Electrical: Radio display not dimming, buttons not illuminating when headlights on

[Wild Turkey]

Summary: Radio doesn't dim despite 12 V on the red/black wire going into it. (A3)

THANKS in advance for any insights on this in my 2001 NSX-T. The only thread I could find on my search was @I_M_Legend thread somewhat similar issue which started out similarly, or perhaps the opposite?

Help Needed - Electrical Issue

My stock radio unit has been acting up lately. It thinks I've turned on the lights when I haven't. I'll be driving along and then notice that the display has dimmed and the buttons illuminated as if I turned on my headlights / switched on parking lights. The display on the climate control is...

www.nsxprime.com

and also:

Car thinks lights are on, but they are not. Loose switch?

My climate control and radio backlight will dim (as if the lights are turned on) randomly and then come back on bright as usual. Sometimes when I turn the car off I'll get the "lights are on" alarm even if the lights are switched off. Any idea what faulty switch or relay may be causing this?

www.nsxprime.com

Workup:

1. (Probably unrelated) Months ago, I changed the brake/tail light bulbs for red LEDs, and the T10 running bulbs in the rear lights as well. I grounded the bulb-out indicator wire in C563 to stop the bulb-out indicator on the dash. This worked fine for months.
2. (Possibly related to this? But I can't see how looking at the wiring diagram.) A few weeks ago, I disconnected the tail light circuit at f5 in C563 and powered the Right tail lights with ignition-on power from C535. I did the same with f5 in C557 for the Left tail lights. According to the wiring diagram as I read it, there was nothing else connected to f5 past that connector, and I wouldn't think disconnecting the previous supply to the running lights would have any affect. Perfect, the tail lights came on with the ignition, as desired, but...
3. Problems - 2:
   1. The brake light-out indicator started coming on again. When I investigated, I discovered that the previously installed ground from the bulb-out bypass at C535 was loose, perhaps bumped when I was repinning C563. I replaced the shunt from the bulb-out bypass, which solved the bulb-out indicator problem.
   2. The radio would dim, sometimes flickering back and forth a couple of times on starting and then staying dim, even if the lights weren't on. Voltage was leaking into that red/black for the radio but nothing else? (The climate control dimming still worked.) If I turned my running lights on, the radio would dim as expected, but would not "un-dim" when I turned the lights off, like there was stray voltage keeping the "lights on" wire hot, or hot enough to trigger dimming.
   3. (I drove like this for a few hours until I could get to trying to fix it.)
   4. The radio has now stopped its always-dimming problem, but now will not dim at all when I turn the lights on.
   5. The climate control unit and clock illuminate fine and dim properly when I turn on the running lights.
4. As a test, I undid the repinning of the tail lights at C563 and C557 back to the OEM wires. The tails now come on normally with the lights switch, but the radio still does not dim. The radio works fine and so do the running lights, just no radio display dimming. There must be voltage on the red/black since it makes the running lights come on, but the radio doesn't seem to get the message.
5. As the last test, I checked for voltage on the red/black that feeds into the radio. Both with the radio connected, and with it unplugged, I measure 12.7 V at the red/black feeding into the radio with the lights on, about 0.5 V with lights off, so it should be getting the signal to dim and illuminate things. So my assumption is that the radio is the problem.

My differential diagnosis includes:

1. Blown fuse? (Can't figure out which one.) Hoping @Old Guy or @drew have some insight. Fuses in the Radio? For the dimming?
2. Crossed a wire and shorted something out in the radio somehow while swapping the wiring at the tail light and the dimming circuit is permanently damaged. This makes no sense as 12 V to this wire should not cause a problem, just put on the illumination. (I can't figure out what accidentally grounding the red/black with the running lights off would do as it's already pretty low resistance to ground through all those running lights. It would blow the fuses 38 & 39 if it was grounded with the running lights on.)
3. Capacitor problem in the radio suddenly presenting in this way, coincidentally at the same time?? @Heineken ?

One more (probably unrelated) fly in the ointment to potentially confuse things: I've read that some folks had problems with this related to the ashtray/lighter lights, especially if they were replaced by LEDs. Neither worked when I got my car, along with the cigarette lighter, so I suspected the connectors might not have been reconnected when the Aux-in was added 2 owners ago. As part of today's investigation, I pulled the console trim. As suspected, neither of the plugs were connected, explaining the no lighter power, no ashtray lights, and no aspirator fan (and presumably no interior temp feedback to the CC unit?) After reconnection, lighter works, it & the ashtray are illuminated, and the aspirator fan spins (silently!) But the radio still won't dim. (I also found what looked like another FM antenna Y'd in and stored in the console??? Also a USA-Spec Ipod adapter that was working OK.)

Does anyone have enough of an understanding of the factory radio to suggest things to check and repair?

