instrument cluster light switch & panel light rheostat

If you could get the right value rheostat, you could enjoy the benefit of the LED's lower current draw. The higher value resistance will allow the current to be controlled over a wider range. You could also add a resistor in series to limit the maximum value although if you are using LEDs with internal current limiting resistors and are designed to handle 13.8V it would not be necessary. I really haven't looked at these retrofit LEDs at all, nor the X1/9 schematic that shows a dimmer circuit so my input is based on the general case. However, in a previous life I used to design LEDs and LED drive circuits - my X was pretty new back then.

My car has a toggle switch with only two positions for the instrument lighting - high and low. The center console has a dimmer that controls the light source into the fiber bundle.
 
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I agree with those who discourage the use of resistors in this case.
I don't think it wise to use resistors as shunts on LEDs in order to simulate the load of an incandescent lamp. This defeats the point of going with LED's to begin with.

I could be wrong, but I believe the dimensions of the 2 rheostats shown above are of the same dimensions, but the electrical connections on the back are quite different.
The electronic version needs power & ground, and supplies an output of varying voltage into 1 wire, which relies upon ground to complete the circuit for the lamps.
The resistive version is nothing more than a relatively high current rheostat.
You should be able to use the "+12 in" wire that feeds into the resistive unit to power the electronic version, and the "lamp out" wire that feeds from the resistive
unit to connect to the Vari-voltage out of the electronic version (2 wires total) and simply ground the Gnd terminal of the electronic version to complete the replacement.
There is no magic to these.
 
Bob, I like what you are saying.
Sounds like I should be able to convert to the 'electronic' dimmer and fit all LED's throughout the dash/instruments. I actually find LED's to be too bright, so a dimmer would be required for my preference.

And agree with you Denny, better to eliminate the "switch" and pass all of these lights through the dimmer only.
 
Not sure what I was thinking......putting another resistor in parallel with the rheostat just increases the current draw on the system and reduces the output variability of the rheostat anyway.
I agree with Bob that increasing the current draw nullifies one of the benefits of the LEDs in the first place and not a logical step.
The LEDs that we are using are designed to handle the 12V, and maybe higher. They are T10 & T5 configuration.
Interestingly, I hooked up both resistive units this morning and neither one seems to give an output on the wiper arm varying from 12V down to 0V. I may be misunderstanding how they work. Since we are dealing with very low currents here, might a potentiometer work in this situation?

Does anyone have the pinout of the electronic dimmer? All of the Fiat wiring diagrams (79 through 82) we have show it as a rheostat, not an electronic device. Our electronic dimmer has the same 3 pins, in the same configuration as the rheostat. It will plug into the same harness directly. One might think that it is a direct replacement, but I am not anxious to blow it up.

The mechanical dimmer is interesting......maybe crude, but it probably works. It's kind of cool to see how some things were done before technology changed it.

Don, I think that designing an electronic circuit that will use the Fiat control knob to do this job is a bit out of my capabilities and have too many other irons in the fire.
 
OK, some progress......after resoldering the ground pin on our electronic dimmer (fractured solder joint), I bench tested the setup, hooking up the converted instrument cluster & the electronic dimmer to a 12V source, following the dimmer connector pinouts in the 79. nothing. I switched the 12V supply wire & the signal wire going to the instrument cluster (at the dimmer connections) and IT WORKS! So, we just need to switch the 2 pins in the harness connector for the dimmer and we should be in business. Because of the physical difference (on the back side) in the 2 different types of dimmers, you also have to turn the harness connector upside down. (In the "installed" position, the ground pin is at the top of the dimmer (rheostat type) and at the bottom of the dimmer (electronic type). Hope this isn't too confusing.
Now we need to go back & look at the dimmer in the 81......thinking that all the dimmers were interchangeable at the time, I need to see which type it really is.
 
7982X said:
I switched the 12V supply wire & the signal wire going to the instrument cluster (at the dimmer connections) and IT WORKS! So, we just need to switch the 2 pins in the harness connector for the dimmer and we should be in business.
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I'm not completely certain if this applies, but LED's are polar. They only function with the current traveling in one direction (+ and - leads), but not the other. So depending on which way you connected the LED's (socket's) wires, the original wiring configuration might be good?

And excellent work!
 
