Volts drop a lot when headlights are on

Make sure your ground connections are clean or take them off clean, apply dielectric greese.

 

Head lights would be a good place to start but I would also do markers and battery to frame and frame to engine grounds.

Also look for corrosion in the connections.

 

You may research how to check your voltage regulator also. It should allow the alternator to offset that. Although maybe not so much at idle. You can also wire your headlights to a relay and directly to the battery. Your old headlight wiring will have the much easier job of triggering the coil of the relay.

 

Any headlights (except LED) consume a lot of power. It's pretty common to see your headlights dim at idle with a stock alternator in a 40. The alternator is not putting out full output at idle. Besides that, the stock wiring harness for the headlights is pretty marginal in terms of gauge; there are (or used to be) several aftermarket headlight harnesses available with much heavier wire and relays, to run (better) headlights straight from the battery. Add age to the equation, and yeah, the connectors and grounds will get dirty and degrade over time like @AlaskanWheeler says. The Second Law of thermodynamics is always working against you.

 

Clean the coil/ignitor bolts and carrier assy. They need a real solid ground, or they pull all the juice in the system

 

I did the cruiser Corp h4 headlights with an hid kit, readjusted my voltage regulator, raising the volts good to go.

 

I have high draw headlights(roundeye’s). I’ve been meaning to install a relay. However, after swapping every bulb in my 76 to LED’s, I’m going with LED headlights next.

Here’s a before LED’s


After. I bet LED headlight would hardly move the needle

 

There’s a good discussion in @Chungas Revenge’s mule thread. BJ40green is involved

This should take you to the start of it(page 64):
76 Fj40 Face Lift

 

Hid is high intensity discharge it's a headlight bulb, a real bright one too. To adjust your vr, remove cover and move the arm up or down to raise and lower voltage. I did this with the truck running and a multi meter.

 

The manual says voltage can be between 13.8-14.8, I just found an in between and was happy with that. Hope it helps.

 

Several years ago, I was working on my 1971 harness and wondered what the real current pull of some electrical items in the Cruiser were. I decided to measure a few electrical things from my collection, since they were out of the truck.

All tests were done with a Fluke 87 multi-meter in current mode. Disclaimer: This was not a scientific test. There are way too many variables involved in these old trucks to say that you will get the same results. All currents were measured while NOT connected to a harness but straight to a battery. This will not take into account the resistance of the harness and switches so the values are higher than what they would be if measured in a working vehicle.

Here are the results:

A 1156 bulb pulls 2.0 amps (Don't remember the brand)
A 1157 bulb pulls 2.1 amps for brakes and .6 amps for tail (Don't remember the brand)
Front heater blower (cleaned and lubed, new resistor) pulls 6.7 amps on high and 4.3 amps on low
Single speed Rear heater blower (Cleaned and lubed) pulls 2.7 amps
OEM Kioti Sealed beam headlight bulb 2.9 amps Hi beam, 2.7 amps low beam. More modern bulbs (not LED) may not pull as much due to improved reflector designs?

Anyway, based on the above, the single largest current pull on an early FJ40 (not including charging a battery) is the Hazard lights! 2.0 amps x 4 bulbs = 8 amps total! So if you ever wondered why the Amp gauge dips into the negative with every flash of the Hazard lights, now you know! One of these days I will expand the list to include the smaller bulbs like dome light, dash lights, gauge cluster lights.

Thread Hi-Jack over....

 

Suggestion: carefully feel (with your finger) the fuse for the headlights in your fuse box with the headlights on (for a few minutes). I burned my finger on my fuse, and this led me to clean the fuse-slot very carefully, and that helped. Obviously, running all the headlight current through the fuse box is a bad idea, but that's how it was done at the time. The least you can do is keep it clean and low-resistance.

 

Bumping up your idle speed 50-100 rpm (if it's on the low side) will do wonders for alternator output, too. Try 725-750, instead of 650.

 

Those numbers seen very low for the headlights. Typically an H4 bulb draws 55/65 watts, which should be closer to 5 amps each.

I also have a pretty large voltage drop when I turn on my headlights, so I think it's pretty normal. The alternator just doesn't put out many amps at idle, so it's all coming from the battery. I've also got 130W piaas on the front bumper, I can't even turn those on unless I'm driving because it'll almost stall my engine at idle.

 

As said before: At idle, the alternator is putting out only a few amps. Enough to keep the ignition and cluster going but not enough to (re)charge the battery, let alone feeding the headlights. You just cranked the engine so your battery is a bit below 12V at this point.
When you turn on your headlights (and running lights), all the power is coming from the battery but the battery is not recharged because the rpm's are too low. Add to that the flimsy wiring (= voltage drop) and you'll notice that your headlights are dim but light up when you rev up the rpm's.
1. Replace battery for a fresh one. This will reduce the dim effect.
2. Adjust the RPM's a bit. The alternator gives more output.
3. Replace the bulbs for LED's. This is a huge improvement.
4. Rewire the headlight circuit with 2 relays directly feeded from the battery.


Rudi