PWM or no PWM?

Swofford · January 27, 2021

Needing information - I am setting up my recently purchased 2020 LT Z71 to be used as a tow dinghy behind our RV. Apparently, there is a bit of information that is needed before I purchase a wiring harness to connect the truck tail/turn/brake lights to the 7-pin connector on the back of the truck. Hopkins makes a kit that is a universal kit, but they are telling me that I have to find out if my truck uses Pulse Width Modulation(PWM) as part of the light system. theirs does not work with PWM. Roadmaster also makes a harness that works with PWM.

The only real information I can find out if/if not a 2020 comes with PVM is a reference that comes from a review that was discussing the U1D Advanced Trailering Package... which my truck does not have. I contacted Chevrolet, but the support person didn't know anything and is saying to contact the dealership and maybe they could help....

Does anyone out there know anything about this? Do all 2020+ trucks use this and the info was essentially a red herring? Or is there a difference and how do I figure out which harness I have to buy?

Thanks for any helpful information.

Larry

Rob Mugs · January 28, 2021

3 hours ago, Swofford said:

Needing information - I am setting up my recently purchased 2020 LT Z71 to be used as a tow dinghy behind our RV. Apparently, there is a bit of information that is needed before I purchase a wiring harness to connect the truck tail/turn/brake lights to the 7-pin connector on the back of the truck. Hopkins makes a kit that is a universal kit, but they are telling me that I have to find out if my truck uses Pulse Width Modulation(PWM) as part of the light system. theirs does not work with PWM. Roadmaster also makes a harness that works with PWM.

The only real information I can find out if/if not a 2020 comes with PVM is a reference that comes from a review that was discussing the U1D Advanced Trailering Package... which my truck does not have. I contacted Chevrolet, but the support person didn't know anything and is saying to contact the dealership and maybe they could help....

Does anyone out there know anything about this? Do all 2020+ trucks use this and the info was essentially a red herring? Or is there a difference and how do I figure out which harness I have to buy?

Thanks for any helpful information.

Larry

I can NOT give you a definitive answer but I can say that the DRLs in our trucks flicker during the day on camera. Similar to old halogen based GMs with DRLs which definitively used PWM to dim them in the day. ie, my 99 Tahoe did this and could be seen on an oscilloscope

A quick run outside showed me that this doesn't seem to happen at night with my DRLs on either my POE cams or mobile device which is 30 fps and 60 fps respectively

I can also say that the brake lights vs running lights activate two separate sets of LEDs as opposed to using PWM for brightness. Also, no flicker noticed on camera

So, my edumacated guess would be that no, the brake lights/running lights do not use PWM. If you can find someone with a scope, it'll show you with 100% certainty though

Jefez71 · January 28, 2021

I can NOT give you a definitive answer but I can say that the DRLs in our trucks flicker during the day on camera. Similar to old halogen based GMs with DRLs which definitively used PWM to dim them in the day. ie, my 99 Tahoe did this and could be seen on an oscilloscope

A quick run outside showed me that this doesn't seem to happen at night with my DRLs on either my POE cams or mobile device which is 30 fps and 60 fps respectively
I can also say that the brake lights vs running lights activate two separate sets of LEDs as opposed to using PWM for brightness. Also, no flicker noticed on camera

So, my edumacated guess would be that no, the brake lights/running lights do not use PWM. If you can find someone with a scope, it'll show you with 100% certainty though

Couldn't have said it better.

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Rob Mugs · January 28, 2021

1 hour ago, Jefez71 said:

Couldn't have said it better.

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And I was partially wrong. After a few cold ones, I realized I could use my camera's slow-mo to confirm. DRL is PWM all of the time as shown. Tails are not, or if they are.. they're faster than 240 FPS. I clicked the unlock button just to show the brake lights as well.

