kramttocs Posted August 4, 2020 Author Share Posted August 4, 2020 Does the truck have LED tail lights? Vacuum speed control doesn't work right without them. You must put a load resister in parallel with the LED tail lights to make the cruise happy. 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? Link to comment Share on other sites More sharing options...
Gary Lewis Posted August 4, 2020 Share Posted August 4, 2020 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? 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...... Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted August 4, 2020 Share Posted August 4, 2020 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...... 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 Link to comment Share on other sites More sharing options...
kramttocs Posted August 4, 2020 Author Share Posted August 4, 2020 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 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. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted August 4, 2020 Share Posted August 4, 2020 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 Link to comment Share on other sites More sharing options...
myrl883 Posted August 4, 2020 Share Posted August 4, 2020 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...... 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. Link to comment Share on other sites More sharing options...
myrl883 Posted August 4, 2020 Share Posted August 4, 2020 Does the truck have LED tail lights? Vacuum speed control doesn't work right without them. You must put a load resister in parallel with the LED tail lights to make the cruise happy. 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! Link to comment Share on other sites More sharing options...
kramttocs Posted August 4, 2020 Author Share Posted August 4, 2020 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! 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. Link to comment Share on other sites More sharing options...
Gary Lewis Posted August 4, 2020 Share Posted August 4, 2020 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. 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. Link to comment Share on other sites More sharing options...
kramttocs Posted August 4, 2020 Author Share Posted August 4, 2020 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. 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. Link to comment Share on other sites More sharing options...
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