What vehicle do you have where the current for the bulbs is conveyed by the switch itself. I would have to believe that's not common these days. Is this an older vehicle?
Voltage Drop
- Thread starter Hokiefyd
- Start date
- Status
OVERKILL
$100 Site Donor 2021
Originally Posted By: Hokiefyd
What vehicle do you have where the current for the bulbs is conveyed by the switch itself. I would have to believe that's not common these days. Is this an older vehicle?
I was wondering the same thing. Ford ran the power through the switch on the fog lights for the Fox body GT's, and they were a fabulous fire hazard. I can only imagine what it would be like with the headlights setup in this fashion
What vehicle do you have where the current for the bulbs is conveyed by the switch itself. I would have to believe that's not common these days. Is this an older vehicle?
I was wondering the same thing. Ford ran the power through the switch on the fog lights for the Fox body GT's, and they were a fabulous fire hazard. I can only imagine what it would be like with the headlights setup in this fashion
Originally Posted By: OVERK1LL
I was wondering the same thing. Ford ran the power through the switch on the fog lights for the Fox body GT's, and they were a fabulous fire hazard. I can only imagine what it would be like with the headlights setup in this fashion
I didn't know that. We had a '92 LX 5.0, so no fog lamps.
I will say, as an aside, that instrument cluster and dash design may be one of my favorites ever. I loved the two large "rocker switches" on each side of the cluster surround. I remember one being for the head lamps and then ours had a blank, probably where the fog lamps would have been. But then I think the other side was possibly hazards and something else. It wasn't all that ergonomical, and the materials used weren't terribly expensive, but it was just a cool design. That was a very legible instrument cluster as well.
I was wondering the same thing. Ford ran the power through the switch on the fog lights for the Fox body GT's, and they were a fabulous fire hazard. I can only imagine what it would be like with the headlights setup in this fashion
I didn't know that. We had a '92 LX 5.0, so no fog lamps.
I will say, as an aside, that instrument cluster and dash design may be one of my favorites ever. I loved the two large "rocker switches" on each side of the cluster surround. I remember one being for the head lamps and then ours had a blank, probably where the fog lamps would have been. But then I think the other side was possibly hazards and something else. It wasn't all that ergonomical, and the materials used weren't terribly expensive, but it was just a cool design. That was a very legible instrument cluster as well.
1986 SAAB 900. The switch burned out often , especially with 85/90 H4's. As I recall Volvo did it as well back then. I think Toyota made a production change to 98 Camrys verses the 97 mode to relay both high and low. The HELLA relay on the SAAB was a latching relay, one pulse through the coil latched it in HIGH beam, the next pulse latched it in LOW. The coil circuit was powered from it's own feed and so designed that pulling back on the wand on the steering wheel turned on the high beams for "signal" purposes.
Last edited:
Most switches just tell the BCM to power up the underhood relay on later cars. I'm running 35w HIDs through factory wiring with no ill effects. I do run capacitors just to help in case my auto lights are on when starting the car to protect the ballast.
Note on voltage measuring and "voltage drop" on late model vehicles.
Almost all late model vehicles use pulse width modulation (PWM) on the headlight (and some other) circuits. During operation of the vehicle the control unit (BCM) energizes the head lamp by pulse width modulating the battery output and coupling the modulating output across the head lamp. As battery voltage changes the pulse width modulation is adjusted to maintain power output of the head lamp. On vehicles with DRL the PWM is used to reduce output of the headlamp in this mode.
A standard inexpensive voltmeter is not capable of measuring this modulated voltage signal accurately. It will only read peak to peak voltage. You must use a true RMS voltmeter to measure the PWM voltage.
Because all late model vehicles must use PWM to control the headlamp output, they will all benefit from a relay harness to feed a halogen bulb system true DC output at full battery voltage.
In many cases the PWM is limited to use on high beams only as part of DOT standards. Remember in the 80s and early 90s when a few vehicles had very bright high beams? And nowdays no vehicles high beams seem very bright (except HID). PWM was the cure to standardize light output.
Almost all late model vehicles use pulse width modulation (PWM) on the headlight (and some other) circuits. During operation of the vehicle the control unit (BCM) energizes the head lamp by pulse width modulating the battery output and coupling the modulating output across the head lamp. As battery voltage changes the pulse width modulation is adjusted to maintain power output of the head lamp. On vehicles with DRL the PWM is used to reduce output of the headlamp in this mode.