Note: the brightening/dimming is the camera trying to adjust. It's not indicative of PWM

I would overall, bet a couple hundo that the rears are NOT PWM

Edited January 28, 2021 by Rob Mugs

kodiakdenali · January 28, 2021

LEDs are current controlled devices, the more current you pass through it the brighter they are. most LED drivers use PWM to regulate that current to the exact target specs of the designer. There's more to it than just for dimming. the current must be regulated or the LEDs will overheat and fail very quickly. this can be done with resistors, but there are some disadvantages to this. it wastes power, it generates heat, and a resistor is not adaptive to changing conditions. an LED driver can compensate for the effects of temperature and input voltage. most high power LEDs are controlled by a PWM based driver board. seeing this is pretty easy if the duty cycle of the modulation is not very high, its easy to see the flickering on a camera like Rob Mugs showed on the DRLs. If the PWM is used just to drive the LED at a specified current (usually near but not at the rated load, for longevity) its quite possible that the duty cycle of the signal is quite high, 90%, 95%, 99%..who knows. if that's the case and depending on the PWM frequency you would either have to get lucky or have a very very high FPS camera to capture the flicker. on Rob's video there is no noticeable flicker on the headlight LEDs, but I would be absolutely shocked if the headlights are not run by a sophisticated PWM based driver board. if not they would be susceptible to dimming with power fluctuations or could be easily damaged should there ever be a high voltage situation. It would require them to run the LEDs with plenty of overhead just in case and it just doesn't make sense to do that. however I bet the LED driver on the headlights runs at a pretty high duty cycle. The electrical schematics for the headlights actually have an icon which does seem to indicate a PWM function, but its on the BCM side of the circuit which makes no sense to me. you don't want to run a PWM signal over a long distance, that wire becomes effectively an antenna, the RFI would be considerable and it makes no sense to do that.

As for how to answer the question definitively...I don't know! if we can't find it documented somewhere, you'd have to inspect the circuit or look for evidence of it with a scope, but I bet they do a pretty good job of suppressing it, with all the LEDs all over this truck I don't notice so much as the slightest change in radio reception when they turn on. they must be filtered or shielded pretty well, as I would expect.

I assume the product you're looking at is compatible with both, why not get the PWM compatible version and not have to worry about it? its possible either way, but when it comes to something like an automotive light, if I see LEDs I would always assume PWM to be on the safe side, even though I've seen some that do just use resistors.

Transient · January 28, 2021

Is a Haynes manual available for our trucks yet? They have electrical schematics in the back of every book. That may be a way to find out.

Honestly, I haven't got the slightest clue. Just throwing this out there in case it does help.

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Swofford · January 28, 2021

Rob Mugs and kodiakdenali, thank you both for your input. I think I should have also stated that I believe my LT is using incandescent bulbs and not LEDs for the rear. I will have to go verify this again. I can understand the need for PWM usage in the LED controls, as kodiakdenali stated, the modulation of power input to the LED provides the different intensity displayed....I can't see how that would work with bulbs, as I don't believe that there is a variance, as wouldn't the filament just burn out with the application of more power?

I plan on calling my dealership today to see if their service manager can also shed light on this and will update this forum post, should they give me any relevant data.

I really appreciate the information.

Swofford · January 28, 2021

Update to all, after I received a call back from the dealership. They said they checked my VIN and also checked into their service information and database concerning the PWM function/capability for their trucks, it appears that my truck does not have this function, at least for the tail lights. They did not go into great detail, but my suspicions are that there is PWM functions active for the front headlights, as those are LEDs, but the function is not used for the rear light, as those are incandescent. Other than hooking up a scope to the wiring, as Rob suggested, I don't know if there is anything other than actually doing the installation and finding out. I also suspect that the aftermarket accessory place that did the quote for all the tow base, lighting harness, and battery disconnect went with the items that earned them to most profit... which is how they do business.... or they assume every Silverado comes with the LED lights on the rear?

Anyway, just for completeness and closure on this item, I will post an update in a couple of weeks, after I have installed the non-PWM harness on whether this worked or not...and to document this for future reference for others...

Thanks again....

Larry

kodiakdenali · January 28, 2021

4 hours ago, Swofford said:

Rob Mugs and kodiakdenali, thank you both for your input. I think I should have also stated that I believe my LT is using incandescent bulbs and not LEDs for the rear. I will have to go verify this again. I can understand the need for PWM usage in the LED controls, as kodiakdenali stated, the modulation of power input to the LED provides the different intensity displayed....I can't see how that would work with bulbs, as I don't believe that there is a variance, as wouldn't the filament just burn out with the application of more power?

