I've been having this really bad issue where the voltage drops whenever the Lights or blinkers are turned on causing the voltage to drop all the way down and the car to start running poor. I've replaced the alternator and checked the fuses but can't seem to find the issue. Its not caused when pressing on the brakes only when the lights are turned on or when the blinkers/hazards is hit. Any help would be very much appreciated as my brother is needing the car here soon.

Start checking power connectors and grounds.
Make sure all the contact points are clean.

Does the voltage drop when you turn on the lights and the engine isn't running? If so, then that rules out the regulation of the alternator and only leaves either a weak battery or bad connections. If the engine starter is working OK then that rules out a weak battery and leaves highly resistive connections. I would inspect both battery cables, the positive connections to the block under the connectors near the battery and the grounds.

The most difficult ground to check, but one that often goes bad due to corrosion, is the ground for the rear wiring harness. It's located on the upper frame rail of the passenger side rear wheel well. The plastic wheel well liner needs to be removed, or at least loosened, to access this ground.

See my Cave, Wire Service
But...
⚠️ Warning: 84 Wiring has many things are 84 only and many later diagrams are invalid for you.

Clean and grease ground ends bolted/screwed to block etc w/ permatex brake grease. (partly because likely need it for brake work too.)
Prevents water etc problems and won't wash out easy. Thin coat is all you need.

Hidden G400 only affects taillights and maybe Cruise parts. Don't bother w/ digging out the inner fender until you find real cause.

Besides checking/fixing grounds, battery cables, etc...
See my Cave, Alternator Sense
Iffy sense wiring will cause many problems.
Many bypass C500 for alt output too because this connector can burn up when have crap contact.

84 has 2 or more Fusible Links to the Starter Solenoid like have problems too. Check/clean all wires to the starter.
Do Not cut fuse links. If "bad" you must replace them properly. see cave.

Note: Low Volt from Alt often causes Battery problems.
Check battery volts and charge until see 12.6-13v and leave battery unconnected from anything overnight. Should stay about same volts next day.
Volt drops then battery is likely bad. Check at walmart AZ etc w/ computer battery tester but even dumb load tester likely fails too.

If you need battery cables, Only use ACDelco battery cables. Aftermarket cables w/ Lead battery ends won't last long w/ side term batteries.
take off bolts and cover on battery ends, Coat them and battery "posts" w/ brake grease, then install.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

[This message has been edited by theogre (edited 03-30-2022).]

I'd like to note that the problem I'm having is only when the car is running and since I have an 84 I can see on the voltage gauge that when I press the blinkers it won't change nearly at all and then it suddenly drops and the car starts dying. This problem also occurs when I turn on the headlights whether they are already up or not. The battery cables aren't the greatest but they aren't rotting or have exposed wire. I hadn't had any of these issues until just recently when I started using it more after winter ended. Is there any specific area where the headlights and the blinkers coincide?

The headlights and the ECM both have fusible links that are connected at the junction block near the battery, under the large connectors. If the connections there are corroded you could have problems like this.

From your clarification I assume the lights and turn signals aren't dimming or turning off but rather only the engine is dying so you may have a bad fusible link F to the ECM (or whatever link letter is used on the 84) as well or in addition.

The fuse block has the source for the gauges fuse and ECM fuse tied together so a drop to the ECM would also show a drop on the voltmeter. It might make sense to check for corrosion in the fuse block, too. I suspect the junction block first since it is subject to weather more than the fuse block.

I made a video of almost exactly whats happening except its much worse when actually driving.

Also, the brake light fuse is hot at all times whereas the ECM and voltmeter aren't so that current path is completely different. That would help explain why the brake lights don't cause the issue.

The 84 doesn't use a Battery Junction Terminal.

Battery power is pulled off of the starter lug.

quote Originally posted by maxalator: I made a video of almost exactly whats happening...

That's weird how the RPMs show to be going up and down exactly in sync (and with absolutely no delay) along with the voltage gauge when the turn signals are on.

The voltage is also shows low for a running engine. Check with a multimeter at the battery while the car is running to get a better picture of what's going on.

[This message has been edited by Patrick (edited 03-30-2022).]

The voltage being around 13 is pretty normal it seems as other 84s seem to hover around there. The battery is new after the one the previous owner put in sprung a leak so I've already ruled out the battery being a part of the issue. I've been doing some looking and cleaning but no dice so I'm going to take apart the steering column as my father has advised to since he seems to think I might be able to find the fix under there.

[This message has been edited by maxalator (edited 03-30-2022).]

quote Originally posted by maxalator: The voltage being around 13 is pretty normal...

The voltage gauge in your video is displaying more like 11.5v.

Before you start ripping your steering column apart... does your 4-way flasher also produce the same symptoms? If so, maybe leave the steering column alone.

