The clock runs on +5 V and +12 V (+14.4 V maximum). There is no +28 V supply in the system. LEDs don't have a voltage rating -- they're current-driven devices. The normal operating current of most LEDs is 20-25 mA. Depending on the LED chemistry and its color, the forward drop of the semiconductor junction is on the order of 1.2 V to 2.0 V. Choose a series current limiting resistor that limits the current to no more than 25 mA. For low-voltage operation, e.g. 5 V supply, one should take the forward drop of the LED junction into account when calculating the resistance value. For operation from the system battery voltage, i.e., 12-15 volts, one can assume the LED forward drop is zero and choose the resistor value on that basis. Don't forget to calculate the power rating of the resistor as well; 1/4 watt will work in most cases for supply voltage under 15 V.

One can get LED replacement lamps that have a built-in current limiting resistor or regulator IC. Such LED lamps do have a voltage rating. The ones with an integral regulator typically have a very wide operating voltage range, perhaps 3 V to 18 V, but dimming control is next-to-nonexistent.

The bulb in the Honda Accord clock module is fed through two silicon diodes, one of which is connected to the +12 V ignition rail and the other to the instrument light rail for low intensity dimmable operation at night. The bulb, therefore, needs at least a 12 V rating, or 13-14 V rating for improved service life. (I measured 13.1 volts across the bulb with the engine running and the headlights off. With headlights on, the dimmer control varies the bulb voltage over the range of 3.8 to 11.1 volts.) Since the bulb is always on when the ignition is on, it will have a shortened life, so you might consider buying several spares when you visit the dealer. I just paid US$3.57 for one in California, including 9.25% sales tax.

The bayonet bulb receptacle in the clock module is designed for an incandescent bulb and is not polarized. If you should put a polarized LED replacement in there without making sure it can operate with either supply polarity, the LED is likely to get destroyed. LEDs will typically survive reverse polarity on a 5 volt supply, but not 12-15 volts, as the junction breakdown voltage is around 6 to 7 volts, depending on the specs.

The LCD display module is what's called an "edge-lit display" in avionics circles, meaning it has a slab of acrylic plastic under it and one or more small bulbs poke into the middle of the plastic through holes or cutouts to flood the layer with light. Incandescent bulbs are ideal due to their omindirectional radiation characteristics. If you want to use an LED, you'll need to choose carefully or customize, as most LED packages are designed to focus the light output into a narrow beam, which is lousy for illuminating edge-lit displays.

Due to the output characteristics of LEDs, they can't be dimmed very well on dimmer circuits designed for incandescent bulbs. If long lamp life is more important to you than smooth dimming, then an LED is a suitable choice as a replacement.

 

The bulb has a plastic "bayonet" base, similar to the way tail light and turn signal bulbs are held in their sockets. Use a flat blade screwdriver to turn it slightly to the left until it stops and it should drop out of the circuit board. The tip of a pocket knife can be used to gently pry the edge of the plastic base if the bulb doesn't want to drop out on its own.

The stock bulb in the clock should have a black or gray plastic base and a blue plastic silicone skin on it to give a bluish backlight to the clock. The hazard warning switch, mounted next to the clock in the same bezel, has a green base and no filter skin. The blue skin on the clock bulb may be partly responsible for its short service life, as well as a 12-volt rating, where it should be rated at 13 volts to 14 volts, the actual bus voltage when the car's alternator is operating normally. If you're in possession of the original bulb, the silicone skin may be (gently!) peeled off and transfered to the replacement bulb.

To see how to get the clock/hazard switch bezel out of the dashboard, see the AUTOTOYS.COM site at http://www.autotoys.com/New_Tech/DIY...ord/accord.htm. The page deals with radio installation and removal, but the photos also show the clock module removal. The bent tip end of a machinist's scribe is useful for getting the bezel started, or if you have a mechanic's pick set for removing o-ring seals, such as those sold by Snap-On Tools, it will work too. The fit of the bezel is so snug in the panel that it may be difficult to get anything thicker than a thin knife blade under its edge to gain a purchase on it. It is held in place with two b a r b e d plastic tabs at the top and two spring metal clips on the bottom. (Sorry about spacing the word starting with "b" in previous sentence; stupid filter in this forum keeps mangling what I'm trying to type, replacing it with asterisks. Do you hear that, Mr. Moderator?) Pry at the bottom of the bezel, not at the top, as the metal clips yield easily, while the plastic tabs at the top may simply break off! Get one side started, then the other. Once the gap has widened enough to get a small screwdriver in, use a flat blade screwdriver to coax the assembly out, or grasp it firmly on opposite sides with two fingers and pull it out of the panel. The clock module and the hazard warning switch are connected to the wiring harness with typical automotive plugs with retainer tabs that must be depressed to remove them. However, it should be possible to replace the clock bulb without unplugging the assembly.

