kramttocs Posted August 3, 2020 Share Posted August 3, 2020 While driving this weekend on the interstate I found that I am having the same issue as reported here. When speed control is on and I use the turn signal it cancels cruise. In that thread Steve mentions a possible cause being voltage backfeeding to the stop lamp switch so I did some testing tonight. All of this testing was with the ignition in run but the engine not running. Also I disconnected the trailer wiring for the brake controller - 3 wire plug at firewall and a splice into the brake switch wire I am checking (the 3.8 one at rest). One of the switch terminals is 12.4 volts and the other is 3.8 If I use the turn signal it ups the 3.8 to 4.9 At the rear crossmember if I disconnect the connector for taillights and license plate lights the 3.8 ups to 12.2. Everything is working aside from the turn/cruise thing. All lights beside the headlights are led. I then tested my 80 F350 and it shows 12.2 and 0 If I use the turn signal the 0 jumps around between 0 and 0.7 This is a flatbed with all incandescent bulbs. The bed/taillight wire on the flatbed is iffy at best. Thoughts on where I should go next? Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted August 3, 2020 Share Posted August 3, 2020 Seems like you have a poor ground in/for your taillights. With the flatbed what kind of lamps are back there? Have you cleaned and greased the sockets? If it's something simple like trailer lights I might consider changing them out for a new pair of whatever is quality these days. Grote was always the fallback, but all things change... Run a solid ground to the frame or all the way forward to avoid finding stray current in the stop lamp circuit when using the turn signals. Have you checked if the 4-ways will trigger the speed control too? Link to comment Share on other sites More sharing options...
Gary Lewis Posted August 3, 2020 Share Posted August 3, 2020 I'm having a hard time figuring out where the 3.8 volts on the stop switch is coming from. Do you have any lights on at the time you are testing? In the schematic it looks to me like there are only two sources of power to the off-side of the stop lamp switch - assuming the switch itself isn't "leaking" First is, as Jim suggested, a bad ground on the stop lights. But that comes all the way back to the instrument panel on your truck. The other is bleeding through in the turn signal switch. The irrational part of me says to replace the LED's with incandescents and run the tests, including the speed control test, again. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted August 3, 2020 Share Posted August 3, 2020 I'm having a hard time figuring out where the 3.8 volts on the stop switch is coming from. Do you have any lights on at the time you are testing? In the schematic it looks to me like there are only two sources of power to the off-side of the stop lamp switch - assuming the switch itself isn't "leaking" First is, as Jim suggested, a bad ground on the stop lights. But that comes all the way back to the instrument panel on your truck. The other is bleeding through in the turn signal switch. The irrational part of me says to replace the LED's with incandescents and run the tests, including the speed control test, again. Doh. I misinterpreted that this is not the *gin pole truck. Link to comment Share on other sites More sharing options...
kramttocs Posted August 3, 2020 Author Share Posted August 3, 2020 Doh. I misinterpreted that this is not the *gin pole truck. Sorry, that may have been confusing. This issue is on the 86 F250. I just used the 80 1 ton as a comparison. I thought about swapping the leds out but since I disconnected the taillights and the voltage jumped to 12 does that not eliminate them as a culprit? I will check the rear crossmember ground. Regarding the hazards - after the second time the turn signal killed the cruise my wife pulled the manual out of the glove box. One of the things she read was that with the hazards on cruise won't work. That was new to me. I am not aware of any lights that where on during the test. I was holding the door switch shut to avoid the buzzer but noticed that pressing it didn't have any impact on the 3.8. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted August 3, 2020 Share Posted August 3, 2020 Sorry, that may have been confusing. This issue is on the 86 F250. I just used the 80 1 ton as a comparison. I thought about swapping the leds out but since I disconnected the taillights and the voltage jumped to 12 does that not eliminate them as a culprit? I will check the rear crossmember ground. Regarding the hazards - after the second time the turn signal killed the cruise my wife pulled the manual out of the glove box. One of the things she read was that with the hazards on cruise won't work. That was new to me. I am not aware of any lights that where on during the test. I was holding the door switch shut to avoid the buzzer but noticed that pressing it didn't have any impact on the 3.8. But it's good to know, because it gives you another data point when troubleshooting the schematic. It's on me. I tend to scroll back to a post while composing my thoughts. But on the phone my screen is limited to a few lines or just one paragraph, so I didn't get it and the thread title doesn't mention which truck. Link to comment Share on other sites More sharing options...
