Dash lights reostat - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
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post #1 of 7 (permalink) Old 08-11-2019, 01:36 AM Thread Starter
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Dash lights reostat

The dash lights on my 1975 280Z only get half bright. As I increase the brightness (turning the knob) the dash lights go out about half way to maximum brightness. I suspect the reostat is dirty. If I am correct I'd like to attempt to clean it. Has anyone else had this problem and had success cleaning the reostat and how did you do it? Thank you

75' 280Z under restoration
Formerly my Dad's car aquired new
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post #2 of 7 (permalink) Old 08-11-2019, 12:06 PM
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You are assuming the dash lights were bright to begin with. They probably weren't.

What is the voltage at a dash light socket?


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post #3 of 7 (permalink) Old 08-11-2019, 01:01 PM
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Unless it's really crusty, the best way to clean the rheostat windings is just "use". Turning the knob fully both directions several times will usually knock most of the oxidation off.

You can try an electrical contact cleaner spray on the windings.

If the lights smoothly transition from dim to bright (no outages or "hiccups" along the way) - the problem is probably not the rheostat. The age of the wiring and connectors (oxidation = resistance) will play a role in the dimming of gauge lighting.

Many people will install higher wattage bulbs. I don't. Most of my gauges have been disassembled at one point or another and I find that the paint they sporadically sprayed inside the cans is anything but white after all those years.

I pull the guts, respray a nice flat white on the inside of the gauge and even with the stock 3.4w bulbs my gauge lighting is nice and bright. (Well, as bright as they were intended to be back in the '70's... I'm old, I grew up on green gauge light, very little "light pollution" in an analog world.)
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1974 "early" 260Z - L28ET - MS3X full sequential fuel and spark, COP=Ford 4.6 coilpacks w/ Nissan Ignitor, diyautotune chopper wheel - FMIC - T04E .63 A/R - 400cc Injectors (too big!), 14point7 Spartan2 Wideband - C/R 5-speed - Coilovers - Big Brake Kit up front, Porterfield friction all around. Mesa, AZ
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post #4 of 7 (permalink) Old 08-11-2019, 01:32 PM Thread Starter
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Originally Posted by cgsheen View Post
Unless it's really crusty, the best way to clean the rheostat windings is just "use". Turning the knob fully both directions several times will usually knock most of the oxidation off.

You can try an electrical contact cleaner spray on the windings.

If the lights smoothly transition from dim to bright (no outages or "hiccups" along the way) - the problem is probably not the rheostat. The age of the wiring and connectors (oxidation = resistance) will play a role in the dimming of gauge lighting.

Many people will install higher wattage bulbs. I don't. Most of my gauges have been disassembled at one point or another and I find that the paint they sporadically sprayed inside the cans is anything but white after all those years.

I pull the guts, respray a nice flat white on the inside of the gauge and even with the stock 3.4w bulbs my gauge lighting is nice and bright. (Well, as bright as they were intended to be back in the '70's... I'm old, I grew up on green gauge light, very little "light pollution" in an analog world.)
Thanks. I guess I'll just keep using the dimmer and see if it improves. The car hasn't had much use for 20 years. I can feel roughness (hiccups in brightness) as I wind the knob up. About 1/2 way towards fully bright it all goes off. I can see the instruments but have a feeling I am not getting all the brightness that I could have. BTW great photo. What do you have in place of the clock?

75' 280Z under restoration
Formerly my Dad's car aquired new
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post #5 of 7 (permalink) Old 08-11-2019, 05:50 PM
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A boost gauge.

Sounds like you need to actually clean or replace your rheostat.

1974 "early" 260Z - L28ET - MS3X full sequential fuel and spark, COP=Ford 4.6 coilpacks w/ Nissan Ignitor, diyautotune chopper wheel - FMIC - T04E .63 A/R - 400cc Injectors (too big!), 14point7 Spartan2 Wideband - C/R 5-speed - Coilovers - Big Brake Kit up front, Porterfield friction all around. Mesa, AZ
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post #6 of 7 (permalink) Old 08-12-2019, 03:19 AM
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Don't know what the back of the rheostat looks like, but: If it's an old style "can" with ear tabs folded over, you can pull the tabs up gently but use a vise grip or pliers to hold the can at the base of the tab where it bends.
This will prevent the bend from bulging out at the OD of the can, which can cause too much stress at the bend area when you try to fold it back in when you're done. It will also make the tab easier to fold back in with a truer fit than if the tab-bend has been distorted.

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post #7 of 7 (permalink) Old Today, 04:44 PM
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As far as a cleaner to use I recommend Deoxit D5 made by Caig Industries. I do a lot of vintage audio gear restoration and use it frequently to clean and lubricate scratchy volume controls. Give it a good spray or two and then rotate the control back and forth fully several times. If it has a small opening or seam you might even be able to spray it in and clean it without disassembly.
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