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Lower idle when headlings turned on

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86-95 Trucks & 4Runners 2nd/3rd gen pickups, and 1st/2nd gen 4Runners with IFS

Lower idle when headlings turned on

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Old 02-10-2018, 08:49 AM
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Lower idle when headlings turned on

I have a '96 T-100 with the 5VZ-FE, automatic 4WD, 145K, runs great. I recently noticed that when I am at a stop light or in Park idling, the motor will idle a little lower when I turn the headlights on or some other high draw accessory like the blower. I also noticed that my dashboard volt meter bounces a little bit back & forth when I have my turn signal on. I have not checked anything yet. I think my alternator is the original and I have a fairly new (like 2+yrs old) Optima yellow top. Any info you can share is appreciated.

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Old 02-10-2018, 10:12 AM
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Red face

Break out the Volt Meter get some real world voltage numbers .

Might just be a poor connection loose corroded

My 2000 3.4 has 225,000 on the original alternator yet
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Old 02-11-2018, 09:59 AM
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Thanks Wyoming, I did a quick visual touch test of the + terminal & cable, looks okay. the + terminal is not even warm to the touch so pretty sure I'm not getting a power sink condition. It starts just as quick as it always has so it doesn't seem like the battery is weak but the low idle condition's got be a little concerned. The truck is 22yrs old so it may just be a symptom of old age. As soon as it stops raining here I will get more into it with my volt meter (no garage to work in).
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Old 02-11-2018, 10:32 AM
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It is not surprising to me that the engine would idle slower with more load (via the alternator). It may even be slow enough that the alternator is no longer able to hold up the regulated voltage, leading to voltmeter bounce with turn signal draw.

You can check the idle spec to be sure. The correct idle should be on the under-hood sticker.
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Old 02-12-2018, 12:24 AM
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Yes, check idle specs like scope103 says^^^


Originally Posted by 8lugnuts View Post
... quick visual touch test ... pretty sure I'm not getting a power sink condition. ... doesn't seem...
Not enough. Need to verify with volt-meter and thorough, not quick, inspection of ALL connections between battery and alternator. ALL Connections should have bare, shiny metal tightly against bare, shiny metal. That is ALL connections between battery and alternator, not just the cliched "check grounds".

The truck is 22yrs old ...
Mine is 31 YO and does not have that issue.

I will get more into it with my volt meter (no garage to work in).
THERE YOU GO!
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Old 02-12-2018, 04:15 PM
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I'll be keeping my eye on this thread. I also switched instrument clusters because my cluster did not have the tachometer/oil gauge/voltmeter. After swapping clusters, my voltmeter flickers side to side when I have a blinker on as well. It never bothered me enough to fix it. My RPMs also are reduced when I turn on my headlights. I figured it was the weak stock wiring because I upgraded the bulbs to Osram 55w instead of the typical 35w (before swapping clusters). I was planning on wiring up relay harnesses for the headlights sometime soon.
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Old 02-13-2018, 07:37 PM
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Forecast for next few days....no dry, warm garage or barn to work in! yarg!

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Old 02-13-2018, 10:23 PM
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Doesn't the 5vze of this era use a motor actuated idle valve, someone correct me if I am wrong and state when this was implemented..

Unless there is an issue with the idle control, out of range limit, the computer should open the control valve to compensate for load from the alternator and increase the airflow.

This is more of a question than solution..
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Old 02-14-2018, 06:10 PM
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It's not UN common for dash lights to dim with blinkers on.
my brake indicator light does that.
Yes, there must be a problem with wiring that causes too much voltage drops to dash circuit when blinkers come on. Maybe wires that are too thin or too long. Also the fact that blinker and headlight circuits take power closer to the battery than the dash (tails) backlighting.
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Old 07-12-2018, 06:13 PM
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I've got the same issue with my 89 3vze. I didn't notice the headlights until recently when I set a new TPS and messed with the idle adjust screw. My buddy and I were setting the idle speed after it got warm and realized turning on the headlights would drop it by about 50-100 RPMs. The ticking voltage meter with every blink of my turn signals has been that way since I got the truck, I just assumed that's the way they were supposed to be...
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