main headlights gone - power to fuse, first position light switch working...

[Ianos]

Hi all

 

Finally got around to investigating the no headlight issue - FYI its a '79 620, and the internet tells me that is a 'non relay' truck.. 

 

The back story is that everything was working fine until recently - then one side was intermittently going, followed shortly be the other. 

I remember at the time having been driving at night on dark unlit roads, so was using the column stalk switch to go between highs and main beams, I had figured that that bit of use might have been the issue - wearing out a terminal or breaking the column switch entirely. . 

 

broke out a multi meter and here is what I know: 

1. the fuse is good - continuity there - its a 15A fuse where the fuse box says it should be a 20A.. I'll get the correct one but for no that doesn't seem to be the problem (see point 2).

2. there is voltage across the head light fuse post ignition

3. however there is no power at main headlights terminal across all 4 headlight units

4. The low lights are working - they seem to have a separate circuit/power source... even with the connecters removed from the headlight until they still have power for the low beams. 

5. there is power going to the dashboard pull out switch, at least for the low beams and buzzer for when the door is opened and lights are on.. 

6. all other fuses are good bar the cigarette fuse which is gone, I'll replace but I don't think that will impact the headlights!  

7. there is power to the other switches, dash lights, dash light dimmer is working correctly, indicators, hazards, etc etc all working.. 

8. engine bay lamp is working fine too 

9. there is continuity/circuit  across the headlight unit terminal/bulbs.. 

 

so - there is power getting to the main light fuse post ignition.. that's for sure.

From there the power goes to the pullout dash light switch which is working in the first position as the low beams come one (pre and post ignition).. I'm going to see if I can get the switch out and swap the terminals to see if that then places the main beams on the first potion and whether they work, which would mean the second position is the fault.. 

 

Another culprit is the dimmer switch, which seems to share the same circuit with the headlights - however, the switch itself seems to be working as the internal dash lights dim perfectly when used.. so eliminating that for now.. 

 

 

Any thoughts, let me know .. for now i.ve no transport after dark... 

 

 

 

 

 

 

 

Edited February 6, 2022 by Ianos

[Ianos]

quite update; 

I was incorrect earlier - only one of the two headlight units has a good circuit - the other is dead. will order two replacements. Either way, I dont believe one bulb/unit being blown would take out the second or effect the power to it - do correct me if Im wrong 

 

  

[datzenmike]

The 620 lights work this way.

 

From the fuse, power goes to the headlight switch but only passes through in the full out No. 2 position.

 

From the 10amp fuse power goes to the headlight switch but only passes to the dash and running lights (parking/tail/marker) in the No. 1 and 2 positions. This is separate from the headlights themselves so we'll ignore for now.

 

 

When the headlight switch is pulled out to the No.  2 position power is passed through to the dimmer switch which selects either hi or low beam headlights.

 

There is a momentary 'flash to pass' position on the dimmer switch. Pull the stalk all the way towards the driver to select high beams in the day time even if the headlights are off. This switch should have very little use on it and is probably in good shape. Try it and see if all four lamps light. If they do, or even one works, then the wiring is OK. If a bulb does not light it's probably burned out. If all high beams come on but not with the dimmer switch in the high beam position, then the dimmer switch is at fault.

 

Remove the steering wheel, and two piece plastic 'clam shell' to get at the dimmer switch and clean the contacts. Buy a can of electrical contact cleaner and spray them clean. You might be able to flush them out from the outside, maybe... or take it apart.

[Ianos]

20 hours ago, datzenmike said:

 

When the headlight switch is pulled out to the No.  2 position power is passed through to the dimmer switch which selects either hi or low beam headlights.

 

 

hi KR

 

thank you for replying and helping! 

 

Thats interesting to read - I knew the column stalk switch was part of the circuit but didn't realize that it is referred to as the 'dimmer' switch. confusingly there an actual dimmer switch for the dash lights so I assumed the headlights were on that circuit somehow. 

