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Voltage Drop at Headlights

SSTJ

SSTJ

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I have the stock wiring harness on a 97 SE.

With the engine at idle, I have 13.85V across the battery terminals. But I have only 12.8V between the passenger headlight plug and the battery's negative terminal.

What do others get, and what is a normal/typical amount of voltage drop between the battery and the headlights?


Update: Shot electrical contact cleaner through the holes of the headlight plugs until they ran clean, and saw a substantial change. Passenger side now gets only 0.3V less than battery. For some reason the driver side was still more than 1V below the battery. Oddly, that seems to be the one with a shorter wire run.

Update: Installed a new headlight switch due to another problem. Now it's the passenger side that is about 1V below the battery.

Update: Installed the Toyota relay harness. Both headlights now get 0.1V less than the battery terminals.
 
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Clean up the grounding points beside both headlight buckets and make sure your battery terminals are clean.

-Mac
 
I can't answer your voltage drop question, because I didn't measure mine.

But the very first mod I did on mine was to run heavy wires from the battery to a couple of relays near the firewall, and then to the headlights. I wanted to get the heavy current out of the multifunction switch before it cooked. There is a lot of thin wiring and that MFS on a factory headlight circuit, so you're seeing the result of that lousy engineering. My stock headlights were immediately brighter. Later, I installed brighter headlights.
 
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Also check your battery to body ground as well. With the engine running and a bunch of high draw accessories turned on, measure the voltage between the negative battery post and a bare spot on the body relatively far away. (Measure to the engine block as well while you're at it.) If it's more than a couple tenths of a volt away from zero, the ground wire is either bad or poorly connected.
 
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macleanflood said:
Clean up the grounding points beside both headlight buckets and make sure your battery terminals are clean.

-Mac
Click to expand...

Thanks. I should have mentioned that was already the case.

KennethS said:

Toyota H4 headlight kit with harness and bulbs (81110-60P70)​


https://toyotaparts.lakelandtoyota.com/oem-parts/toyota-headlamp-assembly-8111060p70

Around $20 gets you new H4 housings with bulbs and a full wiring harness with relays to do what OldBuzzard said above.
Click to expand...

Thanks. I have this kit lying around somewhere.

Steel City 06 said:
Also check your battery to body ground as well. With the engine running and a bunch of high draw accessories turned on, measure the voltage between the negative battery post and a bare spot on the body relatively far away. (Measure to the engine block as well while you're at it.) If it's more than a couple tenths of a volt away from zero, the ground wire is either bad or poorly connected.
Click to expand...

Thanks, will do.
 
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Oldbuzzard, you sure hit the nail on the head with that one. I burned up two headlight switches in my ‘96 RAM 2500 because of the thin/cheap MOPAR wires. I put in the new switch, but also a 30A relay triggered by my headlight output, now the relay carries the load and not my in dash switch. What really did surprise me though, I put an AMP meter in line to see what I’m demanding, because I also use my High beam signal to fire two other 30A relays for 4 100W lights on my bumper(it’s dark out here at nite and deer are everywhere)and on HB it only drew just over 9Amps?
I was surprised like I said, with it melting the switch, over time, I expected more?
Most everything now, I use relays and micro switches, there just used to ground the relay/trigger.
 
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My (previous) '87 F-250 I had added extra running lights plus trailer light wiring to it. The connector on the back of the dash switch melted over time. It was "probably ok" with the stock load, but I should have relayed the whole mess. Lesson learned.
 
Well, I took the opportunity to disconnect and thoroughly clean all main grounding points, as discussed here:
  • Negative terminal to firewall.
  • Negative terminal to engine block.
  • Engine block to firewall and same firewall bolt to hood.
I still get at least 1V less at the headlights than I do across the battery terminals. With engine off, the difference is up to 1.5V.

As mentioned by @machoheadgames over here, that's surely enough to benefit from the relay mod. I'll dig out that Toyota harness and see what difference it makes.
 
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SSTJ said:
Well, I took the opportunity to disconnect and thoroughly clean all main grounding points, as discussed here:
  • Negative terminal to firewall.
  • Negative terminal to engine block.
  • Engine block to firewall and same firewall bolt to hood.
I still get at least 1V less at the headlights than I do across the battery terminals. With engine off, the difference is up to 1.5V.

As mentioned by @machoheadgames over here, that's surely enough to benefit from the relay mod. I'll dig out that Toyota harness and see what difference it makes.
Click to expand...

Should have thought to check the headlight plugs themselves. They were badly gummed up. Shot contact cleaner through the holes until they ran clean, and saw a substantial change. Passenger side now gets only 0.3V less than battery.

For some reason the driver side is still more than 1V below the battery. Oddly, that seems to be the one with a shorter wire run. I'll have to check where the driver-side headlight wires are spliced into the main harness, but of course the relay harness will bypass it in the meantime.
 
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If you are running accessory Hi-Intensity Driving or Fog Lamps just run them off the Battery the ECU won't see a thing - Like wiring a Wench - My wife is rolling in her grave right now - The poor thing - I truly miss Her - However, this can over load the Alternator - So you may need a Bigger one and focus your attention on tight fitting clean grounds -You can never have a Big Enough Alternator,

I was rather lazy having to use the Wench so I had my Wife by my side wearing Hi-Hip-Waiters to pull my cable. It's a Man-Die-Tory thing to have a Wench by ones side at all times with a JEEP TJ.

Forgot to mention Axes, Chain Saws, Slings... It's just so comfortable and relaxing to own a TJ - Need I Go On.
We don't need that Jack Freddy it's time for the Wench ;o)
 
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