Low Voltage Issue

[noabarron3]

Hey y'all. I have a 1992 Blazer I'm working on. POS when I got it and I'm trying to diagnose a low voltage lssue now that it's running.

Alternator tested bad, along with a broken power wire so I replaced the alternator and re-joined the split power wire.

Starter was weak so I went ahead and replaced that and the battery is also brand new.

Would the ground from hood to firewall for the underhood light be a possible cause? It has been disconnected since I bought the truck.

Any other grounds to look for? Could it just need a brand new power wire to the alternator? Maybe a battery cable?

Kind of lost here. TIA.

 

[1998_K1500_Sub]

Where is the "low voltage issue" you speak of? You never said. Please elaborate.

The ground from the hood to the firewall is really only a factor for the underhood light BTW.

A heavy cable from the battery negative terminal should exist, terminated on a capscrew on the engine block.

Often a (~12 gauge) wire exists from the battery negative to the passenger's side fender, attached near the battery using a sheet metal screw (maybe a #14 screw) to the fender.

There are some ground straps from the engine to frame and frame to body, typically found on the passenger's side near the firewall. Those are worth checking or simply replacing as a "service item".

 

[noabarron3]

Where is the "low voltage issue" you speak of? You never said. Please elaborate.

The ground from the hood the firewall is really only a factor for the underhood light BTW.

A heavy cable from the battery negative terminal should exist, terminated on a capscrew on the engine block.

Often a (~12 gauge) wire exists from the battery negative to the passenger's side fender, attached near the battery using a sheet metal screw (maybe a #14 screw) to the fender.

There are some ground straps from the engine to frame and frame to body, typically found on the passenger's side near the firewall. Those are worth checking or simply replacing as a "service item".

When I start it up, volt meter reads between 9 and 12. Headlights are a little dim and the voltage drops lower when I turn them on. I'll replace those grounds. Thanks for the help.

 

[thx1138v2]

You might need to split the insulation on the big cables going to the battery and check them for corrosion under the insulation. Corrosion on the cables can cause a low voltage reading at the gauge. If you find that just replace the cables but don't do that before checking under the insulation.

 

[east302]

When I start it up, volt meter reads between 9 and 12. Headlights are a little dim and the voltage drops lower when I turn them on. I'll replace those grounds. Thanks for the help.

What’s your alternator output measured from back post at alternator? Compare to voltage at battery positive to ground.

 

[Donald Mitchell]

Clean all the grounds while your at it, including those on top of the motor. You'll be glad you did.

 

[noabarron3]

What’s your alternator output measured from back post at alternator? Compare to voltage at battery positive to ground.

This is also helpful, currently in Truckee CA, but when I'm back in Texas I'll give this a shot. Assuming alternator tests out good and voltage at battery to ground is weaker, what does that tell me?

 

[Schurkey]

Use a known-good (trustworthy, accurate, repeatable) voltmeter, tested on some other vehicle.

Engine Off, no accessories turned on. Doors shut, no dome light or stereo on.

Voltmeter + lead on battery +, voltmeter - lead on battery -. Voltage better be 12.6--12.7. Less than that--slow-charge battery, perhaps overnight. More than that--turn on bright headlights for fifteen seconds, turn them off, re-test.

Engine Running, all switchable accessories turned off:

Voltmeter + lead to alternator output terminal. Voltmeter - lead to alternator case. Voltage better be 14.x. Below 13.5, alternator may be defective. Above 15, alternator is defective.

Turn on high-beam lights, HVAC blower to high, windshield wipers on, rear defrost on, any other convenient accessories turned on. Raise idle speed to ~1500-ish.

Voltmeter + on alternator output terminal. Voltmeter - on battery +. Voltage better be 1/4 volt or less. Greater voltage means undersized alternator output cable or corroded connections.

Voltmeter - on alternator case. Voltmeter + on battery -. Voltage better be 1/4 volt or less. Greater voltage means undersized ground cables, loose/corroded alternator bracket(s) or corroded cable connections.

You may/may not need to extend the voltmeter leads using plain ol' primary wire and alligator clips. Voltmeter + on alternator output stud. Voltmeter - on various fuses in fuse-box inside cab. Fuses that have power should show 1/4 volt or less. Greater voltage means corroded connections, or under-size wire. Fuses that aren't powered may show alternator voltage, ignore them.

