67-427ci

I just installed SuperBright LED tail lights in my 67 Coupe. Shortly afterwords I discovered my AudioVox CCS-100 cruise control would no longer engage. After testing all the circuits and going through the troubleshooting guide I first thought the cruise control servo went bad.

After doing some serious web searches I discovered the cruise control sends a residual voltage through the brake light circuit to determine the circuit is functioning. This voltage goes through the brake light filament to ground (but is not enough to light the bulb). Then, when the brake light is activated (i.e. 12V), the cruise control disengages. If the cruise control does not detect a functioning brake light circuit, it will not engage. Apparently the LED does not let a small voltage go through it to ground.

To test this I replaced one of the LED's with a standard 1157 bulb. The cruise control worked immediately. Replaced the bulb with the LED and it no longer worked.

My question is for some of you electrical guru's. Can I install a small resistor (like a 10w, 10 OHM) in the brake light circuit to allow the cruise control to sense a functioning circuit? All I would need to do is one connection.
Thanks, Roy

Plasticman

A resistor is a good idea, but you will have to experiment to determine what resistance you need to provide the cruise control circuit enough feedback.

Brake lights are heavy draw (about 2.1 Amps) which means a very small resistance (6.66 Ohms @ 14 Volts) and heavy wattage (30 Watts). Hopefully your cruise control does not need to see that small a resistance.

Your 10 Ohm, 10 Watt resistor is not large enough (wattage wise). You would need a 20 Watt resistor minimum across a 14 Volt supply. And then air space around it to dissipate the heat.

Formula for wattage is P (power in Watts) equals I (current in Amps) times E (Volts). (P=IE, or I=P/E, or E=P/I)

Ohms law is E (Volts) equals I (current in Amps) times R (resistance in Ohms). (E=IR, or I=E/R, or R=E/I)

Taking your 10 Ohm resistor dropping 14 Volts, the current = 1.4 Amps (14 Volts divided by 10 Ohm).
To find the minimum wattage, multiply the voltage (14 Volts) times the current (1.4 Amps) = 19.6 Watts.

I would start with a much smaller resistor such as a 1000 Ohm (need a 1/2 Watt minimum). If that does not work, try a 100 Ohm, 2 Watt minimum. Etc, until you find the resistance that allows the control to function properly.

Plasticman

67vetteal

And if you successfully install a resistance, when will the circuit know to disengage for braking? Al W.

67-427ci

Thanks for the info. I found someone on the Internet who used two 10 OHM resistors (in parallel). Do you think this may work? Are there any potential problems to doing this? How hot do you think the resistors will get?
Thanks, Roy

John: I just saw the addition to your post. I'll see what Radio Shack has in stock.
Thanks, Roy

Plasticman

Two 10 Ohm resistors in parallel still need to handle the heat. In other words, each 10 Ohm resistor still needs to be 20 Watt rated. Actually, that is a little more resistance (and therefore similar current) of 2 brake lights. I would hope that your cruise control can be fooled by a higher resistance (lower current) load.

Plasticman

Plasticman

Al,

The resistor would act just like a brake light (a brake light is a resistor in effect to an electrical circuit). So he would hook the resistor across the brake light circuit (along with the actual LED lights). And the cruise control would sense that the brake light circuit either has or does not have current flowing though them.

Plasticman

67vetteal

"After doing some serious web searches I discovered the cruise control sends a residual voltage through the brake light circuit to determine the circuit is functioning. This voltage goes through the brake light filament to ground (but is not enough to light the bulb). Then, when the brake light is activated (i.e. 12V), the cruise control disengages. If the cruise control does not detect a functioning brake light circuit, it will not engage. Apparently the LED does not let a small voltage go through it to ground."

This is where I'm lost. I'm of the impression that he was trying to fool the circuit into thinking he had normal bulbs in place for engagement of the Cruise Control.
It's late here on Long Island and I'm off to bed. See ya in the morning. Al W.

Plasticman

Al,

That is exactly what he is trying to do. The resistor would "fool" the cruise control into thinking it has normal brake lights, but the actual lights would be the LEDs. The resistor would still be across the brake light circuit, so instead of lighting, it would just dissipate heat (and fool the cruise control).

