Chevy to GMC headlight harness questions, upgrades

[bambbrose]

Good day everyone.

I have a 72 GMC (technically a Jimmy) that was "rebuilt" sometime in the past out of a Jimmy body and a blazer front end.

I'm putting it back to a Jimmy 4 headlight front end at the moment.

I'm posting here as I'll get more feedback and my question is applicable to the trucks the the blazer/jimmy crowd.


I have a standard 72 blazer headlight harness currently (w/ gauges). I have the original external regulator alternator with the regulator on the radiator support.

I was just about to pull the trigger on a new GMC headlight harness, but I got a quote of 8 weeks lead time as they are out of stock and needing to build new ones. That won't work for me.

That got me digging into the factory service manuals to see the difference. Unfortunately I cannot find a GMC specific wiring diagram, but I did find the Chevrolet diagram, as well as the GMC parts diagram. It would appear that the only actual difference is that a high beam is run off onto a new plug for the GMC.

I'm considering saving the $200 and just rebuilding my harness. Can anybody confirm that I just need to move the high beam wire to a new one of these plugs (attached pic), which I can pick up on ebay for about $6 each?


Can someone please confirm if the outer lamp on the GMC is just a low beam, and the inner is a high beam? Or is the outer a lo/high beam, and the inner just a high beam?



And my final question, is there any reason to upgrade the wiring gauge or similar while I'm in the harness for better light output? What is the limiting factor? I'm sticking with OEM type lamp assemblies, but if there is a way to make them "like new" or better while I'm at it I figured I'd check.


Thanks!

[Stocker]

Originally Posted by bambbrose

Can anybody confirm that I just need to move the high beam wire to a new one of these plugs (attached pic), which I can pick up on ebay for about $6 each?

Can someone please confirm if the outer lamp on the GMC is just a low beam, and the inner is a high beam? Or is the outer a lo/high beam, and the inner just a high beam?

That's a 3-prong connector, which is correct for the your outer lamps. They are high/low, so you need high, low, and ground. Your inner lamps are high beam only, so you can use a 2-prong connector (or a 3-prong, and simply ignore the third). You probably don't need to Ebay those... any parts store should have them.

And my final question, is there any reason to upgrade the wiring gauge or similar while I'm in the harness for better light output? What is the limiting factor? I'm sticking with OEM type lamp assemblies, but if there is a way to make them "like new" or better while I'm at it I figured I'd check.

Yes, do the 'headlamp relay' upgrade, even with OEM lamps. Stock wiring is a bit small, but the main culprit is the headlamp switch. It can't reliably carry full voltage to the lamps. Relays will fix that, and tests prove there is a noticeable difference in light output.


A few links to get you started on relays:

http://www.madelectrical.com/electri...adlights.shtml

http://67-72chevytrucks.com/vboard/s...d.php?t=188478

http://67-72chevytrucks.com/vboard/s...d.php?t=182351

[davepl]

On a hour headlight system the outer does BOTH low beam and high beam, and has a 3-prong connect (ground, low, high). The inner headlight is a simple on/off and just does the high beam, so a two prong (ground, high).

['68OrangeSunshine]

Originally Posted by bambbrose

Good day everyone.

I have a 72 GMC (technically a Jimmy) that was "rebuilt" sometime in the past out of a Jimmy body and a blazer front end....
And my final question, is there any reason to upgrade the wiring gauge or similar while I'm in the harness for better light output? What is the limiting factor? I'm sticking with OEM type lamp assemblies, but if there is a way to make them "like new" or better while I'm at it I figured I'd check.


Thanks!

Another upgrade idea is to use Halogen headlights [I have Sylvania's] in place of Tungsten. I haven't gone to the seperate relayed harness yet but it's not a bad idea. I have a '71 Jimmy.
Be sure your headlamp circuits are well-grounded to the radiator bulkhead. Intermittent connections there played hob with my EVR until I replaced it with a spendy M&H harness. Pulling off the old harness, I found the PO's idea of a splice was to strip the two ends bare, twist them together and e-tape it over. No concept of solder or crimp-connectors!

[bambbrose]

You guys rock, thanks for the fast response.

I'm reading up on the relay upgrade now. Just picked up the pigtails from NAPA.