4 Headlight high beam conversion?

[Daveo91Burb]

Anyone have a good write up on the mod to have low beams stay on when high beams are on? I know there's lots of info on the web, and there are also kits that can be purchased online, but I'm looking for a write up specific to squares if possible. I think it's more or less as simple as adding diodes between low and high feeds, but need to know which diode type to get from radio shack. Or if there's a better way....

Would be great if someone had a good write up it could be made a sticky.

 

[MadOgre]

I just flick the high beam switch on the column! That usually does the trick. Makes all 4 come on every time

 

[MadOgre]

Haha

 

[MadOgre]

No seriously the quad lights are factory wired to have all 4 on at high beam.

 

[89Suburban]

No seriously the quad lights are factory wired to have all 4 on at high beam.

Not on the '88 - '91 mini quad Burbs/K5's.

 

[Daveo91Burb]

Yes, trust me, they don't all come on on my '91 burb or the '89 k5 I had years ago. If you hold the switch back they will all stay on but once you let go it will just be lows or highs. There are $50 kits online that will do this, but there a number of cheaper options for those of us comfortable with wiring and electronics. One way I believe is to use a diode between the lows and the highs so when the highs are on it will feed the lows, but not the other way around. I actually did this on my '89 and it worked fine, but I don't remember all the specifics. Was wondering if anyone on here has done this or similar.....

 

[chengny]

On earlier versions of trucks equipped with the quad headlight system, both sealed beams were illuminated when the dimmer was switched to high. This was possible because the upper sealed beam is a 3 lug (dual filament) base. The dimensions of those lamps (both upper & lower) are 165 mm X 100 mm.


In later models - with quad headlights - the dimensions of the sealed beams used are 150 mm X 92 mm. And that's where the problem lies; there is no such thing as a 3 lug/150 mm sealed beam.

If a 150 mm/3 lug sealed beam were available, the modification (to make all 4 lamps illuminate when on high) you could do it this way:

Install a dual filament lamp in the low beam position, change the harness connector for the low beam lamp (from a dual prong to a 3 prong) and splice the high watt filament lead into the feed for the high beam.

But, since those lamps are not made by anyone, you will have to use a diode to modify your headlights for dual illumination. This modification is even easier than the imaginary procedure described above. Here is a quick crude dwg I made using an old Autosketch drawing program. It is not well detailed because I am very rusty with the program - but you should get the idea (the ground wires are not shown for clarity):

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The diode is cheap and easily available. I am not an electrical engineer but a 40 volt diode rated for 5A (low beam wattage = 50. 50/12VDC = 4.7 amps) should be enough.

Yeah, I just checked around. Here is a link to a physics forum dealing with this issue:

https://www.physicsforums.com/threads/which-diodes-i-need.518780/

A kid is asking about the proper diode required to run a pair of 35 watt fog lamps - essentially what you are trying to do.

Read more: http://www.physicsforums.com

The engineer recommends an SR540 5A 40V diode. It's available from Mouser for $0.54:

http://www.mouser.com/ProductDetail/Rectron/SR540-T/?qs=sGAEpiMZZMtQ8nqTKtFS/GTY/GKvBUTW48csxTNeNy0=

Splice the diode into a length of 10 gauge wire, connect the high beam feed into the low beam feed and you should be done.

Be sure to use good electrical practices:

1. Solder and heat shrink all splices - do not use any type of crimped connectors.
2. Run the new lead from the high beam over to lows along the existing harness and inside the loom.
3. Test the circuit for awhile with jumpers before making it permanent
4. Be sure the diode is wired correctly with respect to polarity


Again, I am not an electrical engineer - so consider the source of this information and proceed at your own risk.

 

[Gator]

This doesn't have any pictures or a huge amount of instructions but it may be of help.

http://www.73-87.com/7387garage/exterior/quad_highbeams.htm

 

[chengny]

He's talking about adding an extra feed into the headlight system - one that would provide an independent power supply to the low beams. But only become hot during high beam operation.

Where that power comes from isn't important. Right or wrong, he apparently decided to tap into his parking/marker lighting circuit. A better idea might be to run a dedicated lead from the firewall junction block. That's what it's there for.

That new wire would be run to the 87 terminal of a standard general purpose relay. Terminal 30 would be spliced into to the normal low beam feed.

On the control side; power to pull the relay in would be tapped off the normal hot wire to the highs and connected to terminal 86 (coil positive). The negative side of the coil (terminal 85) would be led to ground.