A standard inexpensive voltmeter is not capable of measuring this modulated voltage signal accurately. It will only read peak to peak voltage. You must use a true RMS voltmeter to measure the PWM voltage.
Because all late model vehicles must use PWM to control the headlamp output, they will all benefit from a relay harness to feed a halogen bulb system true DC output at full battery voltage.
In many cases the PWM is limited to use on high beams only as part of DOT standards. Remember in the 80s and early 90s when a few vehicles had very bright high beams? And nowdays no vehicles high beams seem very bright (except HID). PWM was the cure to standardize light output.
Originally Posted By: Torrid
Most switches just tell the BCM to power up the underhood relay on later cars. I'm running 35w HIDs through factory wiring with no ill effects. I do run capacitors just to help in case my auto lights are on when starting the car to protect the ballast.
The capacitors are actually increasing the startup DC current load during initial powerup as the capacitor acts as a battery and has to charge up upon recieving initial voltage.
The capacitor acts as a signal conditioner to flatten the 1/2 wave PWM signal which does not allow the relay or bi-zenon actuator magnet to function properly. In your direct feed case, the ballast would be recieving the PWM power and the ballast may not work correctly.
However, the startup current is not as excessive as you would think depending upon the ballasts.
Most switches just tell the BCM to power up the underhood relay on later cars. I'm running 35w HIDs through factory wiring with no ill effects. I do run capacitors just to help in case my auto lights are on when starting the car to protect the ballast.
The capacitors are actually increasing the startup DC current load during initial powerup as the capacitor acts as a battery and has to charge up upon recieving initial voltage.
The capacitor acts as a signal conditioner to flatten the 1/2 wave PWM signal which does not allow the relay or bi-zenon actuator magnet to function properly. In your direct feed case, the ballast would be recieving the PWM power and the ballast may not work correctly.
However, the startup current is not as excessive as you would think depending upon the ballasts.
Last edited:
So, is there a feedback loop with a photocell in each headlamp?
Originally Posted By: HerrStig
So, is there a feedback loop with a photocell in each headlamp?
No, the PWM only keeps halogen headlamp output at a constant level all the time. No dimming at idle and no brightening with increased voltage. Each volt of variance causes a several percent change in lumens output. I guess that is too much variance for the automakers.
Plus a few vehicles use alternator output throttling to vary alternator output voltage based on load and ambient conditions (IE hot weather). So they needed a method to stabilize light output.
So, is there a feedback loop with a photocell in each headlamp?
No, the PWM only keeps halogen headlamp output at a constant level all the time. No dimming at idle and no brightening with increased voltage. Each volt of variance causes a several percent change in lumens output. I guess that is too much variance for the automakers.
Plus a few vehicles use alternator output throttling to vary alternator output voltage based on load and ambient conditions (IE hot weather). So they needed a method to stabilize light output.
Last edited:
Originally Posted By: Jeff_in_VABch
Originally Posted By: HerrStig
So, is there a feedback loop with a photocell in each headlamp?
No, the PWM only keeps halogen headlamp output at a constant level all the time. No dimming at idle and no brightening with increased voltage. Each volt of variance causes a several percent change in lumens output. I guess that is too much variance for the automakers.
Plus a few vehicles use alternator output throttling to vary alternator output voltage based on load and ambient conditions (IE hot weather). So they needed a method to stabilize light output.
Thanks for describing what goes on in a modern headlight system! I guess I mucked that up by building a relay harness since the lights weren't bright enough, even after better bulbs.
Originally Posted By: HerrStig
So, is there a feedback loop with a photocell in each headlamp?
No, the PWM only keeps halogen headlamp output at a constant level all the time. No dimming at idle and no brightening with increased voltage. Each volt of variance causes a several percent change in lumens output. I guess that is too much variance for the automakers.
Plus a few vehicles use alternator output throttling to vary alternator output voltage based on load and ambient conditions (IE hot weather). So they needed a method to stabilize light output.
Thanks for describing what goes on in a modern headlight system! I guess I mucked that up by building a relay harness since the lights weren't bright enough, even after better bulbs.
Originally Posted By: Vikas
Can they use PWM in controlling HID ballast? I would think it would reduce the life of ballast drastically.
No, most all ballasts would not like the non-DC waveform of PWM.
The ballast is primarily a solid state device to strike and maintain the arc at a constant level, regardless of minor DC input fluctuations. The electronics of the ballast determine the energy provided to the HID bulb, not the DC input.
Variable output ballasts are available, but are not common. Only 35W and 55W ballasts are common. All HID bulbs are the same for any wattage.
