Gary Lewis Posted August 4, 2020 Share Posted August 4, 2020 If you suddenly remember at 3am, definitely let me know Could someone with a working cruise take some measurements for me? This is the grey connector on the speed control amplifier above the gas pedal (easy to get to). Key in run and plug connected. Poking the lead up through the holes in the back. LG - 3.9v LB/BK - 7.63 W/P - 12.3 DG/W - 0.02 BK - 0 BK - 0 Key in run with plug unplugged LG - .03 LB/BK - 0 W/P - 12.3 DG/W - 0 BK - 0 BK - 0 I checked all my taillights and everything is working as expected. Can't see any faint glowing or anything like that. I don't have a spare amplifier with me right now or I'd just plug that in an test. I'll test mine tomorrow. But, I can't say that mine is a working speed control since I've not used it yet. But, I do have a spare so might be able to test both. Link to comment Share on other sites More sharing options...
kramttocs Posted August 4, 2020 Author Share Posted August 4, 2020 If you suddenly remember at 3am, definitely let me know Could someone with a working cruise take some measurements for me? This is the grey connector on the speed control amplifier above the gas pedal (easy to get to). Key in run and plug connected. Poking the lead up through the holes in the back. LG - 3.9v LB/BK - 7.63 W/P - 12.3 DG/W - 0.02 BK - 0 BK - 0 Key in run with plug unplugged LG - .03 LB/BK - 0 W/P - 12.3 DG/W - 0 BK - 0 BK - 0 I checked all my taillights and everything is working as expected. Can't see any faint glowing or anything like that. I don't have a spare amplifier with me right now or I'd just plug that in an test. Just found out that while plugged in and getting the 3.9v, if I turn on the headlights it jumps to 5.46v regardless if on high or low beam. Since taillights aren't high or low, maybe this means something. I still don't understand why disconnecting the tail light harness makes it jump to 12v. That seems important. Link to comment Share on other sites More sharing options...
kramttocs Posted August 4, 2020 Author Share Posted August 4, 2020 I'll test mine tomorrow. But, I can't say that mine is a working speed control since I've not used it yet. But, I do have a spare so might be able to test both. Thanks Gary - I appreciate that. At a bit of a loss on this one. Link to comment Share on other sites More sharing options...
grumpin Posted August 4, 2020 Share Posted August 4, 2020 I still don't understand why disconnecting the tail light harness makes it jump to 12v. That seems important. How are you using your test leads? Giving it a ground with the meter? Link to comment Share on other sites More sharing options...
kramttocs Posted August 4, 2020 Author Share Posted August 4, 2020 I still don't understand why disconnecting the tail light harness makes it jump to 12v. That seems important. How are you using your test leads? Giving it a ground with the meter? One lead stuck in the 'supposed to be disconnected' terminal of the stop lamp switch and the other connected to a direct to battery ground wire. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted August 4, 2020 Share Posted August 4, 2020 Just found out that while plugged in and getting the 3.9v, if I turn on the headlights it jumps to 5.46v regardless if on high or low beam. Since taillights aren't high or low, maybe this means something. I still don't understand why disconnecting the tail light harness makes it jump to 12v. That seems important. I'd say that is because 1157's share ground with both filaments. Does turning on the headlights trip out the speed control? Unplugging the harness removes ground altogether, doesn't it? Perhaps your LED's just can't sink enough current to make nice with speed control? Maybe there is a corroded connector or chafed wire some place? Link to comment Share on other sites More sharing options...
Gary Lewis Posted August 4, 2020 Share Posted August 4, 2020 I'd say that is because 1157's share ground with both filaments. Does turning on the headlights trip out the speed control? Unplugging the harness removes ground altogether, doesn't it? Perhaps your LED's just can't sink enough current to make nice with speed control? Maybe there is a corroded connector or chafed wire some place? I think Jim's onto something. Let me say what I think he's saying another way. With the harness connected and with no ground on the 1157 the power goes through the taillight filament of the 1157 then through the stop light filament and out into the stop light circuit and uses every bulb connected to that circuit as a load to ground. But with the harness disconnected there is nothing out there to sink the current and the voltage goes to 12. Jim - Is that what you are saying? Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted August 4, 2020 Share Posted August 4, 2020 I'd say that is because 1157's share ground with both filaments. Does turning on the headlights trip out the speed control? Unplugging the harness removes ground altogether, doesn't it? Perhaps your LED's just can't sink enough current to make nice with speed control? Maybe there is a corroded connector or chafed wire some place? I think Jim's onto something. Let me say what I think he's saying another way. With the harness connected and with no ground on the 1157 the power goes through the taillight filament of the 1157 then through the stop light filament and out into the stop light circuit and uses every bulb connected to that circuit as a load to ground. But with the harness disconnected there is nothing out there to sink the current and the voltage goes to 12. Jim - Is that what you are saying? That's part of it. Leds don't use nearly as much power as incandescent bulbs, but as you know diodes only pass current in one direction (unless you exceed breakdown) 1157 (and 56 for that matter) use the shell as ground. An 1157 bulb couldn't pass current backwards through either stop or taillight, could it? Link to comment Share on other sites More sharing options...
Gary Lewis Posted August 4, 2020 Share Posted August 4, 2020 That's part of it. Leds don't use nearly as much power as incandescent bulbs, but as you know diodes only pass current in one direction (unless you exceed breakdown) 1157 (and 56 for that matter) use the shell as ground. An 1157 bulb couldn't pass current backwards through either stop or taillight, could it? Right. I forgot the LED part. And you are right, they can't pass current backwards. So LED dual bulbs are going to just not work if there's no ground, where incandescent bulbs will do all sorts of strange things w/o a ground, depending on what is in the other circuit. That's going to make troubleshooting circuits with LED's quite different. Interesting! Link to comment Share on other sites More sharing options...
myrl883 Posted August 4, 2020 Share Posted August 4, 2020 Right. I forgot the LED part. And you are right, they can't pass current backwards. So LED dual bulbs are going to just not work if there's no ground, where incandescent bulbs will do all sorts of strange things w/o a ground, depending on what is in the other circuit. That's going to make troubleshooting circuits with LED's quite different. Interesting! 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. Link to comment Share on other sites More sharing options...
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