Daytime running lights dim and dying at idle with Toyota H4 harness + Hella 100/80w bulbs

[fizzlebottom]

I've had the Toyota harness & higher wattage Hella bulbs installed for a few months (headlights are properly aimed, so no one is going blind). Here's the issue:

My Jeep was originally Canadian, thus has DRLs. I recently noticed that at idle, the DRLs go dim and then out altogether. Once I start moving again, they come back to life. Low beams and high beams both work perfectly.

I suspect this has something to do with how the lights are getting power while in DRL mode vs while in low/hi beam mode. When in low or hi beam mode, they are powered through fuses & relays connected directly from the battery. While in DRL mode, I think they're still going through the dimmer switch which was never meant to provide the power these bulbs are asking for.

I've tested the alternator and battery and they're well within spec.

Wondering if anyone has experienced anything similar. Am I going to end up burning out my headlight switch?

 

[Chris]

Not to be obtuse, but does the Toyota harness have relays inline?

Have you tested your alternator by chance?

 

[fizzlebottom]

Not to be obtuse, but does the Toyota harness have relays inline?

Have you tested your alternator by chance?

It does have relays. I actually had to replace both of them because they each failed within 2 weeks of each other. The printed text on both of them was almost completely worn off, even brand new. I tentatively concluded that they came from a poor batch. The new ones from O'Reilly's have been working just fine, though I do have to wonder if the issue is related.

And yes I tested the alternator. It is feeding 13.75-13.8v to the battery when the battery is at full charge. That voltage stayed very steady when I turned on lights & AC.

What are you thinking here?

 

[Chris]

It does have relays. I actually had to replace both of them because they each failed within 2 weeks of each other. The printed text on both of them was almost completely worn off, even brand new. I tentatively concluded that they came from a poor batch. The new ones from O'Reilly's have been working just fine, though I do have to wonder if the issue is related.

And yes I tested the alternator. It is feeding 13.75-13.8v to the battery when the battery is at full charge. That voltage stayed very steady when I turned on lights & AC.

What are you thinking here?

Since you mentioned that the alternator is working well and providing stable voltage, it seems like the problem is more likely related to the DRL circuit.

Your thwoey about the dimmer switch is a good lead. The DRLs operate at a lower voltage, and if they're still tied to that circuit, the higher wattage bulbs might be pulling more current than the circuit is designed to handle, especially at idle when the alternator output can dip slightly. This could explain why they dim and even shut off until you rev the engine and the overall voltage increases.

Have you considered bypassing the DRL system entirely for the headlights? Some people opt to disable the DRLs and run directly through the relays that power the headlights. Personally, if it was me I would bypass the DRLs altogether.

Finally, it might be worth checking the connections and grounds associated with the DRL circuit. A weak ground could also contribute to poor performance at idle. Just a few thoughts I had—hope it helps!

 

[fizzlebottom]

I was reading up on the bypassing the DRL system just earlier today while researching this, and noticed the one thread on here that effectively resulted in "cut the wire coming from the Vehicle Speed Sensor", though it seemed either a few people said it didn't work or I'm just not confident enough in it because folks weren't hooting and hollering that it finally fixed all of their problems.

So, if you have confirmation and confidence that cutting that wire coming from the Vehicle Speed Sensor going into the DRL module will bypass it while maintaining all other functionality, then I can go ahead and do that tomorrow.

 

[Tyler McColl]

Late to the party but want to add for those reading I did this and it completely solved the issue for me. Cut the White/Orange wire from the Vehicle Speed Sensor.