 

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Phil1812 wrote on 8/11/10
snip..snip
The bulb in the Honda Accord clock module is fed through two silicon diodes, one of which is connected to the +12 V ignition rail and the other to the instrument light rail for low intensity dimmable operation at night. The bulb, therefore, needs at least a 12 V rating, or 13-14 V rating for improved service life. (I measured 13.1 volts across the bulb with the engine running and the headlights off. With headlights on, the dimmer control varies the bulb voltage over the range of 3.8 to 11.1 volts.) Since the bulb is always on when the ignition is on, it will have a shortened life, so you might consider buying several spares when you visit the dealer. I just paid US$3.57 for one in California, including 9.25% sales tax
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Good morning everyone. I'd like to continue this discussion, if that's OK. I bought my 17 year old a 99 Accord LX. I noticed the clock backlight wasn't working, so I replaced the bulb. Got that done with no problem. However, it's not acting quite right. When you turn the ignition on, the clock backlight is on and everything is hunky dory. However, when you turn the headlights on, the clock backlight goes out. So, it's like the bulb is burned out again. But, obviously, it isn't. Turn the headlights off, and the backlight turns back on.

Phil1812, after reading what you wrote about the two different sources for the backlight voltage, I want to make sure I understand you. Are there 2 sources for the backlight voltage (ie - 2 input wires on the clock connector)? Or, does the dimmer module take over for the fixed 12V when the ignition switch is turned on (ie - 1 input wire). The dimmer control works fine on everything else, so it's not that circuit. I'm wondering if there is a problem with the harness that brings the dimmer control voltage over to the clock (I REALLY hope that's not the case)? Or, is it possible that the clock module is bad? Does anyone have any kind of diagram showing the pinouts on the clock module connector so I know where to probe to look for fixed/varying voltage, or a board layout so I know what pad or trace to monitor for that voltage? If it's the module, is this problem something that can be user-repaired? I'm a EE, with soldering skills, so I should be able to fix just about anything, if I know how to find the problem.

One last question. Should the emergency flasher switch be lit when the headlights are on? This one isn't, at all. Is this a burned out bulb too? Haven't seen anything on any forums about that. But, haven't looked very hard either.

Any help would be appreciated. Thanks for listening!

 

Yes, there are two separate circuits feeding the clock backlight. That's the reason for the two diodes on the clock circuit board. When the headlights are switched on, the battery rail voltage is removed from the clock module backlight; if it weren't, the higher voltage through the two diodes would always override the lower dimmer voltage and the dimmer would have no effect. If your instrument panel is lit and responds to the dimmer control when the headlights are on, but the clock doesn't, I'd check to see if the clock is OK or the wiring is feeding the variable voltage from the rheostat to the clock. There's a plastic cover on the back of the clock circuit board that one can (carefully) remove to make probing the diodes easy. The hazard warning switch is connected with a separate connector coming off the wiring harness, so I don't know offhand if it is fed off the dimmer circuit like the clock.

The clock backlight is rather dim when running with the headlights on, and may not be visible in daylight, unless you tent it with something dark, similar to the focusing hood used by photographers working with old style view cameras. Do that before undertaking major disassembly.

 

The backlight rheostat is working correctly. The instrument cluster, the heat/ac cluster, and the radio all adjust brightness accordingly. The clock does light up when the ignition is turned on, and it does go out completely when the headlights are turned on. I've verified that in the dark. No hood required! :-) So, that means that it's got to be in the harness (I suspect not) or in the clock assembly. Do you have a connector diagram that shows the different signals on each wire? I'd like to probe the connector with it disconnected and see if the theostat voltage varies withe rheostat position. If it does, then that pretty much indicates that it has to be the clock circuit. If I know the input pin for the voltage, I can ohm out the trace and find the appropriate diode. Do you know if it's an SMT or thru-hole component?

Thanks for getting back to me. I appreciate your taking the time. It's saved me a good bit of research time. If you have any more answers to my questions, I'd appreciate hearing back from you again.

Talk to you later.

 

The wire colors are off a 98-02 shop manual that covers European accords, so the colors may not be identical.

At the clock wire harness there should be 4 wires: Blk, red/blk, yel, and pnk.

Blk is ground, so check for continuity to ground (a something bolted into metal will work).

PNK should always have voltage (for clock memory)

YEL will have voltage when the key is in the I or II position.

RED/BLK will have voltage when the headlights are turned on.

From the wiring diagram, it looks like voltage will be 12V, as they are in line with the fuses. I could definitely be wrong on the voltage.

 

Thanks!! That's what I need. I didn't remember how many wires were actually in the connector from when I removed the clock. With only 4 wires, and one probably being black (GND - that's pretty universal on low DC voltage apps, if I remember correctly), that makes it much simpler to figure things out. Thanks. I'll take a look at it this weekend.

Again, I really appreciate your time and help.

 

Just wanted to get back to you, even though it's been a couple of weeks. I finally found some time to check the circuit. First, there are five wires (not sure what the one +12V source is for) and the colors are different. But, c'est la vie. At any rate, the dimmer voltage is 7VDC down to 72mVDC and is adjustable at the connector input to the clock module and up to the diode input. The +12V constant voltage is good up to the other diode input. They basically diode or'd the two output sides of the diodes together. The dimmer voltage varies inversely with the brightness of the dash. So, it should lower the constant +12V voltage on the output sides of the diodes. But, it doesn't do anything. So, I think you are right. The dimmer circuit diode is bad. I'll take it to work tomorrow and see if I'm right. All I had was a DMM to work with tonight. I'll have scopes and a variable power supply tomorrow. Hopefully, I'll be able scrounge this type diode from a parts bin somewhere.

Also, the emergency flasher switch/module has the same low voltage bulb the clock has in it and that's blown too. So, a new bulb in that will fix it so the flasher switch can be seen.

Just wanted to say thanks again.