So, when wired as above:

If the dimmer is in low, power is as normal to the low beams. No power to the highs.

When the dimmer is switched to highs, power to the lows is secured and normal power is applied to the highs.

When the high beams are energized, so is the relay coil - because of the added wire that connects the high feed to terminal 86 on the relay. The relay then pulls in.

When it pulls in, the contacts (which are connected to terminals 87 & 30) close and power from the outside source is applied to the low beams - from terminal 30 to the spliced connection at the normal low beam feed.

The high beams are fed as normal from the dimmer switch and the lows are supplied by the connection to the junction block via the relay - all the headlights illuminate.

 

[Daveo91Burb]

Thanks, guys - this is the kind of info I was looking for. Looks like I have two options: diodes or a relay. I'm thinking relay might be the way to go - highs and lows will always have their own feed and I won't be worried about pushing too much current through the original wiring by feeding lows from the highs. (I'm sure diodes are fine, too - lots of people have done it including me with no problems) A couple of questions regarding the relay method:

1) One or two relays? They come in a pack of five on Amazon so I'm thinking two may be the way to go, one for each side and separate positive feed from battery or terminal block for both. Drawback would be more wiring, connections, and two inline fuses vs one.

2) Are any diodes necessary for the situation when highs (and lows) are on? Would anything backfeed through the regular low beam circuit? Similarly, when just the lows are on, assuming the relay would be able to handle that backfeed situation?

 

[short_circutz]

A single 40 or 50 amp relay would be more than enough to take care of the electrical current for both sides of the truck. And This method would be preferable to the Diode method As there would not be a chance of overloading the original headlamp wiring with the added current of powering additional lights through the original wiring.

 

[chengny]

When I respond to a posted question, I try not to do so in a frivolous manner. If I am not sure of the system involved, I take the time to research it thoroughly before providing an answer.

For example; in this case I suggested the use of a diode/jumper modification to provide power to both sealed beams via the normal high beam feed. The required voltage/power rating of the diode was provided - as was the AWG size of the jumper lead.

This was provided as an alternative to the commonly used method of using an independent power source supplied to the lows - via a relay operated by the high beam feed.

There has been some concern expressed that the addition of the low beams to the high beam circuit would place an undue load on the factory wiring. Unless there are some special considerations that I am not aware of, this modification should not present an issue. My reasoning is explained below:

The primary high beam feed in a 1991 R/V with quad headlights is supplied on a 1.0 gauge wire. This is to supply a pair of sealed beams rated at 65W (130W). The addition of a pair of low beams rated at 55W (110W) yields a total circuit load of 240 watts.

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As comparison:

On a 1986 GMC K2500 with quad headlights, when the headlights are in high beam mode all four sealed beams are supplied by a 1.0 gauge wire (same size as the high beam circuit described above). The uppers (high beam filament) are rated at 60W (120W L/R) and the lowers are rated at 50W (100W L/R). This yields a total circuit load of 220 watts.

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Using P = I x E (I = P / E), when in high beam mode - we get an estimated current draw of:

240/12 = 20 amps on the modified circuit of the 1991 V2500

220/12 = 18.5 amps on the OEM circuit of the 1986 K2500

Note: The current draw was calculated using a nominal voltage of 12 VDC. However, under normal operating conditions, the voltage available to the headlight circuits is more along the lines of 13.5 volts.

Using that voltage, the calculated current draw on each circuit drops considerably:

240/13.5 = 17.8
220/13.5 = 16.3

These circuits are protected by a dedicated circuit breaker in the headlight switch. That breaker is rated for 25 amps:

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Granted - the load on a 1.0 wire would be at the upper limit.

But, if that size was good enough for the design engineers who spec'd the wiring on the 1986 K2500 - it should be safe to carry an approximate 10% increase in current flow (in your Suburban).

240/220 = 1.09

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[Daveo91Burb]

Thanks, Jerry. I wasn't saying the diode method you proposed was dangerous or would overload the wiring (I actually did some similar calcs to what you posted and came to the same conclusion) but I do think the separate relay method is preferable for a few reasons. In particular I found that I already have the relay I need but I don't have the diodes! And I like the connections better for the relay than for the diodes. I appreciate the interpretation you gave for the relay method - I am looking at the relay now and I see the terminal numbers you mention - makes more sense now. I also plan on taking your advice and using the terminal block on the firewall for this. (I'll use an inline fuse between the block and the relay) I'll post pics when it's done. Thanks again.