I plan on calling my dealership today to see if their service manager can also shed light on this and will update this forum post, should they give me any relevant data.

I really appreciate the information.

I missed the fact that your tails are incandescent. oops!

Swofford · January 28, 2021

13 minutes ago, kodiakdenali said:

I missed the fact that your tails are incandescent. oops!

No, you did nothing wrong... I think I forgot to include that in the original post...

newdude · January 28, 2021

There is no explicit mentioning of PWM or not in the description and operation of the lighting systems on these trucks however, if anything is PWM, its not from the BCM rather its from the logic control module in the lamp itself based on the schematics. So anything with the LED headlamps has built in logic that then outputs the function of the LEDs in the lamps. Same on LED tail lamps.

Quote

Exterior Lighting Systems Description and Operation

The exterior lighting system consist of the following lamps:

Automatic high beam assist

Backup lamps

Daytime running lamps

Front fog lamps

Hazard warning lamps

Headlamps

Park, tail, license, and marker lamps

Stop lamps

Turn signal lamps

Trailer lighting

Low Beam Headlamps

The headlamps may be turned ON in 3 different ways:

When the headlamp switch is placed in the ON position, for normal operation

When the headlamp switch is placed in the AUTO position, for automatic lamp control

When the headlamp switch is placed in the AUTO position, with the windshield wipers ON in daylight conditions, after a 6 second delay

The BCM will also command the low beam headlamps ON during daylight conditions when the following conditions are met:

Headlamp switch in the AUTO position

Windshield wipers ON

Vehicle in any gear but PARK

When the BCM commands the low beam headlamps ON, the vehicle operator will notice the interior backlighting for the instrument cluster and the various switches with backlighting control will dim to the level of brightness selected by the instrument panel dimmer switch.

The body control module (BCM) monitors three signal circuits from the headlamp switch. When the headlamp switch is in the AUTO position, all three signal circuits are open. When placed in the AUTO position, the BCM monitors inputs from the ambient light sensor to determine if headlamps are required or if daytime running lamps will be activated based on outside lighting conditions. When the headlamp switch is placed in the OFF position, the headlamp switch headlamps OFF signal circuit is grounded, indicating to the BCM that the exterior lamps should be turned OFF. With the headlamp switch in the PARK position, the headlamp switch park lamps ON signal circuit is grounded, indicating that the park lamps have been requested. When the headlamp switch is placed in the HEADLAMP position, both the headlamp switch park lamps ON signal circuit and the headlamp switch headlamps ON signal circuit are grounded. The BCM responds to the inputs by illuminating the park lamps and headlamps. When the low beam headlamps are requested, the BCM applies B+ to both low beam headlamp control circuits illuminating the low beam headlamps.

High Beam Headlamps

When the low beam headlamps are ON and the turn signal/multifunction switch is placed in the high beam position, ground is applied to the BCM through the high beam signal circuit. The BCM responds to the high beam request by applying ground to the high beam relay control circuit which energizes the high beam relay. With the high beam relay energized, the switch contacts close allowing battery voltage to flow through the high beam fuse to the high beam control circuits to there respective high beam solenoid actuators located within the headlamp assemblies. With the left and right high beam solenoid actuators active, the solenoid shutters open in each headlamp assembly exposing the remaining portion of the headlamp that was covered by the shutters illuminating the high beams at full intensity.

Daytime Running Lamps

The daytime running lamps will illuminate continuously when the following conditions are met:

The ignition is in the RUN or CRANK position

The shift lever is out of the PARK position for vehicles equipped with automatic transmissions or the parking brake is released for vehicles with manual transmissions

The low and high beam headlamps are OFF

The ambient light sensor is used to monitor outside lighting conditions. The ambient light sensor provides a voltage signal that will vary between 0.2 and 4.9 volts depending on outside lighting conditions. The body control module (BCM) provides a 5 V reference signal to the ambient light sensor and the HVAC control module provides a low reference ground. The BCM monitors the ambient light sensor signal circuit to determine if outside lighting conditions are correct for either daytime running lights or automatic lamp control when the headlamp switch is in the AUTO position. In daylight conditions the BCM applies B+ to both daytime running lamp control circuits to the left and right multifunction light emitting diode (LED) control modules located in each headlamp assembly. The multifunction LED control modules respond the to the B+ input from the BCM by illuminating the left and right daytime running lamp LED’s. Any function or condition that turns on the low beam headlamps will cancel daytime running lamps operation.