Clarification....when plugged in directly to the connector as wired I said "nothing". What I meant was no adjustment in brightness, just full brightness. I also neglected to say that when the cluster was converted to LEDs, we insured that all LEDs were installed so that with +12V applied to the red connector that Bob referred to, all bulbs would light, therefore polarity correct. Not having a schematic of the electronic dimmer, I am just guessing that it has to do with what pin needs to be +12V for the circuit inside of it to work properly. (Sorry, I didn't give all of the details last post)
 
Got it. Interesting about the electronic dimmer's configuration. This is way above my knowledge level.
 
Can't tell you how many times I find out how much I don't know. Working on this stuff is humbling. I like to think that my knowledge level ratchets up a click with everything I get into. Sometimes it doesn't seem that way. I figure that the older we get, working through these challenges should help keep the mind in better shape......sounds good, doesn't it?
 
7982X said:
Not sure what I was thinking......putting another resistor in parallel with the rheostat just increases the current draw on the system and reduces the output variability of the rheostat anyway.
I agree with Bob that increasing the current draw nullifies one of the benefits of the LEDs in the first place and not a logical step.
The LEDs that we are using are designed to handle the 12V, and maybe higher. They are T10 & T5 configuration.
Interestingly, I hooked up both resistive units this morning and neither one seems to give an output on the wiper arm varying from 12V down to 0V. I may be misunderstanding how they work. Since we are dealing with very low currents here, might a potentiometer work in this situation?

Does anyone have the pinout of the electronic dimmer? All of the Fiat wiring diagrams (79 through 82) we have show it as a rheostat, not an electronic device. Our electronic dimmer has the same 3 pins, in the same configuration as the rheostat. It will plug into the same harness directly. One might think that it is a direct replacement, but I am not anxious to blow it up.

The mechanical dimmer is interesting......maybe crude, but it probably works. It's kind of cool to see how some things were done before technology changed it.

Don, I think that designing an electronic circuit that will use the Fiat control knob to do this job is a bit out of my capabilities and have too many other irons in the fire.
Click to expand...
Denny:

I just figured it would be pretty trivial compared to your remote controlled engine cover project!

Don
 
Dr.Jeff said:
Don't stop there; front truck, rear trunk, doors, targa top.
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The above discussion is why I LOVE this Forum. All the X nuts chime in with their ideas on how to make these little buggers even more fun. Will be putting my X up for sale in a couple months, as I have moved to a newer, old toy, but will probably check the Xweb from time to time, to see what’s the hot topic.

Regards,
Gary McCormick
‘79X and ‘98 Z3 2.8L
Carlisle, PA
 
Gary McCormick said:
The above discussion is why I LOVE this Forum. All the X nuts chime in with their ideas on how to make these little buggers even more fun. Will be putting my X up for sale in a couple months, as I have moved to a newer, old toy, but will probably check the Xweb from time to time, to see what’s the hot topic.

Regards,
Gary McCormick
‘79X and ‘98 Z3 2.8L
Carlisle, PA
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I just checked my ‘79 and it has the RESISTIVE type dimmer as shown in pix earlier in discussion. At this point in my pre-sale prep, I guess I’ll leave the upgrade Instrument Panel Lighting project for the next owner.
 
Dr.Jeff said:
Got it. Interesting about the electronic dimmer's configuration. This is way above my knowledge level.
Click to expand...
Me again, reviving old threads. So that rheostat thumb wheel at the base of the control panel - in a 1974 - only is supposed to control lighting that MAY exist to illuminate the words on the switches? Mine isn’t working, likely because the light source is fried. Which, by the way...how does one find that light source: does the entire control panel have to be removed?
Before I tear everything apart, I just want to make sure.
I’m looking through the service manual to learn HOW to remove the control panel.

oh, what the manual refers to as the ‘Idiogram’ is this photo, yes?

34F16B7B-C539-4525-A70B-F777072E78BF.jpeg
 
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tvmaster said:
Me again, reviving old threads. So that rheostat thumb wheel at the base of the control panel - in a 1974 - only is supposed to control lighting that MAY exist to illuminate the words on the switches? Mine isn’t working, likely because the light source is fried. Which, by the way...how does one find that light source: does the entire control panel have to be removed?
Before I tear everything apart, I just want to make sure.
I’m looking through the service manual to learn HOW to remove the control panel.

oh, what the manual refers to as the ‘Idiogram’ is this photo, yes?

