3.4 auto voltage drop when shifting into reverse/drive.

Hello. I've finally hit my limit diagnosing this one alone so I'm hoping someone else can provide some input on this strange electrical gremlin I'm experiencing.
TLDR: Random voltage drops when shifting into drive/reverse that seems to be related to also hitting the brakes while shifting into that gear. For example when I brake coming off an exit ramp I'll frequently see 12.8v-13.0v and sometimes down to 12.1v. It sits steady at 13.5v-13.7v when in park/neutral or when driving with higher RPMs.

Video of the behavior.

Video of the behavior when shifting into drive in January 2021.

The full story
Sometime last month I finished reassembling my 2004 Tacoma following an engine out head gasket replacement. I decided to also install a battery cable upgrade along with a higher amperage alternator. I've been running the stock ~90 amp alternator, Northstar 24F AGM, stock wiring, and an HKB Alt-S fuse voltage booster for the last 4 years. But went with a ~150amp Mean Green alternator, same AGM, 1/0GA copper cables from www.jeepcables.com, a remote 150amp alternator fuse, and the same voltage booster.



Upon first start I immediately noticed the alternator was whining. Figuring it was a bad alternator I ignored it while test driving the truck. I also noticed some odd voltage fluctuations when shifting into reverse. This prompted me to check my positive cables, battery terminal clamps, grounds, and I removed the voltage booster ALT-S fuse for a regular 7.5amp fuse. But the issues remained. So I drove it for a week and noticed while braking from high speeds the voltage will drop down into the low 12s. The AC blower motor will also noticeably fluctuate in speed when these voltage drops occur so it's very annoying driving in stop and go traffic.

I first tired going back to my stock alternator but then the truck tried charging at 16v+ so I quickly shut it off. I tried diagnosing that but decided to just try a third alternator. So I removed the Denso 105 amp Suzuki Vitara alternator and lead acid 24F from my 1999 Tacoma and installed them in my 2004. Same issues besides that odd high charging voltage. Then I installed the 24F AGM and Mean Green alternator in my 1999 and it behaved normally besides the alternator whine. Although it is a 5-speed manual with no AC. Finally I decided to revert back to stock wiring (mostly) I retained the 0/1 engine block ground cable and positive cable to the starter. I'd already added another engine to chassis ground for testing so I ruled out that cable being the issue. But I put the stock fender ground back, the stock alternator power wire back, a stock sized fuse box to battery positive, removed my winch cables, and removed the remote alternator fuse. But this has also not solved the issue and the voltage fluctuations remain.

I could try diagnosing the alternator connector/plug and go back to 100% stock cables down to the engine block ground but it seems pointless with an additional motor to chassis ground. I'm just honestly out of ideas here. Since the behavior has existed between multiple batteries and alternators I'm inclined to say it's a problem with the truck. It also could just be normal behavior with the automatic and I've become accustomed to a manual truck while driving one the past year. But I've had three people say their auto truck doesn't do this.

 

I had one final thought earlier today. I've done multiple large battery cable upgrades on other Toyotas and my 1999. But my 04 was different inside the fuse box and had an "ABS 2" fuse that had a power cable splitting off from the stock battery to fuse box power cable. So I decided to cut it at the split and connect the larger gauge cable to the empty post next to it. Since they're both connected to each other with a metal plate I don't think I've hurt anything. But I figured why not try reverting back to something closer to OEM. So I got another smaller power cable with a similar sized round spade connector and stacked it with the ABS 2 fuse wire. I started the truck up and unfortunately it acts the same.

The ABS 60 amp fuse and 120 amp Alternator fuse both connect to each other at these two posts. Did I somehow cause this strange behavior by cutting that wire and shooting power through the ABS fuse side of things? Maybe, but I doubt it considering the ABS system seems fine. I did notice that everyone with an 03/04 truck seems to just leave the stock gauge fuse box wire in place when they run into this ABS 2 fuse.

 

Some alternators are made to only produce output above a higher rpm then idle.

Without any real world voltage numbers in places beside the voltmeter.

It sure seems like your not getting the Alternator output where it needs to be.

Which causes your output to drop on your gauge.

I run my voltmeter gauge direct to the battery . I never see any system voltage drops with all the loads on.

Up close and person I wish I was closer .

best of Luck!

 

The voltage readings on my OBD2 scanguage are identical to readings directly from my voltmeter at the battery terminals. I’ve seen people talk about there being a 1-2v difference between the OBD2 reading but I’ve never experienced it on my two Tacomas. My truck will sit at the proper voltage in park at ~670-700rpm. But in drive/reverse at those lower rpms it's voltage drops.

Thanks for the input on how your truck behaves. My manual 1999 is the same. No voltage drops when I kick the blower motor on or headlights. Just sits at ~14.3.