Flash to Pass

When the turn signal/multifunction switch is momentarily placed in the flash to pass position, ground is applied to the turn signal/multifunction switch. The turn signal/multifunction switch applies ground to the body control module (BCM) through the flash to pass switch signal circuit. The BCM responds to the flash to pass request by applying ground to the high beam relay control circuit. This energizes the high beam relay, closing the switch side contacts of the high beam relay, applying battery voltage to the 3 pin high beam fuse. Battery voltage is applied from the high beam fuse through the high beam control circuit to the high beam headlamp assemblies. This causes the high beam headlamps to illuminate at full brightness momentarily.

Front Fog Lamps

The front fog lamp relay is supplied with battery voltage at all times. The front fog lamp switch signal circuit is grounded momentarily by pressing the front fog lamp switch. The body control module (BCM) energizes the front fog lamp relay by applying ground to the front fog lamp relay control circuit. When the front fog lamp relay is energized, the relay switch contacts close and battery voltage is applied through the front fog lamp fuse to the front fog lamp supply voltage circuit which illuminates the front fog lamps.

Hazard Lamps

The hazard flashers may be activated in any power mode. The hazard switch signal circuit is momentarily grounded when the hazard switch is pressed. The body control module (BCM) responds to the hazard switch signal input by supplying battery voltage to all four turn signal lamps in an ON and OFF duty cycle. When the hazard switch is activated, the BCM sends a serial data message to the instrument panel cluster requesting both turn signal indicators to be cycled ON and OFF.

The instrument panel dimmer switch controls the brightness of the interior backlighting components. When the instrument panel dimmer switch is placed in a desired brightness position, the body control module (BCM) receives a signal from the instrument panel dimmer switch and responds by applying a pulse width modulated voltage to the hazard switch light emitting diode (LED) backlighting control circuit illuminating the LED to the desired level of brightness.

Park, Tail, and License Lamps

When the headlamp switch is placed in the HEAD or PARK position, ground is applied to the park lamp switch ON signal circuit to the body control module (BCM). The BCM responds by applying voltage to the park lamps, tail lamps, and license lamps control circuits illuminating the park, tail, and license lamps.

Stop Lamps

The brake pedal position sensor is used to sense the action of the driver application of the brake pedal. The brake pedal position sensor provides an analog voltage signal that will increase as the brake pedal is applied. The body control module (BCM) provides a low reference signal and a 5 V reference voltage to the brake pedal position sensor. When the variable signal reaches a voltage threshold indicating the brakes have been applied, the BCM will apply battery voltage to the left and right stop lamp control circuits as well as the center high mounted stop lamp control circuit illuminating the left and right stop lamps and the center high mounted stop lamp.

Turn Signal Lamps

Ground is applied at all times to the turn signal/multifunction switch. The turn signal lamps may only be activated with the ignition switch in the ON or START positions. When the turn signal/multifunction switch is placed in either the TURN RIGHT or TURN LEFT position, ground is applied to the body control module (BCM) through either the right turn or left turn signal switch signal circuit. The BCM responds to the turn signal switch input by applying a pulsating voltage to the front and rear turn signal lamps through there respective control circuits. When a turn signal request is received by the BCM, a serial data message is sent to the instrument cluster requesting the respective turn signal indicator be pulsed ON and OFF. Vehicles equipped with LED turn signals use an additional turn outage detection circuit for detecting when a LED turn signal goes out. This circuit is necessary due to the low current draw of LEDs. The turn outage detection circuit allows the module to alert the driver of the vehicle that a turn signal is out.

Backup Lamps

Automatic Transmission

With the engine ON and the transmission in the REVERSE position, the transmission control module (TCM) sends a serial data message to the body control module (BCM). The message indicates that the gear selector is in the REVERSE position. The BCM applies battery voltage to the backup lamps control circuit illuminating the backup lamps. Once the driver moves the gear selector out of the REVERSE position, a message is sent by the TCM via serial data requesting the BCM to remove battery voltage from the backup lamps control circuit. The engine must be ON for the backup lamps to operate.

