What is the cause of under voltage circuit 87?fault code p2600

Have the vehicle checked for DTC'S - diagnostic trouble code's if you haven't already . The vehicle has two cooling fans an three control relay's that are energized by the PCM - engine computer . Coolant temp sensor tell's the PCM engine temp.
Do you know what a wiring diagram is ? Free one's here http://www.bbbind.com/free-tsb Enter vehicle info. year make , model an engine . Under system click on engine ,then under subsystem click on cooling fans . Click the search button then the blue link. Watch this video.
Relay Driven Dual Cooling Fan Diagnostics

Also ,what are you going by to say it's overheating ? Temp gauge in the instrument cluster ? GM had problems with Stepper motors inside the instrument cluster . 2003 - 2006 . Hooking up a scan tool that can view data would show what the coolant temp sensor is tell the PCM .

Using a infered temp
This would show the real temp of the engine.

DTC P0480 or P0481
Battery positive voltage is supplied to the cooling fan 1 relay from the COOL FAN #1 fuse. The powertrain control module (PCM) controls the cooling fan 1 relay by grounding the low speed cooling fan relay control circuit via an internal solid state device called a driver.
Battery positive voltage is supplied to the cooling fan 2 relay and the cooling fan 3 relay from the COOL FAN #2 fuse. The PCM controls the relays by grounding the high speed cooling fan relay control circuit.
When the PCM is commanding a relay on, the voltage potential of the control circuit should be low, near 0 volts. When the PCM is commanding the control circuit to a relay, the voltage potential of the circuit should be high, near battery voltage. If the fault detection circuit senses a voltage other than what is expected, the DTC will set.
The PCM will monitor the control circuit for the following conditions:
• A short to ground
• A short to voltage
• An open circuit
• An open relay coil
• An internally shorted or excessively low resistance relay coil
When the PCM detects any of the above conditions, the DTC will set and the affected driver will be disabled.

I would say the instrument cluster is probably the problem . If the fans come on when A/C is turned on the system is working.

How to repair GM Instrument Cluster Speedometer Gauge Stepper

Make sure battery has a full charge and battery connections all good. Check fusible links where applicable. Basically, you check voltage and grounds for the system. Check the fuses in the fuse box for voltage, use a test light. Some fuses are hot all the time, some hot with the key on. If the problem is ignition feed, meaning it goes hot with the key on, you may have to check feed to and from ignition switch. I can only help with testing.

If your check engine light is on you should have it checked for DTC'S - diagnostic trouble codes ! An there is no SECURITY light lit on the instrument cluster ? In the underhood fuse / relay box is the CRANK RLY , this relay energizes the starter ! If you pull the relay out of it's socket you'll see four terminals or pin's what ever you like . You'll also see four numbers 85-86, 30-87 . 30 an 87 are the starter side circuit . 30 should have battery voltage if tested with voltmeter or a test light , 87 goes to the S terminal at the starter solenoid . 85 an 86 are the control side circuit , one should have battery voltage an the other is control from the PCM/ECM .If you can use a volt ohm meter you can see what is missing to further you diagnosis , voltage , control ! You mite want to do a voltage drop test of the starter circuit - battery cables ! Check fuse PCM/ABS 10amp left I/P fuse box an PCM (crank) fuse 10amp left I/P fuse box .If you haven't done this type of thing before you may want to take to a ASE certified repair shop ! Here I will post a video on relays ! VEHICLE RELAYS Operation Diagnosis

Could be the alternator , you can have it checked free at most auto part stores . Have them check for DTC'S diagnostic trouble codes as well . The charging system is controlled by the PCM - engine computer , if it see's a problem it will set a DTC and have the insturment cluster turn on the battery light. Plus the way the radio is acting ,it's loosing power .
DTC B1327
The body control module (BCM) determines battery positive voltage through the 10A Cluster/BCM fuse input. The BCM then sends a class 2 serial data message containing the measured battery voltage value. The vehicle control interface module (VCIM/OnStar®) module monitors this class 2 message to determine vehicle voltage.
DTC P0560
The PCM monitors the system voltage to make sure that the voltage stays within the proper range. Damage to components, and incorrect data input can occur when the voltage is out of range. The PCM monitors the system voltage over an extended length of time. If the PCM detects a system voltage outside an expected range for the calibrated length of time, DTC P0560 will set.
DTC P0620
The powertrain control module (PCM) uses the generator turn on signal circuit to control the generator. A high side driver within the PCM allows the PCM to turn the generator ON and OFF. When Generator operation is desired, the PCM sends a 5 volt signal to the voltage regulator via the generator turn on signal circuit. This causes the voltage regulator to begin controlling the generator field circuit. Once the Generator is enabled by the PCM, the voltage regulator controls generator output independently of the PCM by monitoring the battery positive voltage sense circuit. Under certain operating conditions, the PCM can turn Off the generator by turning OFF the 5 volt signal on the generator turn on signal circuit. The PCM has fault detection circuitry which monitors the state of the generator turn on signal circuit. If the fault detection circuit senses a voltage other than what is expected, this DTC will set. The voltage regulator also contains fault detection circuitry. If the regulator detects a problem, the regulator will ground the generator turn on signal circuit, pulling the voltage low. This also causes the PCM to set the DTC.
When this DTC sets, the PCM sends a class 2 serial data message to the IPC illuminating the Charge indicator lamp.

