Voltage-drop with Headlights on? (Generator/Regulator).

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Apr 2, 2007
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Hi All,

Took my Bus to a local meet last night... and it's the first time (in a long time) that I have driven it in the dark.

I have a voltmeter installed up-front and the needle normally sits roughly around the 14V area when daytime driving (at off-idle revs).
However, I noticed last night that with the Headlights on that the voltage reading was around 12.5V ish? (The Generator light did not come on or glow).
I've a 1641 twin port and I'm still running the Generator/Regulator set-up. I'm aware that 14V ish is normal/good for a Generator when without-load... however I've just read in the Bentley Manual that the 'with-load' voltage regulation should still be in the 12.8 - 13.8V area... so mine does seem a bit low... and oddly it did seem to stay pretty stationary at 12.5V and did not seem to increase above that with increased engine RPM?

It was only a short drive, so did not cause any issues... but had I been doing a long drive in the dark, and worst still in the rain with the wipers on... I'm not sure how long running it at just over 12V would have been sustainable?

Is it normal to drop like that and remain at a relatively static low level with the lights/loads on... or would you still expect to see the Generator deliver up and over 13V regardless - especially at higher engine RPMs?

Thoughts and ideas welcome.
Thanks... would that be as follows...

Terminal D+ = Input
Terminal B+ = Output

What would you expect to see on each?
Finally got round to doing some checks today... but a little perplexed by the results 🤔

Started by verifying the generator light/bulb was actually working... Tick.

Then popped the wires off the generator and tested its capability by measuring its output at D+ with my multimeter while momentarily grounding Df... and with the revs up it was putting out upwards of 40v, so no issues there it seems.

Reconnected the wires and turned my attention to the regulator... Checked the output at B+ with my multimeter and found it to be around 14.5v with the revs up... so the same as what I see on the voltmeter guage installed up front on the dash, so all good. Popped the lights on then checking again at B+ the reading was still up around 14 - 14.3v... also good right.

Started scratching my head at this point as this was not what I saw on the drive home the other night up front on the guage at the dash... so did the same test again but this time looking at the guage up front, and found this was as per what I saw before... 14.5v with lights off, then drops to around 12 - 12.2v with the lights on?

So it seems that out back the generator and regulator are working correctly, and the actual voltage in the engine bay and at the battery is only mildly affected by the lights being on or off.

However the guage up front is instantaneously affected by the lights... dropping approx. 2v with the lights on?

So all I'm now thinking is could it be dependent on what the guage up front is connected to for its live feed that is making it behave this way and is misleading me?
I'm not sure what it is hooked into, but say for example it is connected to the ign live of the light switch itself, could that explain it... Because that circuit is technically in use when the lights are on?

Or do I maybe have some deeper issue up front... poor earth/s, high resistance somewhere?

I'll need to dig a bit deeper... and am thinking next stop is to check the voltage readings at the main feed into the fusebox up front... to see what is reaching the front. But ran out of time today. So most likely next weekend now.

However if in the meantime any of you have any ideas or advice... It's very welcomed.

As you said, it depends where you have taken a feed from for the voltage gauge.
If you have taken it from the main feed connection at the fuse box you are actually seeing the voltage there. There will be a voltage drop due to the length of cable from the engine bay and 50 year old connections before you turn anything on.

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