View attachment 38583
Click to expand...
The bulb is mounted on the tunnel behind about where the lighter is. It has a metal cover over the bulb with a fiber going to each switch. Check to be sure the socket has a good ground as well as power.
 
Ideograms are the symbols applied to switches and illuminated in the instrument panel. They are intended to be ‘universal’ like road signs.

A good question on whether the switch will actually adjust the brightness of the LEDs, likely not given it was developed for an incandescent bulb and its characteristics. The rheostat at the bottom definitely won’t adjust the single LED and the newer one won’t fit in an early car. Ugh. I need to pull my cluster to replace the bulbs on my daily X as its instruments are barely visible at night.

The lower console is held in by screws that are quite visible behind the shifter and along the sides near the ‘front’ by the gas pedal and up near the instrument panel. The fasteners are mirrored on the other side. This car is from before all the fasteners were hidden.

The question I don’t have an answer for is the three slider controls that stick out of the panel, whether they and the cable operating quadrants behind them move with the console or not. Don @dllubin or someone with an early car should be able to answer.

I had a 1975 as my first X but it wasn’t mine for long...
 
tvmaster said:
Mine isn’t working, likely because the light source is fried.
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I'm not familiar with the '74 and all of its uniqueness, so I cannot say what's there or how it should work. However I will pass along some general info that might help.

Apparently there were at least two types of rheostats used on the X; one will dim LED's but the other will not. There are some threads that discuss this in detail. However I don't know if the one in a '74 is like either of these (it looks smaller to me?).

I'd suggest removing the center console (control panel) and inspecting the dash illumination system that Don described:
dllubin said:
The bulb is mounted on the tunnel behind about where the lighter is. It has a metal cover over the bulb with a fiber going to each switch. Check to be sure the socket has a good ground as well as power.
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I wasn't sure that the '74 used fiber optics, but now that he has verified that...
The way the fiber optic system works is there is one light source (a single bulb) in the holder behind the console (as Don described). From there the "light waves" are carried by fiber optic lines (looks like fishing lines) to each of the destinations. I don't know exactly where on a '74 those destinations are; just the center controls or also all of the gauges, etc? @dllubin can tell us that. But if that single light bulb is burned out, or the fiber optic assembly (bundle of fishing wires) has come loose from it, then no light can get transmitted to all of the sources. So all of your problems may be a single simple fix. But that bulb is where a LED can be swapped in to further improve the system (it will provide much more light waves into all of the fiber optic lines).

While in there you can also clean up all of the electrical connections. And you can find out if your particular rheostat will control a LED bulb. If not then maybe revert back to a new "standard type" bulb, or bypass the rheostat (but that may be TOO bright). Because as @kmead said it doesn't look like you can use one of the newer style rheostats on your '74.

Once you have sorted out the fiber optic system, you may find only one or two places that should illuminate but still don't. That is likely because the other end of the fiber optic has come loose from its holder. They have a little receptacle that holds the 'fishing line' up to the back of what should light up. And those can come out of place, not allowing the light to pass.
 
I would be surprised if the two position instrument cluster light switch has enough series resistance in the low position to significantly dim LEDs. However, the rheostat is wired as a voltage divider, not a series resistor, and that could dim LEDs. That's why I would replace the light bulb/fiber thing with individual LEDs in the console switches and the heater ideogram if switching the instrument panel to LEDs. Then you could use the dimmer for everything.
 
dllubin said:
I would be surprised if the two position instrument cluster light switch has enough series resistance in the low position to significantly dim LEDs. However, the rheostat is wired as a voltage divider, not a series resistor, and that could dim LEDs. That's why I would replace the light bulb/fiber thing with individual LEDs in the console switches and the heater ideogram if switching the instrument panel to LEDs. Then you could use the dimmer for everything.
Click to expand...
Good point. I wasn't even thinking about the 'two position' selector switch, only the rheostat dimmer. And not knowing exactly where the fiber optics reach on a '74 I didn't know which items get "dimmed" and which don't. This whole system on the X's is very bizarre to me. I'd much prefer a more simple and common arrangement with a light bulb at each point and a single dimmer that controls all of them...along the lines of what you described. At that point the 'selector switch' could be eliminated completely.
 
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