Dash lights & headlights crossed?

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gregpenn
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Dash lights & headlights crossed?

Unread post by gregpenn » Wed Jan 10, 2018 4:15 pm

Help please, 65 Monza convert 140, I have no dash lights, but when I hit the dimmer switch for bright Iights the brights work and the fash lights come on. Hit the dimmer switch to turn the brights off and the dash lights go out. I have installed a new headlight switch, but that didn't fix anything.

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Jerry Whitt
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Re: Dash lights & headlights crossed?

Unread post by Jerry Whitt » Wed Jan 10, 2018 4:30 pm

May have more than one problem. Try a step by step process.

1. Remove the dimmer switch. Quite often they are in very poor condition. Many time the plug is really corroded. Clean up the plug. There are 3 terminals on the plug. With a continuity light, pull on head light switch and make sure there is a hot lead to one terminal.

2. Install a new dimmer switch. Most auto parts stores still have them in stock. Usually just a couple dollars. With new switch in place, check operation of the high and low beams a dash lights.

Let us know your test results. If needed, we can advise further.
Jerry Whitt
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65 Monza, purchased new
65 Corsa convertible

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bbodie52
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Re: Dash lights & headlights crossed?

Unread post by bbodie52 » Wed Jan 10, 2018 5:46 pm

In the Monza instrument panel, the fuel gauge, instrument illumination lights, turn signal indicator lights, and the high beam indicator light all rely on chassis ground for a ground return path. As shown in the illustration below, there is a single ground strap that provides a ground for all of these components in the instrument panel (necessary because the instrument panel itself is made of non-conductive plastic). This ground strap is secured to the dashboard metal frame (chassis ground) by a single screw. If that screw is loose, missing or corroded your entire instrument panel may not be properly grounded, which can cause the fuel gauge and panel lights to malfunction or to only function intermittently. (Only the two engine warning lights are not dependent on an instrument panel ground).

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Brad Bodie
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terribleted
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Re: Dash lights & headlights crossed?

Unread post by terribleted » Wed Jan 10, 2018 10:17 pm

Want to be smart? Unplug the dimmer switch wire plug. Look in at the contacts in the plug and the contacts on the dimmer switch. If you see any corrosion at all get a new switch pigtail from Clark's and install it and a new dimmer switch and forget about the common dimmer switch issues for many years to come. It really sucks when you are driving along at night and you hit the dimmer switch and all your lights turn off and maybe won't turn back on due to corrosion at this switch. Corrosion here is very common.
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66vairguy
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Re: Dash lights & headlights crossed?

Unread post by 66vairguy » Wed Jan 10, 2018 11:53 pm

gregpenn wrote:
Wed Jan 10, 2018 4:15 pm
Help please, 65 Monza convert 140, I have no dash lights, but when I hit the dimmer switch for bright Iights the brights work and the fash lights come on. Hit the dimmer switch to turn the brights off and the dash lights go out. I have installed a new headlight switch, but that didn't fix anything.

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The dimmer switch is probably fine if the headlamps work. It's most likely the instrument panel ground Brad spoke of. Why? When you select high beam with the dimmer, it not only runs the HI headlamp filaments, it connects to the high beam indicator bulb in the dash which is connected to the dash light ground. If that dash panel lights ground is open then the dash lamps ground through the high beam bulb and headlamps to ground.

Oh - Yes there is a ground strap on the instrument panel, but don't assume it's properly connected to the instruments housing. After all these years things get removed, swapped, mis connected.

gregpenn
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Re: Dash lights & headlights crossed?

Unread post by gregpenn » Thu Jan 11, 2018 5:52 am

Thanks everyone. Ahhh, the hi beam indicator- couldn't figure out how in the world the headlights circuit could be connected. Fixed 2 problems- it was the ground strap, but also a connector at the firewall was loose, M&F have no locking tabs on the side to insure connection. I've got all my lights back now.

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66vairguy
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Re: Dash lights & headlights crossed?

Unread post by 66vairguy » Thu Jan 11, 2018 9:52 am

Glad it all worked out. Your problem is called a "ground loop". Grounds are just as complicated as the positive power supply side of things. I worked in aerospace and spent time working on ground planes to eliminate problems.

BTW - I install a ground wire from the instrument panel to the body front bulkhead. That dash housing ground strap is notorious for causing problems.

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bbodie52
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Re: Dash lights & headlights crossed?

Unread post by bbodie52 » Thu Jan 11, 2018 10:11 am

:think: A faulty primary ground path can introduce some really strange and unpredictable results, as the current seeks a path to return home (to the battery negative terminal). In your case, the primary ground path had failed, so the instrument lights didn't work until a secondary path was introduced when the high beams were engaged (including the high beam indicator lamp circuit). Odd behavior in instruments, noisy audio equipment, and unreliable or intermittent electronic components can all be produced by faulty ground paths.
Brad Bodie
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gregpenn
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Re: Dash lights & headlights crossed?

Unread post by gregpenn » Fri Jan 12, 2018 5:12 am

I'm learning. Thanks again, all.

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