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MG MGB Technical - Halogen headlights and generators

I have a 67 MGB with a positive ground system and a generator. I was thinking of upgrading my headlights to a Halogen light system. If I do this I would take the power off of the starter Solenoid and use two relays to switch power to the lights. While the wiring is fairly simple to do, I have a concern about the ability of the generator to handle the load of the Halogen lights. Does the original generator have the capacity to handle the load of Halogen lights and still run the car, heater fan and window shield wipers at the same time? Will I burn out the generator? I am not interested in converting to a alternator. Besides the heater fan and window shield wipers the only other electrical item on the car is the original positive ground AM radio.

Thanks Jim
Jim Lema

Use Halogen bulbs and lamp units of the same wattage as what you have now and you will see a dramatic increase in light (lux) with no extra current draw.
Daniel

You won't burn out the generator as the control box has a current limiting relay to prevent it. However if the demand exceeds the output of the generator the voltage will fall, and if you exceed it by too much for too long you could eventually flatten it enough to cut the engine. If you do uprate them and increase the current as a result then relays would be beneficial to protect the main lighting switch and also get more voltage to the lights.
Paul Hunt

Jim I will give you the knowledge that you need to assess this for yourself. The dynamo will produce a maximum of 35 amps (adjustable by the C/Box) as most do, to find the amps you are using when running diferent electrical items you need to know their Wattage and also a little formula
Amps = Watts divided by Volts.
Example 60 watt headlamp would be 60W divided by 12V equals 5 Amps.
When you work them all out you will find that a dynamo is more than capable of running your needs plus it will have a little in reserve for recharging the battery after starting. Whilst an alternator will charge at about 45 amps it also has the ability to do this at lower revs than a dynamo and thus someone using their car in a winter in congested traffic with everything on may gain from a conversion.
Bob

35 amps does not seem like a lot of power. How much does the heater fan, windowshield wipers, fuel pump and other running lights draw?

Jim
Jim Lema

Bob,
Please correct me if I'm wrong, but I beleieve that an MGB generator maxes out at 22 amps. The cut in is at 1585 rpm.
David

You can weld steel with 35 amps!!!!!!!! With everthing turned on my ampmeter shows a 20 amp draw.
Dauntless

The Workshop Manual does state that the control box limits dynamo/generator output to 22 amps, this is done to prevent loads greater than that from melting the dynamo and should not be fiddled with. The GT HRW probably takes more than anything but the starter, and sitting with the foot-brake on and indicators flashing takes a lot more than without. A dynamo is probably not charging at idle, which is why the warning light is usually glowing or flickering, whereas an alternator will charge - depending on load - down to about 600 rpm and the normal idle speed for most alternator equipped cars is more than that.
Paul Hunt

Correct me if I am mistaken but wasn't the original point about the current draw of Halogen bulbs compared to standard bulbs. Secondly there is no increase in current draw on like for like wattaged bulbs whether Halogen or not but the Halogen produce significantly more light, even more so the new Xenon bulbs.
Daniel

Paul you are quiet correct the limiting factor of any electrical motor / dynamo is the heat generated, but I must confess that I thought all dynamos were 35amp and I was surprised when you suggested MGB control boxes were set to limit the current at 22 amps? After checking it would appear the lucas C40 is regulated to 22 amps so I must stand corrected.
Bob

Here's sometheing to put in your pipe to smoke... The halogen H4 light bulb & shell assembly is commonly used to replace the 7 inch bulb. (easily found on Ebay, too) We can even convert to HID lighting and draw a third of the current compared to standard halogen. I have no idea how robust the HID ballasts would be in an antique wiring system, but it can be done! I imagine the prices will drop in a few years, ($400-500 now)but imagine how unique your car would be! Now if I could only read the instrument panel at night. Cheers, O. Jibril (mgb77brg@aol.com)
O. Jibril

This thread was discussed between 02/01/2003 and 05/01/2003

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