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headlights dim when press gas, brighten when let off gas

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headlights dim when press gas, brighten when let off gas

 
Old 04-29-2018, 02:00 PM
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Default headlights dim when press gas, brighten when let off gas

ever since i swapped my 00 civic with a jdm itr b18c r motor with the stock jdm p73 ecu
i noticed at night when i have my headlights on only when iím driving when i press the gas my headlights dim , and then when i let off the gas the headlights brighten up again.. itís very strange.. i have 0 gauge grounds from tranny to chasis
from battery to chassis and from valve cover to chassis
i also have 0 gauge power wore from alternator fused to battery.. i have the oem alternator wire hooked up with it as well

now i donít think it has anything to do with the grounds because when i has my old motor with these grounds it was not doing that.. i doubt its the alternator because i donít have a battery light on nor is my battery draining since i drove it pretty far.. iíve read it could be somthing with the jdm ecu causing it..

also it only did it it one night so far.. right after i washed my car

Last edited by eddie7978; 04-29-2018 at 05:33 PM.
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Old 04-30-2018, 10:02 AM
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Default Re: headlights dim when press gas, brighten when let off gas

I've run into this exact situation, but have yet to have time to check it out. My swap: 97 Civic LX with USDM 2000 Si B16A2 running a USDM P30 ECU OBD2a. Since it's easy enough to source the parts, I'm going to try to replace the under-hood fusebox which contains the ELD and hope that helps.

See Honda-ELD for a bit about how the ELD works.
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Old 04-30-2018, 03:58 PM
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Default Re: headlights dim when press gas, brighten when let off gas

hmm i doubt its anything that was previously on the car before i did my swap since it never happens before
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Old 06-25-2018, 07:48 AM
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Default Re: headlights dim when press gas, brighten when let off gas

well i just tested my alternator and battery are both good all grounds are fine.. but i canít test it while iím driving can i??
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Old 06-26-2018, 10:56 AM
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Default Re: headlights dim when press gas, brighten when let off gas

Originally Posted by eddie7978 View Post
well i just tested my alternator and battery are both good all grounds are fine.. but i can’t test it while i’m driving can i??


So, I finally had time to check out mine, and as far as I can tell it's acting like it should and within spec despite the lights slightly dimming and interior blower fan slowing down a little. With my swap, I am using a brand new Honda alternator for OBD2a Del Sol VTEC, brand new Honda battery, brand new under-hood fuse box with ELD direct from Honda and an USDM OBD2a P30 ECU. My grounds and wiring are as good as factory as well. So I just have to accept that this is just how it works.

For example, if I turn on the A/C compressor, it kicks over to the high-charging system because the ELD senses the the car is pulling too many amps to use the low-charge system.

It's only happening between ~ 10-45mph which is apparently normal according to that link I posted above ^. Under 10, it's always using the high-charge system and if I accelerate past ~42mph, it will kick back to the high-charging system and my lights will get a little brighter and fan (if it's on full) will get a bit faster.

The idea is that if you're under very little load with both engine and or electrical system (like just cruising at 30mph in 4th or 5th) the ELD will inform the ECU that there is a low load and to kick the alternator into a low-charge state, which will improve fuel economy while delivering just enough juice to the battery to keep it charged. Under low-load, between 10-45mph with the a/c compressor off, you should see the alternator kick down to it's minimum, which that article says is 12.3v. If it's below that then it could be a problem with the alternator signal wiring or something else.

You can test it while driving with a bit of ingenuity. You can tape a multimeter backwards to your windshield so you can see the readout and run the leads to the battery to watch the voltage go up and down when the ELD talks to the ECU, which in turn talks to the Alternator. I've yet to do this, but will likely do it this weekend to test. Since the High/Low switching is normal, the light dimming just a tiny bit between 10-45mph doesn't really bother me too much for now.
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Old 06-26-2018, 03:49 PM
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Default Re: headlights dim when press gas, brighten when let off gas

yea it doesnít bother me too much but i just wana know if my alternator is putting out the right amount of power to the engine.. and would like to figure out the cause but if not then o well thatís life .. would an aem volt gauge work iím thinking about getting one
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Old 06-27-2018, 12:42 PM
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Default Re: headlights dim when press gas, brighten when let off gas

Originally Posted by eddie7978 View Post
yea it doesnít bother me too much but i just wana know if my alternator is putting out the right amount of power to the engine.. and would like to figure out the cause but if not then o well thatís life .. would an aem volt gauge work iím thinking about getting one
I believe it would work.
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Old 06-27-2018, 12:45 PM
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Default Re: headlights dim when press gas, brighten when let off gas

Also check this paper. Super interesting and really breaks down how the dual-mode charging works: http://opensiuc.lib.siu.edu/cgi/view...text=auto_pres
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Old 08-23-2018, 05:00 PM
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Default Re: headlights dim when press gas, brighten when let off gas

wait so your saying it wonít do this with the ac on??
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Old 08-24-2018, 08:50 AM
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Default Re: headlights dim when press gas, brighten when let off gas

Originally Posted by eddie7978 View Post
wait so your saying it won’t do this with the ac on??
Correct. having the A/C on will bypass the ELD and put the alternator into high-charge mode. So if you have A/C on, you should not see your lights bright and dim while pressing the gas.

