RPM idle drop when headlights on

[airschooled]

Richen the idle a tiny bit and try again. Sometimes the "power" gain from a slight increase in idle fuel can mask the drop. Vanagons use an idle timing stbilizer, you could hook one of those systems up if it really bothers you.

That should buy you time to fix the charing issue

Robbie
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[telford dorr]

A little math:

Assume the alternator is supplying its full 70 amps, and the output voltage is 14 volts.

70 amp x 14 volts = 980 watts

980 watts x (1 hp / 746 watts) = 1.31 hp

Assume the alternator is 80% efficient:

1.31 hp / 0.80 = 1.64 hp

So the fully loaded alternator should add a 1.64 horsepower load to the engine (the fan load won't change, loaded or unloaded).

The question is: how much should this load slow down an idling engine?
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[mikedjames]

Changing the alternator pulley for a smaller one will spin the alternator faster and allow the alternator to draw even more power from the engine.

If it works properly it should make the effect of turning on the lights even more noticeable as a bigger drop in engine RPM...

Fitting a bigger pulley will increase the cut in speed, so the warning light is more likely to be on at idle, but the RPM will not drop as much.

Your choice.

Fitting a higher capacity alternator will also increase the possible power drawn from the engine at a given RPM .

Alternatively, tweak the idle RPM up a little.
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1979 Westerly GK24 24 foot racer/cruiser yacht Forethought of Gosport.
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[Timewizardbutterfingerkri]

Have the same thing happen on my 74 super.
Will run at 950to 1000 rpm no lights then goes down to 850 to 800 with the lights on. I have added an extra set of hella fog lights wich could be resulting in the added load to the alternator. Havent had a problem tho! Might be normal?
Car stays on and everything lights up great.
I have heard of the use of a smaller pulley on the alternator to spin it faster but have not looked into it. Maybe give that a search good luck!

[Wildthings]

Don't know what oil your are running, but I strongly recommend synthetic oils if you have hydraulic lifters. Synthetics don't have the large additive packages that breakdown over time causing the buildup of all kinds of scuzz inside the engine.

[KlassicBus79]

I had a big scare today. I drove the bus into work today with the lights on the whole way and the rpm was very steady when I stopped at red lights.
On my way home it started fine and drove fine, but I had to go into another place so I shut it off for a couple of minutes.
When I started it back up it sounded awful. It was the same type of noise as the loose valve seat I had last year (expensive). So I took it home and changed the oil and filter and prayed. I took it for a short drive no change.

So with the stethoscope I determined it was number 1 cylinder so I let it sit for a bit and then adjusted the valves on number 1.
Started it back up and noise was gone. I suspect it was dirt in the lifter.

Also took it for an alignment today.
So busdaddy it will be a couple of days before I can get back to the hooking up another vehicle to my bus because I have to fly for work a couple times this week and I'm gonna need my beauty sleep.
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79 Cal Transporter 2L

[busdaddy]

So that indicates a slightly lean idle, good......better than too rich but not an explanation for the electrical load drop.

What happens if you connect up to another car with booster cables?, leave the other car turned off and give the systems 5+ minutes of bus running to equalize before proceeding with the drop test.
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[KlassicBus79]

When I pump the brakes rapidly (3-4 pumps per second) with headlights on or off it takes about 6 pumps to start making the rpm drop and after about 15 to 20 pumps it drops from steady idle of 950 to about 800. With the back hatch open I can even hear the change in the exhaust.
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79 Cal Transporter 2L

[busdaddy]

Out of curiosity what does the idle speed do when you pump the brakes rapidly?
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[KlassicBus79]

I thought I would go over all the suggestions again in this post after I blocked the port and didn't see the big change in rpm I was expecting. The headlights still seem to be the main culprit. So I hooked everything back up and I installed the clamps on the hoses.

I decided the easiest and least expensive one to try for now was to adjust the idle mixture screw.

So here is the result; on about a 1/4 turn clockwise on the idle screw.
Idle smoothed out with not much of an increase in rpm (around 950) lights off.
Lights on with brakes applied idle dropped between 25 and 50 rpm, but was steady at that rpm with no fluctuation.
Short drive and every time I stopped it seemed a lot better.