Trailer Lighting (Without U1D)

Backup Lamps

For backup lamp operation, the backup lamp relay is supplied with battery voltage at all times. With the engine running and the transmission in the reverse position, the transmission control module (TCM) sends a serial data message to the body control module (BCM). The message indicates that the gear selector is in the reverse position. The BCM energizes the backup lamp relay by applying battery voltage to the backup lamp relay control circuit. When the backup lamp relay is energized, the relay switch contacts close and battery voltage is applied through the backup lamp fuses to the backup lamp control circuits which illuminates the backup lamps. Once the driver moves the gear selector out of the reverse position, a message is sent by the TCM via serial data requesting the BCM to remove battery voltage from the backup lamp relay control circuit.

Park Lamps

When the headlamp switch is placed in the HEAD or PARK position, ground is applied to the park lamp switch ON signal circuit to the body control module (BCM). The BCM responds by applying voltage to the park lamps, tail lamps, license lamps, and trailer park lamps control circuits illuminating the park, tail, license, and trailer park lamps.

Stop Lamps

For stop lamp operation, the left and right trailer stop/turn signal lamp relay’s are supplied with battery voltage at all times. The brake pedal position sensor is used to sense the action of the driver application of the brake pedal. The brake pedal position sensor provides an analog voltage signal that will increase as the brake pedal is applied. The body control module (BCM) provides a low reference signal and a 5 V reference voltage to the brake pedal position sensor. When the variable signal reaches a voltage threshold indicating the brakes have been applied, the BCM energizes the left and right trailer stop/turn signal lamp relay’s by applying voltage to the left and right stop lamp relay control circuits. With the left and right trailer stop/turn signal lamp relay’s energized, the relay switch contacts close and battery voltage is applied through the left and right trailer stop/turn signal fuse’s to the trailer stop lamp control circuits which illuminates the trailer stop lamps.

LT LED lamps:

Edited January 28, 2021 by newdude

JimCost2014 · January 30, 2021

On 1/27/2021 at 2:08 PM, Swofford said:

Needing information - I am setting up my recently purchased 2020 LT Z71 to be used as a tow dinghy behind our RV. Apparently, there is a bit of information that is needed before I purchase a wiring harness to connect the truck tail/turn/brake lights to the 7-pin connector on the back of the truck. Hopkins makes a kit that is a universal kit, but they are telling me that I have to find out if my truck uses Pulse Width Modulation(PWM) as part of the light system. theirs does not work with PWM. Roadmaster also makes a harness that works with PWM.

The only real information I can find out if/if not a 2020 comes with PVM is a reference that comes from a review that was discussing the U1D Advanced Trailering Package... which my truck does not have. I contacted Chevrolet, but the support person didn't know anything and is saying to contact the dealership and maybe they could help....

Does anyone out there know anything about this? Do all 2020+ trucks use this and the info was essentially a red herring? Or is there a difference and how do I figure out which harness I have to buy?

Thanks for any helpful information.

Larry

Here is maybe an overlooked question, but can you actually dinghy tow a T1? I know with the 4WD K2's, you had to have the two speed transfer case with the 4LO feature, so you could put it into neutral.

Transient · January 30, 2021

Here is maybe an overlooked question, but can you actually dinghy tow a T1? I know with the 4WD K2's, you had to have the two speed transfer case with the 4LO feature, so you could put it into neutral.

Here's another question: Can the RV handle the weight of towing a T1? I have a neighbor with a class C that tows a 2-door Wrangler. Says he close to max. Might need a B or an A.

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Swofford · January 30, 2021

JimCost2014 and Transient... concerning the ability to be towed and towing... Mine is a 2020 LT Z71, with the 2-speed transfer case... which has the ability to be put into neutral mode so it can be flat towed. Additionally, my RV is a Class A and had the capability to tow the truck...and the tow bar I have is rated for over 6500lbs...and my truck sticker has it at 5200lbs... so if I don't put a ton (ok, actually a 1/2 ton) of stuff in the truck, I should be good. I was towing my 2-door Wrangler behind this RV without any hint that it was actually behind the RV.

I may find out that my research is faulty, but I will have to cross that bridge when I come to it.