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Discussion Starter #1
'96 4x4 ext cab
5vzfe

Really stumped by this one! I've had this truck for 2 yrs and done a whole lot of work on the thing myself.

Recently I noticed when I turn the headlights on the temp gauge indicates a high temp. Additionally if I'm running a few electric components like the radio or fan and I turn on the directional the temp indicator needle will flick back and forth in sync with the blinker.

I have an obd2 app and I can see that the engine is actually running at normal operating temps. Based on these symptoms I was thinking the alternator might not be generating enough power. Using obd2 to look at running voltage I could see that the engine would do about 14.0 v with no electrical components on, but if I switch on the headlights it would drop to around 12.5 and if I added any combination of radio, blower fan, or power windows the voltage would drop to as low as 11.9 v even at high rpms. The headlights also seem kind of dim.

The battery is relatively new so I decided to replace the old autozone alternator with a new denso unit. To my horror after replacing the alternator the described symptoms remained.

I plan to get the battery tested, but I am reluctant to believe that the battery is the problem at this point. This low voltage condition appears to be slowly worsening. The truck is also just now throwing codes p0171 and p0301indicating lean upstream O2 and a misfire respectively. Nevertheless the truck is running smoothly and I can not find a vacume leak.

Have you experienced something like this?
Please help!
 

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1997 T100 RWD Auto
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165 Posts
It sounds more like you have a short in the wires going to the temp gauge, or possibly the wire going to the little sender (not to be mistaken for the two-wire temp sensor that is located towards the front of the engine).

Do you know where the sender unit is? You cannot really see it that well but you can reach back there and feel it...check if that wire is plugged in.
Here is a video where they guy changes it out so you can see where it it
 

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1998 T100 SR5 2WD
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One of the first questions I ask when there are electrical gremlins is: do you have a trailer light adapter installed? If so, remove it from the circuit and see if the issues still exists. Bad trailer light adapters are known to cause electrical issues when they go bad. ;)

The next question would be is: how good yer ground connections? How good are yer battery connections? Snap some piccies of the battery connections and post them. ;)

Ground points to check are the left fender, engine ground (just below the oil filter), firewall near the passenger side and connected to the rear of the engine, and the brown wires on the air plenum near the diagnostic connector. ;)

BTW, what is yer idle RPM running at? ;)
 
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1997 T100 RWD Auto
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..do you have a trailer light adapter installed? If so, remove it from the circuit..

That! First thing I did when I got my truck was pull all those wires for the trailer hookup (someone had done all sorts of half-splicing for who-knows-what) and it fixed all kinds of issues that were seemingly unrelated.
 

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1998 T100 SR5 2WD
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As far as yer misfire goes, double check yer spark plug to make sure there aren't any issues with it. Also, double check the boot on the coil on plug (COP) to make sure there aren't any cracks or issues with it. I usually do the squish test or look fer carbon tracks on the plug or boot. ;)
 

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1998 T100 SR5 2WD
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Yer P0171 is usually a vacuum leak. What are yer fuel trims (short and long) reporting at idle after warming up to operating temp? ;)
 

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Discussion Starter #7
I appreciate the feedback on this. I do not have a trailer light adapter plug. I checked my engine grounds on the left and right and they both look solid to me. Battery posts are plenty tight and free of corrosion.

Idling around 600 rpm when hot

I guess I'm not understanding the loose wire thing for the temp sender. If that connection was bad wouldn't I get no reading on the temp gauge? Can someone explain this? Is the hypothesis that I have exposed copper that's getting a 'contaminated' voltage signal when other electrical components go on?
How does this explain the low running voltage?


A more pointed hypothesis would be appreciated. I don't really know what I'm looking for.

🤙
 

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1998 T100 SR5 2WD
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Thanks fer the piccies! :)

With weak or poor grounds, the electrical components will seek another path to ground. This can cause the issues you currently have with yer vehicle. ;)

Looking closely at the left fender ground, I can see some green corrosion on the wires. I would go ahead and clean up the battery terminal connection (remove that rust on the wing nut, washer, and threads) as well as the left fender contact point. I'm not a fan of the wing nuts and would replace them with a proper hex nut and tighten them down to about 14 ft lbs. I would go ahead and replace that ground wire with the proper terminals and some good 8 or 10 gauge stranded copper wire. You might want to install a proper battery tie down. A battery bouncing around will can shorten a battery's life from the shock. ;)

It probably wouldn't hurt to pull the engine block ground and clean it up really well. Through the years of oil and gunk could affect the contact point of the ground. ;)

Were you able to locate the firewall ground on the passenger side? In the OEM configuration, there will be a black quick disconnect. ;)

Here's a couple of videos on measuring yer grounds and voltage drops. ;)
 

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1998 T100 SR5 2WD
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Any suggestions to a tie down?
You could git the OEM one from the dealership or find one in the parts yard near you. Even a ratchet strap would be better than nothing. ;)
 

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1998 T100 SR5 2WD
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@neal_logic

BTW, yer fuse box should really be covered by something as debris and moisture can affect the electrical contacts. Ideally, the proper fuse cover can be found in a parts yard. ;)

I just noticed you have two 20A fuses installed in the fuse box. Those fuses should be a 10A (AC) and a 15A (HAZ/HORN). Yer taking a big chance in burning up the wiring harness should anything go wrong. The proper rated fuses are meant to protect the wiring. If yer popping fuses, you need to determine why and what is causing them to pop. ;)
 
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