Can they use PWM in controlling HID ballast? I would think it would reduce the life of ballast drastically.
No, most all ballasts would not like the non-DC waveform of PWM.
The ballast is primarily a solid state device to strike and maintain the arc at a constant level, regardless of minor DC input fluctuations. The electronics of the ballast determine the energy provided to the HID bulb, not the DC input.
Variable output ballasts are available, but are not common. Only 35W and 55W ballasts are common. All HID bulbs are the same for any wattage.
Originally Posted By: Vikas
So, if your car came with so called "modern" headlight system with PWM, you better not try to put aftermarket HID system on it! (Well, at least not without smoothing out the PWM output via capacitor and relaying it.)
That's what I had to do with the relay harness for the existing halogens on the Cruze. The PWM'ed daytime running lamp function was playing havoc with the low beam relay. A capacitor to smooth out the PWM made the relay happy.
So, if your car came with so called "modern" headlight system with PWM, you better not try to put aftermarket HID system on it! (Well, at least not without smoothing out the PWM output via capacitor and relaying it.)
That's what I had to do with the relay harness for the existing halogens on the Cruze. The PWM'ed daytime running lamp function was playing havoc with the low beam relay. A capacitor to smooth out the PWM made the relay happy.
Originally Posted By: Jeff_in_VABch
Originally Posted By: HerrStig
So, is there a feedback loop with a photocell in each headlamp?
No, the PWM only keeps halogen headlamp output at a constant level all the time. No dimming at idle and no brightening with increased voltage. Each volt of variance causes a several percent change in lumens output. I guess that is too much variance for the automakers.
Plus a few vehicles use alternator output throttling to vary alternator output voltage based on load and ambient conditions (IE hot weather). So they needed a method to stabilize light output.
I suspect that ought to make the battery last longer since overcharging can kill them quickly. Also, Halogen cycle bulbs do not like low voltage. The Toyota DRL which used the H4 low and a series resistor was NOT a great idea.
Originally Posted By: HerrStig
So, is there a feedback loop with a photocell in each headlamp?
No, the PWM only keeps halogen headlamp output at a constant level all the time. No dimming at idle and no brightening with increased voltage. Each volt of variance causes a several percent change in lumens output. I guess that is too much variance for the automakers.
Plus a few vehicles use alternator output throttling to vary alternator output voltage based on load and ambient conditions (IE hot weather). So they needed a method to stabilize light output.
I suspect that ought to make the battery last longer since overcharging can kill them quickly. Also, Halogen cycle bulbs do not like low voltage. The Toyota DRL which used the H4 low and a series resistor was NOT a great idea.
JHZR2
Staff member
Originally Posted By: HerrStig
I suspect that ought to make the battery last longer since overcharging can kill them quickly. Also, Halogen cycle bulbs do not like low voltage. The Toyota DRL which used the H4 low and a series resistor was NOT a great idea.
Really? Because my 98 chevy pickup uses low voltage on the high beams as DRLs, and Ive not touched a single bulb on the truck, having owned it since new. All original still.
Best bulbs Ive had!
I suspect that ought to make the battery last longer since overcharging can kill them quickly. Also, Halogen cycle bulbs do not like low voltage. The Toyota DRL which used the H4 low and a series resistor was NOT a great idea.
Really? Because my 98 chevy pickup uses low voltage on the high beams as DRLs, and Ive not touched a single bulb on the truck, having owned it since new. All original still.
Best bulbs Ive had!
Originally Posted By: JHZR2
Originally Posted By: HerrStig
I suspect that ought to make the battery last longer since overcharging can kill them quickly. Also, Halogen cycle bulbs do not like low voltage. The Toyota DRL which used the H4 low and a series resistor was NOT a great idea.
Really? Because my 98 chevy pickup uses low voltage on the high beams as DRLs, and Ive not touched a single bulb on the truck, having owned it since new. All original still.
Best bulbs Ive had!
I think even back in 98 they used PWM to dim the bulbs. That's different from reducing the voltage.
Originally Posted By: HerrStig
I suspect that ought to make the battery last longer since overcharging can kill them quickly. Also, Halogen cycle bulbs do not like low voltage. The Toyota DRL which used the H4 low and a series resistor was NOT a great idea.
Really? Because my 98 chevy pickup uses low voltage on the high beams as DRLs, and Ive not touched a single bulb on the truck, having owned it since new. All original still.
Best bulbs Ive had!
I think even back in 98 they used PWM to dim the bulbs. That's different from reducing the voltage.
- Status