myrl883 Posted August 3, 2020 Share Posted August 3, 2020 But it's good to know, because it gives you another data point when troubleshooting the schematic. It's on me. I tend to scroll back to a post while composing my thoughts. But on the phone my screen is limited to a few lines or just one paragraph, so I didn't get it and the thread title doesn't mention which truck. The most common cause for the cruise to cancel when you turn a signal on is that the turn/brake bulb on the opposite side of the truck doesn't work - ie, if it happens with the left signal, the right side isn't working. Could be as simple as a burned out bulb, can also be a poor ground to that lamp or a problem with the turn signal switch. Link to comment Share on other sites More sharing options...
kramttocs Posted August 3, 2020 Author Share Posted August 3, 2020 The most common cause for the cruise to cancel when you turn a signal on is that the turn/brake bulb on the opposite side of the truck doesn't work - ie, if it happens with the left signal, the right side isn't working. Could be as simple as a burned out bulb, can also be a poor ground to that lamp or a problem with the turn signal switch. That's good to know. Think I have a less common cause going on Did the ground wire directly to the taillight ground with no impact so I started pulling fuses. Fuse 6 is the culprit circuit: Speed control; 4wd indicator; aux battery control;digital clock;rear window defrost;feedback carb circuit Sadly, you'll notice that Speed control is in that circuit which makes it a little hard to eliminate and test cruise at the same time I put the fuse back in and pulled the gray connector out of the bottom of the speed control box and saw the same behavior - the voltage dropped to 0 at the stop switch. With that connector out if I use the turn signals I see a flickering 0.1v which I figure is an acceptable leakage. I also confirmed that with that connector out I don't see the 12v show up if I disconnect the taillight harness. So the taillights are dropping it 9v? This isn't normal, right? All seems weird. I don't have another cruise truck at my house to compare. This is also all going off the assumption that the bump from 3.8 to 4.9 (when using turn signals) is enough to cancel cruise. Will start testing the pins of the amplifier. Link to comment Share on other sites More sharing options...
grumpin Posted August 3, 2020 Share Posted August 3, 2020 That's good to know. Think I have a less common cause going on Did the ground wire directly to the taillight ground with no impact so I started pulling fuses. Fuse 6 is the culprit circuit: Speed control; 4wd indicator; aux battery control;digital clock;rear window defrost;feedback carb circuit Sadly, you'll notice that Speed control is in that circuit which makes it a little hard to eliminate and test cruise at the same time I put the fuse back in and pulled the gray connector out of the bottom of the speed control box and saw the same behavior - the voltage dropped to 0 at the stop switch. With that connector out if I use the turn signals I see a flickering 0.1v which I figure is an acceptable leakage. I also confirmed that with that connector out I don't see the 12v show up if I disconnect the taillight harness. So the taillights are dropping it 9v? This isn't normal, right? All seems weird. I don't have another cruise truck at my house to compare. This is also all going off the assumption that the bump from 3.8 to 4.9 (when using turn signals) is enough to cancel cruise. Will start testing the pins of the amplifier. I somehow remember having this problem or reading about it. Ended up being the turn signal switch. Wish I could remember better! Link to comment Share on other sites More sharing options...
kramttocs Posted August 4, 2020 Author Share Posted August 4, 2020 I somehow remember having this problem or reading about it. Ended up being the turn signal switch. Wish I could remember better! 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. Link to comment Share on other sites More sharing options...
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