 

Perhaps check in the area of the neutral safety switch and the reverse light switch .

Solenoids drawing to high?

Something in the transmission causing a current draw but not enough to open a fuse.

then you might have already done this.

Good luck!

 

Since it does it in drive I’m not too concerned about reverse lights but I had that thought too.

I did have to replace/repin every electrical connector on my transmission (cracked brittle plugs) but I took photos and was very cautious. Plus I’d assume if I’d pinned it wrong I’d be getting a code or improper behavior from the transmission.

I can try reseating all the connectors but the truck is too warm to touch right now. Fluid level was proper but maybe I have the shifter linkage out of adjustment and it’s not properly shifting into gear or something? I’ve had it out of adjustment in the past with no issues.

 

It could be even wires rubbing in the harness something pinched shorting to anther circuit but not to ground.

 

That’s a depressing thought but possible. Not sure how many more weekends of troubleshooting I have left in me with this triple digit heat. I’ll have to dig into it next weekend and see if reseating plugs helps at all.

 

I really wish I was closer I would love to help.

Best of luck .

 

Definitely could be transmission related. Like if I hit the brake and shift to reverse the voltage drops. But then if I shift into drive from neutral without hitting the brake (parking brake holding me) the truck is fine. Stays at ~13.7v. Hitting the brakes in an auto sends a signal to the transmission if I'm not mistaken so that sort of makes sense.

That's about it for tonight. Also tried reseating both transmission plugs.

 

The alternator has to be grounded… you would think that the mounting bolts would enough but it is worth checking. With your volt ohm meter set the scale to the smallest scale, usually 20VDC, put one lead on the negative battery cable then the other on the negative clamp, you are checking for a voltage drop, anything over .3v DC is too great. Do this between the alt case and the negative battery post and for all of you other wires. Is the chassis to P/S head in place?

I do not have a manual for an 04 Tacoma so i do not know if the following applies. Modern auto charging systems use the PCM to regulate the charging of the alternator for emission purposes, no to low charge at idle speeds.

 

I just can’t comprehend why it would’ve suddenly become an issue. Then again I did remove the alternator bracket and torque it back on to spec during the head gasket job. So I guess that’s a change by removing it at all. I’ll try this tomorrow for sure.

The passenger side head ground was replaced with a larger gauge wire but I was suspicious so I’ve reinstalled the factory one with no changes in behavior.

I have an 03 manual which is identical to my truck but I’m not following exactly what you meant at the idle speeds part.

 

Alternators usually have an internal voltage regulator controlled, well, internally, modern cars control the charging voltage from a signal from the PCM, like I said, is yours one of those, dont know. Many a tech has chased a false trail only to discover that that is how it is designed. The voltage drop test is the test to use.

 

Attempted to do a more thorough voltage drop test. The alternator test and two grounds were about all I could do with my voltmeter that doesn't have clamps. Alternator casing to negative battery terminal read roughly 18mV/.018v. I ran an extra ground cable to the battery anyway from the alternator mounting bolt but as expected it didn't help. I've ordered voltmeter cables specific to my model that have clamps so I'll be able to go after remaining grounds later this week.

I've posted the charging system wiring diagram for an 03/04 truck below. It seems to point to the alternator regulator (which after taking apart a few Tacoma alternators over the years makes sense) but I'm not an electrical expert. At this point if I want to solve this issue and actually drive the truck I'll need to sink some time into learning.

 

Yep, internally regulated, no PCM control. Those voltage drop readings are OK, no problem there.

 

Does your charge light come on with the ignition switch in run position to make sure the light works .

It would not be the first time the nut on the B terminal of the alternator was loose causing out put problems

Being your in Texas you should not really see the corrosion I deal with but it is always possible on the 3 pin alternator plug.

 

Do you still have the HKB Alt-S fuse installed? If so, remove it and replace it with the stock 7.5 fuse then re-check.

 

Yeah I removed it after first start when I noticed all the voltage fluctuations. Didn’t have a Toyota fuse (just cheap Chinese ones) but I’ve swapped the Toyota ECU 7.5 fuse in it’s place just to be sure.

 

Yeah it does. 3 pin looks visually fine but I did rewrap the plug into the harness like it’s done stock and even folded the excess wire length up the same.

Not worried about the B terminal just because no corrosion and I’ve tried 3 alternators between this Mean Green, the stock, and Denso 105 amp off my other 1st gen.

 

So if its still doing it with HKB out, I vote put it back in, verify the alternator is good, verify batt is good and send it along it's merry way. By all means monitor of course.

Voltage readout is going to fluctuate a little with RPM/load change, right? Unless I'm missing something, that's how I see it. I need to watch your video again but it didn't seem like a huge jump the first time I watched it. Seemed fine imo.

FWIW, I'm still running that HKB fuse. No issues. I'm still on my stock alternator.