Have a good sleep, I am off too!
Plasticman

bweaver999

Why not just put a light bulb in the circuit? A dash light would likely do the trick.

Plasticman

While you are correct that a bulb would work, the cruise control may require the same current flow as the original 2 brake lights. A small bulb like a dash light may not work (may have too much resistance).

The idea behind a resistor vs. the bulb is that a bulb has a finite life and a resistor does not. Why put a bulb in there knowing that it will need to be replaced, when a resistor is a one time deal.

Plasticman

67-427ci

I went to Radio Shack this morning and purchased some resistors.

1k-Ohm, 1 watt, metal oxide
100-Ohm, 1 watt, metal oxide
100-Ohm, 10 watt, wirewound

I'll start with the 1k first and see what happens. By the way, if I replace 1 of the 4 LED's with a standard 1157 bulb the cruise control works perfectly.
Roy

1COOL60

Actually, two 10 ohm resistors in parallel produces a total resistance of 5 ohms. The resistance of two parallel resistors is calculated as the product of the two values divided by the sum of the values. Therefore, 100 divided by 20 equals 5 ohms.

I would approach this problem from the point of determining the minimum amount of current needed to get the cruise control working. If no information can be found that states how much current is needed, I would assume a small amount, such as 50 milliohms. So if you run 50 milliohms across a 12 volt drop (the resistor), you would need a 240 ohm resistor. The resistor will need to dissipate 600 milliwatts of heat, so I would suggest a 1 watt resistor.

MidYearRoadster

what brand leds did you use and where did you get them? were they expensive? thanks for the info

67-427ci

They are made by SuperBrightLeds.com. Check out this thread for a comparison to incandescent bulbs. I used the 1157-LX3 (Red) They are kind of pricey at $24.95 each, but are a lot brighter and come on faster.
Roy

Plasticman

You missed what I said. Two (or twenty) resistors in parallel each still need to dissipate the full current through each of them and the voltage across them. One 10 Ohm resistor across 14 volts needs to dissipate the 1.4 Amps through it. 14 Volts times 1.4 Amps equals 19.6 Watts each! Add additional 10 Ohm resistors, and each resistor still needs to dissipate about 20 Watts. No magic there! Yes the total resistance drops to 5 Ohms, but only if he needs that low a resistance.

As far as determining the minimum resistance needed, I think I said that too! The 1K resistor I suggested is a pretty high resistance across a 14 Volt circuit. If it works then great, if not, then he will need to decrease the resistance until the circuit does function.

Plasticman

Plasticman

Roy,

That is good news, and tells us that you don't have to go lower (in resistance) than that of the single 1157 bulb (about 6 Ohms - see below). Hopefully it will still work with a much higher resistance.

Note that the 1157 bulb is rated at 12.8 Volts for the 2.1 Amp draw. Therefore its resistance is 12.8 divided by 2.1 = 6.09 Ohms.

Good luck,
Plasticman

67-427ci

Plasticman: I tried the resistors I purchased today and none of them did the job. As a test, I soldered leads on a 1157 bulb and it worked immediately. The residual voltage in the circuit is only 1.8 volts. The cruise control must be calibrated to look for the resistance of the bulb (???). Radio Shack has a 10 Ohm, 10 Watt resistor. Do you think it might be worth a try?
Roy

Plasticman

Roy,

As a try only, since as I stated earlier, 10 Watts is not sufficient to prevent them from burning up (really need 20 Watt rating min.). I would purchase two 10 Watt resistors, and try them wired in series with each other (that would equal 20 Ohms, and the current draw would be .7 Amp (when across a 14 Volt supply) which would require a 9.8 Watt rating minimum). If that did not work, then a single 10 Ohm would be the next try, and if that did work, then find some with a 20 Watt rating minimum.

Good luck,
Plasticman

67-427ci

I think I found exactly what I need. Check out this resistor at eAutoWorks.com. It is a 6 Ohm, 50 Watt LED Light Bulb Load Resistor Kit.
Roy

Plasticman

Roy,

Agree that should do it!

Plasticman