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Stop Lamp Switch Voltage

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kramttocs

Stop Lamp Switch Voltage

Administrator

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?

Scott
'Camano' 1986 F250 Supercab XLT Lariat 460/C6
'Chanute' 1980 F350 C&C 400/NP 435 - Gin Pole

But there ain't nothin' wrong with the radio

ArdWrknTrk

Re: Stop Lamp Switch Voltage

Administrator

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?

Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.

Gary Lewis

Re: Stop Lamp Switch Voltage

Administrator

In reply to this post by kramttocs

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.

Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI

ArdWrknTrk

Re: Stop Lamp Switch Voltage

Administrator

This post was updated on .

Doh.
I misinterpreted that this is not the *gin pole truck.

Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.

kramttocs

Re: Stop Lamp Switch Voltage

Administrator

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.

Scott
'Camano' 1986 F250 Supercab XLT Lariat 460/C6
'Chanute' 1980 F350 C&C 400/NP 435 - Gin Pole

But there ain't nothin' wrong with the radio

ArdWrknTrk

Re: Stop Lamp Switch Voltage

Administrator

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.

Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.

myrl883

Re: Stop Lamp Switch Voltage

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.

Ford Parts Monkey since 1985
1981 F100 Flareside - Black, 302-4V Roller/AOD
1986 F150 Flareside - Medium Fire Red 302/AOD
1989 F150 Standard Cab 4x4 - Dk Shadow Blue 302/AOD
1993 F350 4x4 Crew Cab - 7.3 IDI/ZF-5
I think it's a sickness...

kramttocs

Re: Stop Lamp Switch Voltage

Administrator

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.

Scott
'Camano' 1986 F250 Supercab XLT Lariat 460/C6
'Chanute' 1980 F350 C&C 400/NP 435 - Gin Pole

But there ain't nothin' wrong with the radio

grumpin

Re: Stop Lamp Switch Voltage

I somehow remember having this problem or reading about it. Ended up being the turn signal switch.

Wish I could remember better!

Dane
1986 F250HD SC XLT Lariat 4x4 460 C6-Sold
1992 Bronco XLT 4x4 351W E4OD
1998 GMC Sierra SLE K1500 350 4L60E
Arizona

kramttocs

Re: Stop Lamp Switch Voltage

Administrator

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.

Scott
'Camano' 1986 F250 Supercab XLT Lariat 460/C6
'Chanute' 1980 F350 C&C 400/NP 435 - Gin Pole

But there ain't nothin' wrong with the radio

Gary Lewis

Re: Stop Lamp Switch Voltage

Administrator

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.

Gary, AKA "Gary fellow": Profile

kramttocs

Re: Stop Lamp Switch Voltage

Administrator

In reply to this post by kramttocs

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.

Scott
'Camano' 1986 F250 Supercab XLT Lariat 460/C6
'Chanute' 1980 F350 C&C 400/NP 435 - Gin Pole

But there ain't nothin' wrong with the radio

kramttocs

Re: Stop Lamp Switch Voltage

Administrator

In reply to this post by Gary Lewis

Thanks Gary - I appreciate that. At a bit of a loss on this one.

Scott
'Camano' 1986 F250 Supercab XLT Lariat 460/C6
'Chanute' 1980 F350 C&C 400/NP 435 - Gin Pole

But there ain't nothin' wrong with the radio

grumpin

Re: Stop Lamp Switch Voltage

In reply to this post by kramttocs

kramttocs wrote

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?

Dane
1986 F250HD SC XLT Lariat 4x4 460 C6-Sold
1992 Bronco XLT 4x4 351W E4OD
1998 GMC Sierra SLE K1500 350 4L60E
Arizona

kramttocs

Re: Stop Lamp Switch Voltage

Administrator

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.

Scott
'Camano' 1986 F250 Supercab XLT Lariat 460/C6
'Chanute' 1980 F350 C&C 400/NP 435 - Gin Pole

But there ain't nothin' wrong with the radio

ArdWrknTrk

Re: Stop Lamp Switch Voltage

Administrator

In reply to this post by kramttocs

kramttocs wrote

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?

Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.

Gary Lewis

Re: Stop Lamp Switch Voltage

Administrator

ArdWrknTrk wrote

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?

Gary, AKA "Gary fellow": Profile

ArdWrknTrk

Re: Stop Lamp Switch Voltage

Administrator

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?

Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.

Gary Lewis

Re: Stop Lamp Switch Voltage

Administrator

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!

Gary, AKA "Gary fellow": Profile

myrl883

Re: Stop Lamp Switch Voltage

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.