Voltmeter + to dashboard ground--most any dashboard steel structure. Voltmeter - to alternator case. Voltage better be 1/4 volt or less. Greater than that indicates poor grounding, corroded ground straps, or corroded connections.

Turn off all accessories, shut off engine.

 

[Supertex]

Use a known-good (trustworthy, accurate, repeatable) voltmeter...

So ...this is kind of an old thread, but your specifics appealed to me. I recently 'resurrected' my 93, and part of that process was adding the dual 16(?) inch fan/shroud combo from a 2005ish half ton. I've seen them hit 50A+ on startup, and then settle down to 22A-25A...each.

My issue is a little strange. It seems as though my alternator runs out of wind as it heats up, or ...something. And it does heat up. Like, pronto. It gets blister you hot, sometimes faster than the headers.

I bought a USB charger that replaces the cigarette lighter, and it has an LED voltage readout that I've verified to be correct. I use that for my 'inside' measurements. For under hood measurements, an old Wavetek multimeter I've had since before this truck was new. I've added an 8ga direct from the charge post to the B+. And this is a CS144 - supposedly the 130A version. Adding a ground from alt case to B- didn't seem to have any impact at all. Battery and Alt differential is .1v or less both + and -. Voltage drop to the fuse panel (lights and AC on) was interesting. Most places I see -.5v. That's gauges, turn/bu, PCM/IGN, Inj, 4WD, and brake. Third column was weird. HTR-A/C showed -.74, courtesy and parking lights were showing -.9v, and the panel lamps measured -1.8v, and stop being -.3. I suppose the gauge cluster connector is ...crappy?

Before I added the 8ga, I would see maybe 13.2(inside) on startup. Left to idle, it would eventually drop down to 12.4ish. Take off driving, it would go up to 12.8 and remain fairly stable there, even with accessories, lights, blower on high. However, pull into a drive through and idle, it would drop to 12 or even 11.8, and seems like it would drop more the longer I sat there. Turn off the blower and drop the headlights, it's back up around 12.2. Fast idle would bring it back up to 12.8.

Since the 8ga addon, I see 14.4 at the alternator charging lug and 14.2 inside, but only on a cold start. It eventually settles down to about 13.5 at the lug and 13.2 inside - without turning anything on. Take off driving, and it will stay about 13.4. Kick on the lights...down to 12.4 cruise, 11.9 idle. Add the A/C...12.2 cruise, 11.2 idle. Pull into my drive, leave lights and AC on, pop the hood and I have 12.3ish at the charging lug. Also at the lug, fast idle will bring it up to 13.2 or so. That's grounded to the alternator case.

At idle, I put my snap-gauge on the charge wire that disappears into the harness (where does that go, exactly?). It's showing about 55 amps outbound. When I check the 8ga to the B+, I see 14ish amps headed that way at idle, and revving up I saw it as high as 50 amps. When I check the cabling from B+ to the starter, and its secondary that runs the distribution block, I see close to 38 amps outbound. So as near as I can tell, my demand is around 93A at idle with A/C max and lights... hot... no wipers, window motors, stereo, etc. I'm guessing the 14A I see from charging lug to the B+ is part of the 38 amps I see leaving the battery. Would I be correct in thinking the alt at idle is producing about 69A, and the battery is providing the rest of the 93?

I've suspected the alternator for a few weeks now, and this is where I've been leaning: https://www.mechman.com/alternators/buick/lesabre/3-8l/1986-1991/250-amp-fits-6-12-cs144-cast-case/ No small drop in the bucket for just an alternator, but that's 200A at idle, fits my brackets, and should have headroom for a few stereo amps.

But seeing .2v difference inside to out when voltage is up, and as much as 1.2v difference when voltage is low, makes me wonder what the hell is going on. Am I about to throw money away with a new alternator? I'm really pretty confused.

Hep meh!

 

[Pbeanbag]

Hi Supertex, in my searches for fuel pump issues I came across a video from a guy who had a low voltage inside the cab ( like 2 volts lower) he thought it was a bad ground and did everything suggested. he decided to return to checking all connections and found that the main power supply to the firewall cluster had (from the factory) been installed with the rubber cover under the washer and nut. this caused and arcing and low voltage acrooss all the power terminals. he removed the rubber ,cleaned up the contact area and reinstalled the wire and nut. Problem fixed!