 

As to the column stalk aka dimmer switch - i tested it out and it doesn't seem there is power getting to it. . I will have another look today if I get time but I think I tested voltage across the two contact points hidden beneath the clam shell casing, and got nothing across either positions (neutral/main beams or High beams). 

 

Currently my thinking is the pull out switch with 1 and 2 positions is the issue - and will do further investigation. 

 

any other thoughts, do let me know - , I was using that column stalk (dimmer) 'flash to pass' function before and it was working fine - but currently zip. i'll clean it out and see if that helps but without voltage to the contact points Im not holding my breath!

 

thank you!

Ian 

Edited February 7, 2022 by Ianos

[datzenmike]

The 'flash to pass' is wired directly to the fuse for headlights and has nothing to do with the headlight switch. It works independently of it. If 'flash to pass' is not working it's either the fuse (unlikely because the low beams work) or we're back to the dimmer (hi/low) switch.

 

On the dimmer switch... Red/Blue wire is power from the fuse. Red/Yellow fends power to the high beams when the lever is pulled all the way towards the driver.

[Stoffregen Motorsports]

Problems with headlights on non-relay equipped vehicles is common as all the power travels through the switch and abuses the contacts, which wear out over time. It is possible to "fix" the switch.

 

Start by unbending the metal tabs that hold it together and carefully prying the phenolic board from the metal housing. Once inside, there will be springs and possibly some balls and some either copper or lead/aluminum contacts. It should be easy enough to identify which position is bad by visual inspection. Cleaning everything should be enough to get it working again, but you may have to take drastic measures and add a bit of solder to one or more of the contacts or by bending things around so that there is more pressure on them.

 

It might be wise to check continuity across the contacts before you disassemble the switch. If you find a dead circuit, then go ahead and try the repair.

 

I just recently had a problem with the heater blower switch in my old Chevy K30 yard truck. Took the switch apart (like I described above) and found a mud bee nest inside the switch. Cleaned it out and reassembled it. Works like a champ now.

 

 

[datzenmike]

You can bend them once and back into position. I did it a second time but scary close to snapping the tabs off.

[Ianos]

Thanks v much - 

 

this is super useful information - 

 

I believe I tested the two contact points on the dimmer switch and neither were getting power.. however the fuse is 100% working, it has continuity though it - that said, just thinking about it more - maybe its the fuse box its self - many the contact points of the fuse box are not contacting correctly - but again you point out, the low beams are working  and if they share that fuse than that cancels that. 

 

I am putting my money on the headlight dash no. 2 position not working - unless I can prove there is power getting to the dimmer switch, it seems to be the only possibility remaining. 

The difficulty is getting that switch out - its really awkwardly placed in up behind the fuse box, plus the knob seems to not want to budge off its switch column!

 

thank again - will keep you posted 

 

[datzenmike]

Try the 'flash to pass'. If the high beams light then there is power to the hi/low switch. If they do not light there is the possibility that both high beams are burnt out.

[Stoffregen Motorsports]

Right. Test the dimmer switch while activating it. The flow of power may not be there if sitting static.

[Ianos]

23 hours ago, Stoffregen Motorsports said:

Right. Test the dimmer switch while activating it. The flow of power may not be there if sitting static.

excellent point 😅 i'm clearly not an auto electrician

[Stoffregen Motorsports]

Diagnosing anything follows the logic "you don't know what you don't know", so poking and prodding anything and everything is not unprofessional.

[Ianos]

the flash to pass isn't working in any case so either both high beams are gone or the column switch isnt getting power and/or the column switch is broken. 

I'm searching for correct replacement units for the headlights and will order a pair - 

 

plan:

1) replace both headlight units

2) clean out the column dimmer contacts and switch

3) pull out the headlight dash switch and clean/test position 2 - likely try to replace with a new unit. 

 

any further ideas or input  greatly appreciated.