Stop Lamp Switch Voltage

[kramttocs]

Yes it does. LED was my first guess at all this and part of why I did the test against my 80 that doesn't have leds (see first post).
I went over all of the harness when reinstalling them and while I can't say that there wasn't something I missed or couldn't see, they all looked good.

Needing a resistor, just like for the flashers, makes sense.

That means that someone with incandescent taillights could do the voltage test on the grey speed control connector (plugged in with key on) and they should see:

W/P -12v
LG - little, if any, voltage

Then with the taillight harness unplugged they should see the same as me with mine unplugged (taking leds and the load out of the equation):

W/P - 12v
LG - 12v

Right?

[Gary Lewis]

Interesting!  Now I'll need to swap out some bulbs as I have LED's on one side and incandescent on the other.  So, I wonder......

[ArdWrknTrk]

Like I said, I think the LED's just can't pass enough current to play nice with speed control.

By installing a ballast you're right back to taxing the harness and switches, that you were trying to avoid by using the LED's  

[kramttocs]

Haha yep.
Honestly I can live with cruise dropping out when using the turn signal. They are optional now anyways, right?

Right now I really just want to ensure I don't have a potential fire hazard. Speed control isn't all that important to me.

Jim - the LED's can't pass enough or they pass too much?
I was thinking they were letting too much through.

What I need to do at lunch and if Gary could do the same with his spare that would be great, is to check the resistance on the amplifier between the W/P and LG terminals. I think that would answer some questions I have.

Because I still don't understand why the LG at that grey connector (C717) only has voltage when the connector is plugged in. That would indicate the voltage is COMING from the amplifier via the W/P wire which just seems odd as I would think LG would be an 'input/trigger' wire that tells the speed control to stop.

[ArdWrknTrk]

kramttocs wrote

Jim - the LED's can't pass enough or they pass too much?
I was thinking they were letting too much through.

Cant pass enough current.
LED's are SEMIconductors

Free electrons are always looking for the easiest path to a lower energy state.
That path might be through your speed control

[myrl883]

Gary - If you have vacuum cruise on the vehicle with LEDs on one side, the cruise should shut down when you turn on the signal on the side with incandescent bulbs. If you try it, let us know the result - I'm very curious!

I plan on using the '92 & newer electronic cruise on my '81 F100, hopefully I won't have any problems, because EVERY bulb on that truck is an LED, and I don't plan on using any load resistors.

[myrl883]

The simple answer is that the cruise module is pulling a ground through the stop filament of the brake light bulbs when the system is operating. If you brake, the system stops anyway, but when you use the working turn signal (assuming that one side is burned out), the module sees voltage instead of ground. It's this positive voltage signal that causes the cruise to shut off, because it can't pull a ground through the burned out bulb.

It's a long story as to how I learned this... BUT, this is where my years in the dealership community has paid off!

[kramttocs]

Thanks Myrl. This is very interesting to me and I am dying to know the how behind it's functionality.

Do you know why the LG coming out of the amplifier has ~3.8v on it when in park but with the key on? The LG wire only has voltage on it when the connector is plugged in so either it's getting that voltage from the W/P (hot in run) wire or else it's joining a circuit elsewhere yet to be determined.

I checked the terminals on the amplifier with the connector unplugged and I don't see any continuity between the LG and W/P terminal.

My plan after work is to remove the green wire from the connector, plug the connector back in, turn the key to on and see if the LG has voltage.

[Gary Lewis]

Ok, let's see if I got what you need.  First, there's no continuity between the LG & W/P terminals on my spare amplifier.

Then, I ran four tests:
     W/P     LG
1. 11.9v  .02v  LED on left & 1157 on right.  LG: Left turn = .02v  Right turn = 7.8v
2. 11.9v  .0      1157's on both left & right    LG: Left turn = .05 - .23v  Right turn = .09 - .25v
3. 11.9v  .66v    LED's on both left & right    LG: Left turn = 7.8v  Right turn = 7.8v & dash lights flash!
4. 11.9v  11.6v  No bulbs or LED's, trying to simulate pulling the connector - I hope.

[kramttocs]

I owe you one Gary (add it to my tab)!

So for test 3 and a right turn on test 1, your cruise would cut out also when applying the turn signal. That fits.

And test 4 gives me peace of mind even though you've all been saying that's how it works

I'd still like to figure out why you only have .66v and I have 3.8v on test 3 when no turn signal applied.

[ArdWrknTrk]

Gary,
What do you get if you read from a powered LED to ground?

Volts, amps?

[Gary Lewis]

Jim - I'll check tomorrow.  I've come into the house for a cuppa and cookie now.  But you want to know what the voltage and current are on the stop light LED.  Right?  I'll have to rig up a way to push the brake pedal for that as my digital voltmeter won't react fast enough in turn signal mode.

Scott - No problem, happy to help.  So apparently the speed control depends on the resistance of the bulbs being there.  Interesting.

And, if you figure out why I have .66v and you have 3.8v in Test three, please also figure out why my dash lights flash and, if the door is open, the headlight buzzer sounds - only on right turns, not left.  

[ArdWrknTrk]

Don't make a special setup for me.

I thought if you had it all out it might be interesting to note what was actually passing through the LED's (both taillamp and stop lamp)

But I'm sure we can see that in the advertised wattage.