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Old 07-11-2018, 10:10 PM   #1
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Location: La Grange, Texas
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7-pin versus 8-pin headlight switch - 1970 Jimmy

I have been working getting all my lights working and the headlights do not come on. Got a nice police escort the other night for last mile of 25 mile no headlight run at midnight.

I have studied the wiring diagrams and read all the threads.

My dinner switch is fully functional. All the wiring tests for good continuity between the NEW headlight switch and the dinner. The two 12-volt fused and unfused power hots also test positive for voltage.

When I put the old switch in headlights work fine. (But that switch has a busted reostat) Dimmer switch works beautifully for Low and High beams. All good (except panel lights from broken reostate)

When I plug new switch in, all the side marker and dash lights work, just nothing on the headlights. No power down to the dimmer switch coming out of the switch.

I have followed the testing YouTube and the switch tests good on all pins for continuity and power voltage and resistance per test instruction.

The only thing I can see is that the switch from NAPA is an 8-pin switch. And my switch and harness is a 7-pin switch. The number three and the number 7 are not used and no wire in the harness.

Is it possible that there is something unique between the 7-pin and the 8-pin switch that provides a ground or is missing something that keeps the 8-pin switch from working on the 7-pin harness? There is obviously an extra #7 spade on the switch, but no wire in the harness so just goes to air. (My old switch does not have the #7 spade.)

When I test the new switch on the bench with power jumpers the #6 headlight spade for the light blue wire to the dimmer wire gives full voltage. So I am pretty sure the switch is making the necessary contact internally.

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Old 07-12-2018, 01:07 PM   #2
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Re: 7-pin versus 8-pin headlight switch - 1970 Jimmy

I believe that there is an auto-resetting circuit breaker internal to the head light switches.
If that's the case and the breaker is either defective or is breaking for a reason it might turn the lights out when the switch is on with the lights, but might work when the lights are not connected (when you are checking continuity.

Perhaps unplug side's headlight connector and see if the new switch will run one of the lights. If it will but not run both, I'd think it was a defective switch.

Of course if you've already done a headlight relay modification then this likely won't apply.
1970 C10 LWB 350SBC, SM326 (on tree), 3.07 open (assumed), PS, PB, Idiot lights
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Old 07-12-2018, 05:42 PM   #3
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Location: La Grange, Texas
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Re: 7-pin versus 8-pin headlight switch - 1970 Jimmy

Well, I replaced the NEW switch with a second NEWEST switch and works like a charm. The first NEW switch tested with both power and continuity and resistance, so I really don't think the switch was bad, unless as you point out there is some form of circuit breaker or there is an internal wiring configuration that somehow puts a ground in the circuit.

The 8-Pin part number for NAPA ProFormer is HL6554SB Switch / Commutateur Interruptor and oddly enough it is $5 bucks cheaper.

The 7-Pin part number for NAPA ProFormer is HL6613SB. This was not the correct switch for my 1970 GMC Jimmy.

I actually believe it is just important to get the correct switch for your year model and harness configuration and I am under the impression, unverified, that the switch over (no pun intended) was around 1970. Only way to tell is to look on your existing switch and see if there is a spade in the number 7 position indicated on the switch, and if there is then it is an 8-pin switch. If NO Spade, then it is a 7-pin switch.

Now my wiring connector did not have a wire from the harness into that #7 spade nor did it have one in the #3 position, but that possibly is of no consequence.

Hope this helps anyone having this issue down the road.

Thanks ZoSopp for the input.

Attaching picture of my old switch with the 8 pins along with a diagram from American Autowire labeling the spade functions. (Not advertising and I have not actually bought anything from them, but they posted this diagram and it was helpful to me - so figure i needed to give credit.)

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Old 07-13-2018, 12:06 AM   #4
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Re: 7-pin versus 8-pin headlight switch - 1970 Jimmy

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1965 C30 dually rolling chassis getting built with a '67/'68 body, am collecting parts for body and inside cab and 250/292 engine brackets pieces right now 2018! text 737 0942 in 217 anytime if you have anything in central IL for me!
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