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Discussion Starter · #1 ·
That dimmer switch that controls the brightness of the instrument lamps can deteriorate over a number of years. I had my panel out over the weekend in order to install the radio and LED lamps -- I had not been satisfied with the lack of illumination. When I tested the LEDs I was less than impressed. Then I found that, even with the dimmer on full-bright, I had a voltage drop of about 2.5 volts. I disconnected the dimmer and ran a jumper between the two connectors and found immediate improvement.
 

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1970 Opel Gt - Purchased July 1972 - Chartreuse - restored - 3000 miles as of 02-16, 2021 -
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That is really good information as I too have not been satisfied with the brightness of the gauges. I would like to be able to see them better during the day. You said, " I disconnected the dimmer and ran a jumper between the two connectors and found immediate improvement."
So are you saying that you can just permanently remover the dimmer switch and put in a jumper wire as a fix? Or was that just a diagnostic test and if so what is the fix. This question comes from an electrically challanged forum member who appreciates your patience and reply. Thanks Michael.
 

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Discussion Starter · #3 ·
Well, sort of both. The dimmer switch has two connectors: one is the gray/green wire that is "hot", supplying voltage to the instrument lamps via the dimmer; the other connection consists of three wires, gray, that supply juice to the lamps after it has run through the dimmer rheostat. There is a junction block on the top of the instrument panel, between the speedometer and the ammeter/oil gauge that receives juice from one of the gray wires from the rheostat and connects at this junction with other gray wires, each feeding the other instrument lamps. With the dimmer set at "full bright" (not to be confused with Governor Fulbright), I tested the voltage at that junction block and found about 9.0-volts. I then disconnected the two terminals from the dimmer switch and connected them with a jumper wire, thus taking the dimmer out of the circuit. Testing at the junction gave me about 10.5-volts. This was with the car shut off and after I had been testing lamps for a while, and I suspect that I had discharged the battery somewhat.

I left the jumper wire in place.
 

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1973 Opel GT
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I wonder how hard it would be to recondition the switches? Once the rivets are removed, they should come apart pretty easily.
 

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why not just put a duplex connector and keep it on all the time....isen't it connected to the side of the fuze pannel when the key is not on....LED's don't draw much power.
 

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I wonder how hard it would be to recondition the switches? Once the rivets are removed, they should come apart pretty easily.
Personal experience with taking the switches apart, the plastic is so damn brittle that you do any fidgeting with the parts inside, either the spring breaks or the plastic components that are on the spring breaks. I just try to stock up on spares when I see them for a good price on ebay. Spray the heck out of them with a plastic safe contact cleaner, then spray down with WD40 and put them in a zip lock bag for later use. I've never had a dimmer wear out, but have had plenty of problems with corrosion on the grounds causing dim lamps. On my rebuilds I put a direct to battery ground in the wire harness for all the body common ground points. Also 1 for the fuel tank, and 1 going to the group on the speedo. I started doing this on my VW's projects and the Opel is very similar in wiring and PITA body grounds.
 

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Opeler
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Dimmer Switch
I tried cleaning off the contacts on the instrument dimmer switch, but could not get it to dim very well. Also I plan on using LEDs. To make this work, I got a $2.50 PWM dimmer off Amazon which uses a potentiometer to dim. I got a slide pot with similar resistance, and replaced the wrapped wire with the internal strip from the slide pot. End result is a pretty smooth dimmer that works with both LEDs and incandescent bulbs! Just need to figure out where to mount the small PCB.
 

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On my rebuilds I put a direct to battery ground in the wire harness for all the body common ground points. Also 1 for the fuel tank, and 1 going to the group on the speedo. I started doing this on my VW's projects and the Opel is very similar in wiring and PITA body grounds.
To me the extra ground loop is not a good idea. Ground loops can cause current to flow in unexpected paths, which can cause odd problems, especially for electronics. Google ground loops. Apologies if I misunderstood what you are doing.
 

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Opeler
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To me the extra ground loop is not a good idea. Ground loops can cause current to flow in unexpected paths, which can cause odd problems, especially for electronics. Google ground loops. Apologies if I misunderstood what you are doing.
Agree:
It is usually better to attach chassis grounding mounting points rather than a direct to battery NEG ground for accessories. The argument I read against battery grounding is based on the battery being a "noisy" place to ground.
THE BEST PLACE TO GROUND ACCESSORIES
MARCH 2, 2019
Despite common belief, the battery ground is NOT the best place to ground accessories. The battery ground to body/engine/chassis is certainly required in order to complete the circuit which makes up the vehicles ground path. However, it is not the best place to install your ground wire for various accessories.
The issue here is related to ground path, and everything else in the vehicle that ultimately uses the same ground path, which all makes its way back to the (-) side of the battery.
For simple-circuit accessories (lights, relays, electric motors), battery ground is perfectly fine, though anything with a processor or that has high sensitivity to electrical noise or ground path should never use battery ground. The reason here is due to the battery being the return ground path for everything electrical in the vehicle. There can be much electrical noise, or simply high current flow at the battery, which can wreak havoc on your hi-tech or sensitive device/accessory.
The best place to ground electronics like these is actually to the engine itself. Due to the nature of the engine’s construction, there is going to be very little current flow through nearly any ground point you choose on the engine. We recommend the engine block, cylinder head, intake manifold (if aluminum or cast iron), or transmission to engine bolt. Any of these locations will provide you with a great grounding location for your accessories. You do, however, want to keep any related wiring as far away from the ignition system or plug wires as you reasonably can to avoid creating a new issue with ignition noise.
You can also consider this. The OEM manufacturers ground very little directly to the battery. They engineer and create ground locations throughout the vehicle to work for what the vehicle comes originally equipped with and they are not all in the same place. They are spread out. Not just for cost savings purposes, but also for the purpose of not having everything funneling into the same ground path at one point, which can cause problems.
For the purpose of installing gauges that use senders which monitor various engine vitals such as temperature, pressure, and EGT, another good reason for engine ground is to ensure that you have a completed, non-interrupted ground loop shared between the sender and the gauge for the most accurate readings.
 

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To me the extra ground loop is not a good idea. Ground loops can cause current to flow in unexpected paths, which can cause odd problems, especially for electronics. Google ground loops. Apologies if I misunderstood what you are doing.
The electronics of the 70's are not like that of today. By putting a ground directly on top of the wires that are grounded thru the body is just another direct path back to the battery ground. The car body is basically a giant wire going to the battery ground. But when the wires touching that body corrode you now have a natural resistor between the wire and the giant wire going back to the battery. Either way, these cars electronics unless changed, are extremely simple. Heck even the clock in a GT is a WIND UP clock running on springs and gears. The only electronics in the clock it is a solenoid that winds up the main spring once every 60-70 seconds.
 

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True about 70s cars being simpler but weird things, like a light glowing when it should be off, can still happen.

I’ve had an aversion to ground loops ever since a lithium battery from a distributed energy management system board blew up in my face. The cause was reverse current applied to the battery due to different ground potential between the first and second floor of a high rise.
 

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ouch that'll piss you off for a while
 
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