I will drive it to work tomorrow and will let you know if this worked out.
Thanks for all the suggestions.
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79 Cal Transporter 2L

[Wildthings]

[airkooledchris]

For what it's worth, I also run a 79CA with federal heater boxes, and with or without the sensor connected you won't see any difference in this behavior (I only disconnect mine when I hit the highway, connected while in town.)

not to say I think it's definitely the battery, it's just that the battery seemed to really make the difference in my case for RPM drops under electrical loads.

[KlassicBus79]

My bus is a 79 California bus that has been switched to federal heater boxes, as far as I can tell the FI is mostly stock as the part numbers seem to match up, although there is no oxygen sensor (wire is still there and it is on my to do list).
I don't have access to a CO tester off hand.
The battery can possibly be changed out when Momma Bear decides that I can have some more money for the bus. Already spent my allowance for the spring

I also noticed that my rpm drop corresponds with both the lights and brake application when I come to a stop. I found that the lights have a voltage drop but the rpm doesn't fluctuate too badly and that the brakes don't affect it too bad by themselves either. It is when they are done together.

I bought some clamps for the rubber t where the brake booster hose goes to the intake plenum. Maybe there is an air leak there.
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79 Cal Transporter 2L

[airkooledchris]

How is the battery itself? I had the same issue as you and when I finally put in a new battery the behavior went away.

[SGKent]

It doesn't sound all that abnormal depending on how the engine is tuned as you are close to the cut off speed of the alternator. You might try tweaking the idle mixture a bit richer or leaner to see if the engine runs a little smoother when under load. Use a CO tester if you have access to one. If you have duals instead of FI, check the sync on the carbs. Very late buses with an automatic have an idle speed stabilizer on them that may need adjustment too.

I agree with Wildthings that the engine slowing shows that the alternator is taking the load. And as suggested, maybe there are larger amperage bulbs in the headlights.
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[telford dorr]

Sounds like everything is working properly, except that the lights seem to be pulling a lot of current.

Anything unusual about the lights?

If you can find a suitable meter (something that can measure upwards of 40 amps), I'd recommend measuring the alternator output with lights off and on.

One of those magnetic hold-it-near-the-wire type meters could work for this.
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[KlassicBus79]

This morning battery voltage no lights 12.6 with lights on 11.8
Started it and at 950 rpm 13.6 with lights on and 14 with lights off.

I also checked the starter terminals and I have 6 wires on there so there seems to be an extra one on there if I have the big red alternator wire going directly to the battery instead of the starter. Will have to do some more investigation.
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79 Cal Transporter 2L

[telford dorr]

[KlassicBus79]

Going out this morning to do voltage checks right at the battery. Might even wire it up so I can see voltage right from the battery while driving. Last night was a just a quick check with a voltmeter wired to a cigarette lighter plug.

I did notice from the schematic that the heavy gauge red wire that attaches to the alternator is supposed to go to the starter. On my bus it goes right to the battery. Same point electrically just a different spot. I replaced that wire ( it had a cut in it from the tin where the grommet slipped off) when I installed the alternator and put on new lugs and dual wall heat shrink.
I routed it to the battery because I assumed it was going to the right spot. Didn't check Bentley mostly because it worked previously and I didn't want to get my Bentley all dirty with grease and grime.

I replaced the starter when I replaced the alternator and made sure all the wires and terminal lugs at the starter were cleaned or replaced.

Unfortunately I didn't count the wires or follow them back too far.
So now I have to check to see if the PO left an orphan wire on there.


Update

12.9 volts at the battery not running. 14.06 volts at the battery when started and at 950 rpm. 13.6 volts after a short drive and warm.
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79 Cal Transporter 2L

[Wildthings]

The rpm drop is telling you the alternator is taking load, so I don't think there is necessarily anything wrong with the alternator. Are you running high wattage lights or extra bulbs which could increase the load on the alternator? You could use a clamp on ammeter to see what the actual load on the alternator is. If the load is normal, maybe tweaking the idle mixture screw a bit would keep the idle from dropping as much.