Voltmeter shot to 18v. Instrument cluster on dash started going haywire. Battery was spitting, so i believe the voltage going into it was too high. Headlights were going bright/dim. Mileage and turnsignal lighting on dash went on/off. Is this an alternator issue?

again,
all clusters made for vast decades have regulator there.
if not the meters go nuts. . as alternator, goes from 13v to 15v all gauges the needles wag. facts. (is this the secret problem?)_
where is it, varies on all cars made the is no law to make that happen.It will be where GM put it. of lost wow hard to find 34 years later;.
some are just blob in the dash screwed to a bracket.
some are on the cluster rear back PCB some are inside if rear PCB of the cluster.(means Dismantled fully )
on modern cars it has Gas discharge or LCD diesplays that try to mimic days of old needle meters and are 5vdc regulators for logic power. So we still have it, in fact the clusters now are computer that talk (communicate) on the CAN_BUS. network. but IT's still there.
for the same reasons (battery voltage goed from 10v to 15vdc. )

You never told why this HUNT started or a real problem with a cluster.? the oldest gauges were thermal, then later all are really voltmeters, scaled different. ALL can have needle stick this old.
I have repaired many.
The real GM factory service manual should cover this.
not some chilton rag books useless junk.
most times the real schematic that show no IP, regulator there
means it is in the works, in side the cluster deep.
but is there.

Headlamps flicker as well ? too much A/C voltage ripple , alternator problem , diode going bad . The instrument cluster could have a fractured solder joint , instrument cluster could be bad .

if flashing not a fuse need to check circuit wiring or connections for short or connection problems

NO , there is not a fuse just for those two thing's ! There are two power supply's for the whole insturment cluster . You could need a new cluster or the ECM isn't sending this info. to the cluster . Could have DTC'S - diagnostic trouble codes stored in the PCM or the insturment cluster . Without knowing how this works (insturment cluster gets it's info.) Your best bet , take it to a qualified repair shop that has the knowledge and test equipment to diagnose this problem.
Engine Oil Pressure Gage
The instrument panel cluster (IPC) displays the engine oil pressure as determined by the engine control module (ECM). The IPC receives a serial data message from the ECM indicating the engine oil pressure. The engine oil pressure gage defaults to 0 psi (0 kPa) or below if:
• The ECM detects a malfunction in the engine oil pressure sensor circuit.
• The IPC detects a loss of serial data communications with the ECM.
For the 4.2L LL8, the ECM uses an engine oil switch that is open when the oil pressure is normal and closed when the oil pressure is low. At normal pressure, the engine oil pressure gage will display 275 kPa (40 psi). At low pressure, the gage will go to 0 kPa and the low pressure warning will be displayed in the driver information center (DIC). At high pressure, the gage will display 551 kPa (80 psi). Any values between these three values are calculated values determined by the ECM.

Voltmeter
The instrument panel cluster (IPC) displays the vehicle voltage based on the information from the body control module (BCM). The IPC uses the serial data information from the BCM to provide a voltage level indication on the voltmeter.

DTC P0521: Engine Oil Pressure (EOP) Sensor Performance

DTC P0522 Engine Oil Pressure (EOP) Sensor Circuit Low Voltage
DTC P0523 Engine Oil Pressure (EOP) Sensor Circuit High Voltage

Volt Gage Inaccurate or Inoperative
2
Install a scan tool.
Turn the ignition ON, with the engine OFF.
With the scan tool, perform the instrument panel cluster (IPC) gauges output control.
Does the volt gauge move up and down when commanded?

Did you check primary voltage at the ignition coil? Pink wire, goes hot with the key on, any voltage there? That voltage comes from ecm 1 fuse.


I haven't looked for info on your instrument cluster, yet.

Instrument Cluster Diagnostic Trouble Code (DTC) Index DTC Description Source Action B1317 Battery Voltage High Instrument Cluster Go To Pinpoint Test C . B1318 Battery Voltage Low Instrument Cluster Go To Pinpoint Test A .
A1 CHECK THE BATTERY CONDITION

• Carry out the Battery - Condition Test to determine if the battery can hold a charge and is OK for use. REFER to Section 414-01 .
• Did the battery pass the condition test?

Yes
GO to A2 .

No
INSTALL a new battery. REFER to Section 414-01 . TEST the system for normal operation. A2 CHECK THE GENERATOR OUTPUT

• Carry out the Generator On-Vehicle Load Test and No Load Test. Refer to Component Tests in this section.
• Did the generator pass the component tests?

Yes
GO to A3 .

No
INSTALL a new generator. REFER to Section 414-02 . TEST the system for normal operation

The GEM has nothing to do with it !
Odometer/Trip Odometer
The odometer and trip odometer are displayed in the integrated circuit LCD display. The instrument cluster records the total mileage of the vehicle using information received from the PCM, and electronically displays this information in the LCD. If this information is not received, the LCD displays all dashes. How did you prove that instrument cluster isn't bad ?
Did you have a factory scan tool hooked up , check for DTC'S - diagnostic trouble code's ? View data to the instrument cluster etc...
Instrument Cluster Diagnostic Trouble Code (DTC) Index DTC Description Source Action B1202 Fuel Sender Circuit Open Instrument cluster Go To Pinpoint Test B . B1204 Fuel Sender Circuit Short to Ground Instrument cluster Go To Pinpoint Test B . B1205 EIC Switch-1 Assembly Circuit Failure Instrument cluster INSTALL a new instrument cluster. REFER to Instrument Cluster in this section. TEST the system for normal operation. B1342 Instrument Cluster Is Defective Instrument cluster CLEAR the DTCs. REPEAT the self-test. If DTC B1342 is present, INSTALL a new instrument cluster. REFER to Instrument Cluster in this section. TEST the system for normal operation. B1359 Ignition Run/ACC Circuit Failure Instrument cluster Go To Pinpoint Test A . B1317 Battery Voltage High Instrument cluster Go To Pinpoint Test N . B1318 Battery Voltage Low Instrument cluster Go To Pinpoint Test N . U2199 Invalid Data for Engine Coolant Instrument cluster Go To Pinpoint Test D .
Your best bet ,take it to a qualified repair shop.

• The speedometer/odometer is inoperative

• Powertrain control module (PCM).
• Instrument cluster.

• Go To Pinpoint Test F .

Instrument cluster maybe !

Checked Every fuse in the fuse box (no small task).This includes #4 - Power point Instrument Panel, #14 - Day time running lamps (auto lamps doesn't work nor does auto lock), #27 - ignition switch run feed, #35 - Instrument Cluster, #41 - Instrument Cluster, #42 - Delayed Accessory, #45 - Ignition switch run/start feed, #102 - Door locks body security module, #103 - Ignition switch, #110 - Ignition switch, & #116 - Ignition switch. But I also check all other ones as well, they all register OK. AC panel & radio work. Turn signals work. Mileage does not work, trip computer does. So, basically, everything works except auto head-lights, auto locks, and the dash cluster (no MPH, RPM, mileage display, Fuel, Temp, Tran Temp, and Oil Pressure).

It could be the voltage regulator for your instrument cluster is the problem.