor, which is determined by the lead-acid part of the system. Of course, the temp isn't taken there, but the 700 at least doesn't take the temp under the exhaust manifold. And your aftermarket regulator may not operate exactly as Bosch planned either -- not saying there's anything wrong with that.


--
Art Benstein near Baltimore

There is no psychiatrist in the world like a puppy licking your face.
-Ben Williams

Hiya Art!

Interesting graph. And at -30C maybe 15V charging is appropriate.
I was at my friends place in northern VT and it hit -40F, Brrrrr
The charts don't help much at -40F, everything is nasty at that temp.

Bill

Hi Gregg,

This is not a problem. Or it is just a little one. ;-)

I think it is not a problem. If the voltage was as low as 12.8, then I would be concerned the belt or harmonic balancer is slipping due to having to start with a bigger load. That (the belt anyway) happens on my cars sometimes, but thankfully, accompanied by the appropriate squealing noise. Not all slippages squeal, especially those greased by leaking cam seals, etc.

The small differences arise in voltage drops that occur between the point of regulation (sensing) and the point of measurement. When you turn the lights on, I think you are drawing that current from a point nearer your gauge's connection, with respect to the highest voltage in the system at the alternator B+ terminal.

A mental subtraction of expected losses in the harness wiring is called for. I would give the wiring a 1-volt benefit of the doubt (14 to 13) before getting, well, excited.



--
Art Benstein near Baltimore

Women and cats will do as they please, and men and dogs should relax and get used to the idea. -Robert A. Heinlein

Thanks, Art.

I get the drift of what you say. Or a hint of the drift. (My brother is a EE, so I am not totally clueless.)

I suppose I should drag out the factory wiring diagram for the car (money WELL spent), and trace the routes for reading voltage... Midwinter reading coming up.

P.S.

I like you too! :-)

Do you think the wiring is that small?
Unless there are LARGE loads (and undersized conductors), the voltage should be pretty much the same all over.

Do you think the wiring is that small?

Yup, and I like Gregg. I wouldn't steer him wrong or attempt to BS him with some armchair theory -- at least not above my sig line. The voltage won't be the same all over, and those conductors are likely to be "right sized," not undersized or oversized.

The problem I have is not knowing the 700 or where its voltmeter is actually connected. So the science is lacking without the quantities. How much time lapses when headlamps are cued after the high-voltage startups.

I'd be interested to see some of the voltage drops in that S60* of yours, so much later in the copper shedding game. Did Volvo employ the power bus and CAN switching at the load? I know what the design challenge is, but have no clue what Volvo's done to meet it, post RWD.

*for example, check across a brake light bulb, rear window defogger, tank pump, or other remote load compared to alternator voltage.


--
Art Benstein near Baltimore

A dog teaches a boy fidelity, perseverance, and to turn around three times before lying down. -Robert Benchley

Well, the battery is in the trunk and it starts OK.
There is some kinda bus that seems to terminate at a big bolt under a cover under the hood.
I am not sure my voltmeter is good enough to check that kinda stuff.
I should probably buy a new one. I indeed can tell the difference between
12, 12.6 and 14 though.
IOW, it does most of what I need to keep a D-jet running!

IOW, it does most of what I need to keep a D-jet running!

I've browsed some of the archival stuff on the D-jet controller. I remember the critical element being the MAP sensor, and being very impressed by the effort put into restoring and maintaining the system. The design of it was breaking news, too, in automotive circles.

If I was to hazard a guess, I'd picture a Simpson 260. You're right. Used in the manner I suggested, you'd be splitting hairs on the scale unless you could stretch the probes to measure actual drop between source and load, on its 1V scale. That's where a DVM's eye-saving resolution helps - you can conveniently avoid the stretching. And when I chose those loads for testing examples, I was thinking distant loads, but the alternator is still up front on the motor, I suppose.

My daughter's '02 BMW has the battery in the trunk, and a post, like you describe, under the hood for jump starting. Just a couple weeks ago, I learned it must use that load switching, copper-saving, car area network approach when I noticed her tail lights flickered. Barely perceptible, until compared with steady, full on brake lights.

Those tail lights had the same 21W bulbs as the brake, turn, and backup lights, but were mimicking 5W lamps by being switched on and off at a rapid rate, just like the frequency valve on a k-jet lambda car. That was the flicker I noticed. My DVM wasn't much good at perceiving it either, but an analog meter would see the average, and the oscilloscope I found even more handy (my Simpson's red test lead dry rotted) proved the pulse-width scheme.

It must have been a painful compromise in the quest to reduce weight and expense of copper wire only to run a heavy cable from trunk to engine for cranking.
--
Art Benstein near Baltimore

BTW, don't buy the $25 DVM Walmart sells as "Innova Autoranging." It can't be taken out of that mode.