From the paper I posted above are the conditions that have to be met to go into low-charging mode via the ELD:

The ECM uses the low output mode when the engine is starting or if all of the following parameters are met:
  • electrical Load below 15 Amps (varies with vehicle),
  • vehicle speed between 10-45 mph or at idle while in drive,
  • engine speed below 3,000 rpm,
  • coolant temperature above 167įF (75įC),
  • A/C Switch Off
  • intake air temperature above 68įF (20įC).
What this is saying that once the engine is warmed up and lightly cruising with very low load, the computer will start lowering the alternator output, which will will reduce drag on the engine and increase fuel economy. This lowering of alternator output will cause your lights to dim slightly.
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Old 08-24-2018, 05:04 PM
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Default Re: headlights dim when press gas, brighten when let off gas

so this is normal your saying then? because it only started doing this after i swapped my motor..
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Old 08-27-2018, 07:21 AM
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Default Re: headlights dim when press gas, brighten when let off gas

if it is normal.. it never did it before on my stock motor.. only on my new jdm swap.. so iím guessing my alternator is good and grounds are good too.. this is normal?
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Old 08-27-2018, 08:01 AM
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Default Re: headlights dim when press gas, brighten when let off gas

Originally Posted by eddie7978 View Post
if it is normal.. it never did it before on my stock motor.. only on my new jdm swap.. so iím guessing my alternator is good and grounds are good too.. this is normal?
That's what so vexing about about this issue. I don't think mine did it before the swap but I honestly didn't pay all that much attention.

With my swap (2000 Si B16A2 / 1997 Civic LX), I've since installed new: alternator (OEM from Honda), new battery (OEM from Honda), new under-hood fuse block with ELD (OEM from Honda), and stock USDM OBD2a P30 ECU from 97 Del Sol VTEC. I've double checked all my grounds, which are all new OEM, and my wiring. It's all correct and within spec. So I have no choice at this point but to think this is just how the ECU is handling the electrical load under these narrow conditions. I have a spare USDM OBD2a P30 ECU that I checked as well. Same issue.
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Old 08-27-2018, 10:02 AM
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Default Re: headlights dim when press gas, brighten when let off gas

okay.. what if you upgrade to a highout put alternator??
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Old 08-27-2018, 01:24 PM
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Default Re: headlights dim when press gas, brighten when let off gas

Originally Posted by eddie7978 View Post
okay.. what if you upgrade to a highout put alternator??
You could also just bypass the ELD. Search around as there are quite a few threads out there.
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Old 08-27-2018, 01:46 PM
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Default Re: headlights dim when press gas, brighten when let off gas

and what would that do??
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Old 08-27-2018, 03:26 PM
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Default Re: headlights dim when press gas, brighten when let off gas

wow i just read this thread it has a lot of good info

https://honda-tech.com/forums/honda-...assis-2642854/
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Old 08-27-2018, 03:48 PM
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Default Re: headlights dim when press gas, brighten when let off gas

is it safe to bypass the eld ?? they said to just depin the wire from the ecu.. or if u get hondata u can do it through there too..
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Old 08-28-2018, 07:29 AM
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Default Re: headlights dim when press gas, brighten when let off gas

can u read that thread and tell me what u think?
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Old 08-28-2018, 10:27 AM
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Default Re: headlights dim when press gas, brighten when let off gas

Originally Posted by eddie7978 View Post
can u read that thread and tell me what u think?
It sounds like there are a multitude of issues but the most credible seem to be related to the two master power / ground connectors under the intake manifold. One blue, one grey, which house shared power and ground. Sounds like there's a Honda TSB for wires rubbed down to the copper, causing CEL 20 for ELD. I don't have this code but I'll check out those connectors out this weekend.

See: https://sparkys-answers.com/2009/04/...use-blown.html

See also from that thread: https://honda-tech.com/forums/honda-.../#post46406917

As I've mentioned before, make sure you're not drawing amps directly from the battery for something like an amp, stereo, alarm, etc. The ECU needs to account for all used amps in order to switch the ELD correctly.

Lastly, with a multimeter, check Fuse #15 (under dash, 7.5a)





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Old 08-28-2018, 01:48 PM
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Default Re: headlights dim when press gas, brighten when let off gas

yea i mean my alternator is charging fine.. battery light not on .. car never dies.. no cel..
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Old 08-30-2018, 05:27 AM
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Default Re: headlights dim when press gas, brighten when let off gas

could it have anything to do with me running a jdm itr ecu?
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Old 08-30-2018, 09:13 AM
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Default Re: headlights dim when press gas, brighten when let off gas

Originally Posted by eddie7978 View Post
could it have anything to do with me running a jdm itr ecu?
Doubtful. I have a few ECU's that i've tested. 2 stock USDM OBD2a P30s and a JDM ITR P73. They all do it.
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Old 08-30-2018, 10:23 AM
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Default Re: headlights dim when press gas, brighten when let off gas

it has to do somthing with the swap i didnít touch anything else and it never did it before.. so ecu jumper harness or alternator cuz that is different too.. thatís pretty much all thatís different besides the motor
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Old 08-31-2018, 08:40 AM
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Default Re: headlights dim when press gas, brighten when let off gas

Originally Posted by eddie7978 View Post
it has to do somthing with the swap i didnít touch anything else and it never did it before.. so ecu jumper harness or alternator cuz that is different too.. thatís pretty much all thatís different besides the motor
Yeah, I mean, there's a lot to consider. I've seen some people talk simply about the wire gauge size being slightly different for the Alt-C wire between D and B series. It could be the B-seires alternators or engines pull or put out slightly different amps. We all take these B-series swaps for granted because it's mostly all plug and play but there really are lots of little differences between an OEM d-series car and a b-series car. Wire sizes, group power/grounds, shielded wires, etc. My guess it's it's somewhere in the Alt-C wire signal or common harness ground that's slightly off, just enough pull excess amps outside the ECU's knowledge.
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