Topic: Dimmed cluster lights, won't go very bright, need help to troubleshoot

[gtcs1]

Hi, on my newly acquired 73 vert, the dash lights when put at max setting can go only at about 20% (for cluster section) of full brightness and the ctr section ( radio and temp control) goes a wi bit better to 33% .

Any advice on what could be wrong there, like rotary  switch, circuit, etc. Not very nice/safe for night driving...

Thanks

[c9zx]

Corrosion on bulb/sockets/pcb /switch. They are dimmer than I think they should be even when they are working as designed.Chuck

[gtcs1]

Would that affect all of the bulbs, this is what puzzle me. If it was one or two zones, i could understand.

 Specially the car is in perfect condition with only 27000 miles.

Thanks

[67gtasanjose]

The basic description of the instrument/dash light wiring (for MOST vintage 60's-70's American built vehicles):

12v power is switched on using the taillight circuit of the headlamp switch and then goes through the underdash harness to the fuse panel into the instrument light fuse (approx 3amp) back to the headlight switch (again in the main underdash harness), then through the rotary resistor (back into the underdash harness) and on to the cluster and other instrument lamp items (eg, radio, heater, etc.)

Ground path varies, often the cluster will use the ground path all the gauges use,  usually from the main underdash harness, the radio light uses grounding path the radio uses and heater panel (if equipped) more often is grounded with the dash harness but can be grounded with a grounding bulb socket in the control panel.

It is a really simple circuit. This outline should guide you to discover the problem.

My guess? The rotary resistor built into the headlight switch. An unlikely 2nd guess would be a loose or bad ground connection.


Richard

[c9zx]

The switch will but, each bulb and socket is independent. I owned a 71 in 76 and I complanied about the brightness level then. I suppose, with experimentation, a resistor value could be determined to place in parallel with the rheostat in the switch to increase the current to the lights. Just a thought. You could use LEDs but, color choice could be an issue.

[67gtasanjose]

...I suppose, with experimentation, a resistor value could be determined to place in parallel with the rheostat in the switch to increase the current to the lights.

If you determine which wire is returning from the (let's call it a 3amp fuse for reference) instrument fuse, and which wire from the headlight switch feeds the instrument lamps, a simple jumper wire would do this task. No resistor (a jumper wire provides No Resistance) equals instrument lamps in the brightest setting. A wiring diagram that is EXACT for your application would help identify the correct wires. Randy @ Midlife Harness Restoration might be able to chime in on the correct pins in the headlight switch connector. It is fairly uniform for most Fords of the era, but the wiring color codes differ between years and models.

Richard

[midlife]

If you determine which wire is returning from the (let's call it a 3amp fuse for reference) instrument fuse, and which wire from the headlight switch feeds the instrument lamps, a simple jumper wire would do this task. No resistor (a jumper wire provides No Resistance) equals instrument lamps in the brightest setting. A wiring diagram that is EXACT for your application would help identify the correct wires. Randy @ Midlife Harness Restoration might be able to chime in on the correct pins in the headlight switch connector. It is fairly uniform for most Fords of the era, but the wiring color codes differ between years and models.

Richard

12v power is switched on using the taillight circuit of the headlamp switch and then goes through the underdash harness to the fuse panel into the instrument light fuse (approx 3amp) back to the headlight switch (again in the main underdash harness), then through the rotary resistor (back into the underdash harness) and on to the cluster and other instrument lamp items (eg, radio, heater, etc.)

Richard: you almost got the circuitry correct.  The headlight switch takes battery power and routes it through the rotary resistor on the headlight switch and then routes it to the fuse box.  The other side of the fuse box directly connects to all dash lamp circuits. 

Two primary sources of voltage loss: (1) at the headlight switch, particularly the rotary switch.  It's a terrible design and at its best, probably loses about 2 volts due to contact losses.  (2) corrosion at the fuse box itself can add 1 or 2 ohms of resistance.  Each bulb runs about 15 ohms when lit, so every additional place of resistance robs voltage for the bulb by putting resistance (and heat) elsewhere.  The third place where you can get bulb dimness is having a bad ground: always sandpaper your ring connectors and make sure you have good metal where you connect it to the chassis.  A fourth (did I say there were only two primary sources of voltage loss?  my bad...) area is corrosion or tarnish on the bulb socket contacts.  If at all possible, get some 300 grit scuff pads and have at the contacts. 

From 64.5 through 73 (and probably longer) Ford did not change the basic design or wiring colors.  I've seen some early 60's Fords with the same wiring scheme, but am less familiar with them. 

[gtcs1]

Hi,

So I can check for these grounds or corrosion at the fuse box.  For the rotary switch, is it possible to clean it, or is it all internal sealed, and the only way to fix that is to use a new switch?

Thanks

[midlife]

Hi,

So I can check for these grounds or corrosion at the fuse box.  For the rotary switch, is it possible to clean it, or is it all internal sealed, and the only way to fix that is to use a new switch?

Thanks

It's on the exterior of the switch, so simply remove the switch itself and clean it as best as you can.  If you have a volt-ohm meter, you can watch the resistance of the rotary switch change as you rotate the knob.  Maximum voltage is when the resistance is lowest: best when less than 1.0 ohms.

[67gtasanjose]

Thanks Randy, I hadn't actually looked at the circuits for old Fords in particular, so as I mentioned, it was a typical 60's-70's design though. I figured you would know more of the exact particulars so I am happy you noticed this thread. The particulars are different maker by maker, but all of them seem to use the same basic principals. As in any wiring problem, the best first search would be a correct wiring diagram for exactly what you are working on.

As far as the resistor in the headlight switch, it is the visable coil on a ceramic ring that rotates when you reinsert the shaft going to the knob. I assume you know about the shaft release button, but if not, it will be on the side of the headlight switch opposite where the wiring plugs in. Pull the headlight switch all the way out, depress that sping button on that side of the headlight switch and lightly pully the shaft the rest of the way out. Now the switch will come all the way off the car.
 
If I remember corretly, the dimmer rheostat can be serviced a little to clean and tighten it's contacts. You would need to remove the headlight switch first. I will not say how to service it without first looking at my own (it's a 67, primarily the same thing) and see how best to service it. I've done it before to correct a lamp flicker while turning the dimmer, but it has been many years ago.

Richard

[dave6768]

Your eyes are getting old.  One night I had my reading glasses with me...all of a sudden I could clearly see everything in the dash.  In the end I ended up taking out all the blue shields.  I don't drive much at night, so no big deal.  No judge has ever taken off any points.