Yeah the hazards cause the same issue but its even worse than the blinkers. I'm not even going to attempt to make a video of it because I feel I could damage something. The voltage gauge almost always reads right around 13 even when running a bunch of things at once. Its reading at the 11.5 mark because I hit the blinker and it was causing my draining issue

[This message has been edited by maxalator (edited 03-30-2022).]

You "Claim" is only HL bulbs and Turn light cause this is Wrong.
Dash gauge reading 13v w/ engine running is Low Volts and is Not Normal even for 84 Fiero.
Tack bouncing is common because used same power and ground in the dash as volt gauge and more.

Even if a dash gauge reads wrong... You can't trust them to find the problem.
Use a real meter on Battery, alt output, and other places.
Should see ~ 12.6-13 engine off w/ full charge battery, 14.5-15 engine running.

Engine and body has many "ground" bolted/screwed to them, between frame and engine, and more in engine bay, in cabin, in front of car.
Not all "Grounds" are power grounds but return lines like for ignition and "one wire" O2 sensor.
1 large ground between engine and frame carries major Alt Ground and often have problems or is gone.

Iffy Alt Sense wiring can do similar as bad grounds or cause high volts.
Alt Output Goes Thru 84 Fiero C500 in a round about way and crap connection there can Burn Down your car.

Link A and C500 E4 runs Most of the car including the dash, ECM, Engine, AC and Heater, other Lights like the turn and brake lights.
Only HL Bulbs and Motors get power by Link B connected to the starter same way as Link A covering in Alt Sense page.
(Link splice w/ Alt Out says goes to EFI is only Standby Power to save ECM code(s), radio clock and setting.)

read wire service but location(s) of whatever maybe different or don't have for 84.
If you can't find 84 data elsewhere... go to alldatadiy.com to get Correct Data For 84 Fiero. They have Same Data as GM FSM unlike crap Haynes and others.

If someone have "modified" the wiring... you likely still need same to find just what they did and is it safe.

Once you find this problem... Continue cleaning/fixed grounds. Crap grounds often cause Dim HL bulbs and more problems.

quote Originally posted by olejoedad: The 84 doesn't use a Battery Junction Terminal.

Yup. Some people can't be bother most 84 Fiero wiring is different then later years.
Like Has the Same problems in other GM vehicles w/ fusible links to the starter causing problems and alt output and others going thru C100 that = to C500 in 84 Fiero. The links and ends mounted so low often get dirty/rusty, cooked by exhaust, and worse. 84 Fiero, V6 and V8 front engine vehicles have exhaust running near them and Road salt etc can eat them.

These need same cleaning as grounds above. If ends or links are "bad" then must Replace Them right or expect more problems.

The dash gauge does in fact read wrong and the actual voltage from the alternator and battery is reading correct on a multimeter. In my video the tach is bouncing at the exact same time as the voltage gauge due to the motor struggling with the voltage changing. If I was driving the car and hit the turn signal I wouldn't be able to accelerate due to the turn signal draining all of the voltage. I tested it with the hazards and the cars voltage drops down intensely and struggles to keep the idle at even 600.

You took dash out/apart?

Big plugs on back often have problem were hits copper on back board. the "glue" fails allowing the copper to bend/break that can short out whatever the "pin" connected too.

HL Switch can have problems it's own.

But Those should Not cause the main problem(s) that's killing the engine.
Iffy Grounds and Fusible Links may have enough Volts and Amps at one test point and barf in other sections of car.

Pull out cig lighter and test volts there and leave meter connected while try turn lights etc engine off and on.

If mostly/only happens while driving... Check motor/trans mounts and done bone. Crap mounts let engine to move way over spec that often pulls/pinches wires, hoses including vac hoses, etc. Can loosen or break exhaust and intake parts too.

[This message has been edited by theogre (edited 03-31-2022).]

I got my headlights fixed but the blinkers still cause the same issue and now I can get it to happen when only the accessories are on. All the gauges bounce when the blinkers are on and I think something is shorting under the dash or around the steering column. I'm not exactly sure what the problem is but I'm going to take it all apart to clean everything and slide it back together hoping for the best.

quote Originally posted by olejoedad: The 84 doesn't use a Battery Junction Terminal. Battery power is pulled off of the starter lug.

Good point. I forgot that.

The weather was finally good enough to work on the car today and I managed to get the issue fixed and I ended up finding that it was just a connector that wasn't all the way seated for the instrument cluster. Thank you everybody for your insightful information on my issue.

Do the rest to clear/fix grounds etc soon.
Just One iffy "Ground" in the "right place" can cause this and worse problems.

Example: Many hear are 2 front grounds for HL etc but only 1 of those ground the rad fan.
Many blame "bad" "weak" HL bulbs but iffy ground or fuse link B and rest sending power often makes HL bulbs dim.
If you put hotter bulbs, iffy wiring will have more problems and can burn down the car.
Rad Fan gets low volts because of crap wiring try to draw more amps and does same thing.