 

[Old Guy]

The only observation that I can make is that the dimmer circuit in the head unit seems to have a very low turn on threshold. It seems that it does not need 12 V to initiate dimming. If you look at the wiring diagrams, you will see that fuse #38 (which provides the dimming signal) and #39 are common off of the taillight relay. The taillights share a common ground with the brake lights and if the ground is compromised (high resistance) application of the brake lights can raise the voltage at the ground point which then raises the voltage on the head unit A3 lights on sensor wire triggering dimming. That is my hypothesis for the brake related dimming action that others have mentioned. The low turn on threshold also makes the circuit susceptible to dimming from sneaks provided by other sources such as the CCU.

However

After I replaced the bulb-out bypass, the radio stopped its always-dimming problem, but now will not dim at all when I turn the lights on.

The change from always dim to never dim is a different problem. The first thing I would do is check to confirm that A3 (dimming signal) on the 16 pin head unit connector is getting 12 V when it is plugged in to the head unit and the lights are switched on. Do this by back probing the plug. Some connection on that circuit may have been disturbed / broken. If you are getting 12 V on the back probe test, carefully check the plug and the A3 male pin on the head unit side for damage. If that shows nothing and you are fairly confident that the 12 V lights on signal is successfully making its way into the head unit I think that points a finger at an internal head unit problem. I have never had to repair anything on the inside of the head unit so I can't advise on the internal operation of the dimming circuit. @Heineken may be able to advise.

 

[Wild Turkey]

Yes, the strange change from always dimming to never dim is what perplexed me as well, trying to figure what could have changed, while I was working on the wiring at the rear lights, really only the red/black wire and the shunt grounding the bulb-out sensor. 12 V should not be a problem since that's what the light circuit supplies when the running lights are on. Could accidentally grounding that wire with the lights off somehow actually result in backwards current through the radio from the constant power wire? Could that backwards current damage a transistor or something?
The check for voltage at A3 (red/black) with the running lights on was what I was trying to describe in step 5. It seems fine. There is 12.7 V when the running lights are on but the unit does not illuminate/dim, which is why I was also suspecting the head unit.

 

[Old Guy]

Grounding the fuse #38 or #39 circuits when the taillight relay is off should not damage the dim sensing circuit on the head unit. If you are confident that the A3 terminal is good and the 12 V signal is getting into the head unit (no plug problems) then I think your problem is inside the head unit.

Unless somebody confirms that this condition is a symptom of capacitor leakage damaging the circuit board traces, I might be inclined to leave it alone. The whole dim versus bright display is a not particularly valuable feature as far as I am concerned. To get into the head unit you need to remove the face plate and display unit which is attached by a ribbon cable. The ribbon cable has provision for detachment; but, if you are not careful you can bugger it up.

 

[warrenw]

Your head unit is dying.
That was one of my first symptoms. Next was the display not working, then nothing.

 

[Old Guy]

If @warrenw is correct then you probably do want to extract the head unit to examine it. If the damage is related to electrolyte leakage from capacitors early intervention to minimize additional damage is the best path. Whether you want to attempt a repair or send it to @Heineken who is the only one that I am aware of that will attempt to repair the head units is your decision.

 

[Wild Turkey]

I'll open it up and have a look - little to lose I guess. Ironically, I spoke to @Heineken last summer about having my radio done prophylactically, but shipping it to him in Germany would require me to pay import taxes to send it to him and export taxes when he sends it back to me $$$$ so I don't think that's going anywhere. I should talk to @Big McLargeHuge - I think he might have replaced some caps in his radio when he was doing the caps on his amps.

 

[The King]

Doesn't Brian K do repairs on this side of the pond?

 

[Old Guy]

Doesn't Brian K do repairs on this side of the pond?

My recollection is that Brian K does not repair the head unit, just the amplifiers. Heineken and I think Kaz were the only ones posting about head unit repairs. I know that @drew was fiddling around with the head unit; but, I don't know whether he was offering up repairs. At one time I recall that there was a commercial business that claimed to do head unit repairs in the US; but, I don't think that option still exists.

Ship to New Zealand, England or Germany. Those seem to be the options.

 

[Wild Turkey]

You're right, @Briank doesn't do radios any more; I didn't know kaz did. My other radio had the caps replaced by "Factory Car Stereo Repair Inc" in Longwood Florida in 2007. They're still in business, repairing radios, but they don't do NSX radios anymore so NZ and Europe seem to be the only options, with VERY expensive shipping / taxes to get something done.

I applied 12V directly to the A3 pin in case the connector was the problem. Still no illumination. I opened it up looking for burnt/leaking caps or anything else but I can't see any issues. I removed the cassette player but did not remove bottom circuit boards as I didn't want to undo the solder on the lower attachment screws. I did notice that one of the ribbon cables to the front display wasn't plugged in all the way, but fixing that didn't help. Helpful pictures:

Radio Head Unit - Capacitor Replacement - Blogs - NSXCB Forums

Radio Head Unit - Capacitor Replacement Heineken's Garage

www.nsxcb.co.uk

I think it must be some transistor that controls both functions since both are missing - the display doesn't dim, and the buttons & knobs don't illuminate. I'm at the end of my ability to repair it and open to any insight.