GMC Headlite (poor lo-beam brightness!)

[GMC Jim]

GMC uses a 4 headlite system - PAR46 bulb; 5.7" diameter. Original lamps were#4000 lo-beam--37.5/60.0 watts and #4001 hi-beam--37.5 watts.
These trade no's were replaced by H5006 lo-beam--35.0/35.0 watts (very poor lo-beam brightness!!) and #5001 hi-beam--50.0 watts.
By contrast; Chev trucks use 2 - #6014 lamps--60.0/50.0 watts ( a definite improvement on lo-beam brightness! )
In searching for a lamp to replace the low output lo-beams; I find that J.C. Whitneysells a lamp to replace the lo-beam 35.0/35.0 watts with a 60.0/55.0 watt H-4 halogen lamp.
Part # 135ST9319P @ $15.99 each. 100/80 & 130/90 watt H-4's are also available for off-road use.
Plan on replacing the existing (lo-output) lo-beam lamps with the J.C. Whitney subsitute. Would appreciate any comments or furhter information on improving light output on GMC units.

['68OrangeSunshine]

Yeah, the 7" PAR (parabolic aluminized reflector) of the Chevy headlamp is a more efficient design than the 5.75" that the GMC uses, especially at Lo beam. The big punch in that system is when all 4 lamps are burning on brights.
Besides basic troubleshooting like checking harness continuity, cleaning contacts and lamp condition, the easiest way is to go to Halogen lamps. There are some aftermarket rally type headlamps available in the 5.75" format, but they look expensive. Over $50 bucks each, X 4?
I have heard that putting the 4 headlamps on a separate relay circuit -- like foglamps -- is a way of getting more and cleaner power to your headlights, Also takes the strain off a 30+ year old factory headlamps & underdash harness. I havn't done it myself, so I can't say anything about how to set it up.

[GMC Jim]

will post to this forum; the results of my intended mod to the J.C. Whitney lamps... not all that expensive....
My comments on checking for voltage drop;......Rigid marine specs allow up to 3% voltage drop for "critical" applications, i.e. transceivers,loran,GPS, etc..... . So... I would use this procedure before going to the time and trouble to re-wire the circuitry involved. Start engine and set engine speed for a fast idle at minumum.... so that alternator charges.... measure voltage with a digital meter at the battery; then measure voltage at the load you are analyzing. In this case at the low and high beam terminals at the headlite lamps themselves. Differnce in voltage at the battery and the lamps will be the voltage drop. I have saved myself many hours of re-wiring by determining voltage drop beforehand on industrial, marine and auto applications.