Or it may be more wise to have a qualified repair shop check it out .

P0620 - Generator Control Circuit Malfunctionhttp://engine-codes.com/p0620.htmlForum CodePossible causes- Failed Generator/Belt
- Battery charge
- PCM controlled generator
- PCM failure
- Open or short generator circuit When is the code detected?The PCM has fault detection circuitry which monitors the state of the generator turn on signal circuit. If the fault detection circuit senses a voltage other than what is expected, this DTC will set. The voltage regulator also contains fault detection circuitry. If the regulator detects a problem, the regulator will ground the generator turn on signal circuit, pulling the voltage low. This also causes the PCM to set the DTC.

Older Model Computer Codes (OBDI) - IMPORTANT! Codes may be different for newer vehicles starting circa 1996.
11 No ignition reference signal detected during cranking OR timing belt skipped one or more teeth; OR loss of either camshaft or crankshaft position sensor.
12 Battery or computer recently disconnected
13 MAP sensor or vacuum line may not be working
14 MAP sensor voltage below .16V or over 4.96V
15 No speed/distance sensor signal
16 Loss of battery voltage detected with engine running
17 Engine stays cool too long (bad thermostat or coolant sensor?)
OR
17 (1985 turbo only): knock sensor circuit
21 Oxygen sensor signal doesn't change (stays at 4.3-4.5V). Probably bad oxygen sensor
22 Coolant sensor signal out of range - May have been disconnected to set timing
23 Incoming air temperature sensor may be bad
24 Throttle position sensor over 4.96V
25 Automatic Idle Speed (AIS) motor driver circuit shorted or target idle not reached
26 Peak injector circuit voltage has not been reached (need to check computer signals, voltage reg, injectors)
27 Injector circuit isn't switching when it's told to (TBI)
OR (MPI) injector circuit #1 not switching right
OR (turbo) injector circuit #2 not switching right
OR (all 1990-) injector output driver not responding
- check computer, connections
31 Bad evaporator purge solenoid circuit or driver
32 (1984 only) power loss/limited lamp or circuit
OR
32 EGR gases not working (1988) - check vacuum, valve
OR
32 (1990-92) computer didn't see change in air/'fuel ratio when EGR activated - check valve, vacuum lines, and EGR electrical
33 Air conditioning clutch relay circuit open or shorted (may be in the wide-open-throttle cutoff circuit or normal on early 80's models if you don't have air conditioning)
34 (1984-86) EGR solenoid circuit shorted or open
OR
34 (1987-1991) speed control shorted or open
35 Cooling fan relay circuit open or shorted
OR
35 idle switch motor fault - check connections
36 (turbo only) Wastegate control circuit open or shorted
37 Shift indicator light failure, 5-speed
OR
part throttle lock/unlock solenoid driver circuit (87-89)
OR
solenoid coil circuit (85-87 Turbo only)
OR
Trans temperature sensor voltage low (1995 and on)
41 Alternator field control circuit open or shorted
42 Automatic shutdown relay circuit open or shorted
OR
42 Fuel pump relay control circuit
OR
42 Fuel level unit - no change over miles
OR
42 Z1 voltage missing when autoshutdown circuit energized (The Z1 voltage is the voltage of the circuits fed by the autoshutdown relay. This typically includes fuel pump and switched-battery feed to the ignition coil)
43 Peak primary coil current not achieved with max dwell time
OR
43 Cylinder misfire
OR
43 Problem in power module to logic module interface
44 No FJ2 voltage present at logic board
OR
44 Logic module self-diagnostics indicate problem
OR
44 Battery temperature out of range
45 Turbo boost limit exceeded (engine was shut down by logic module)
46 Battery voltage too high during charging or charging system voltage too low
47 Battery voltage too low and alternator output too low
51 Oxygen sensor stuck at lean position (this may be tripped by a bad MAP sensor system causing a rich condition. If you get hot rough idle and stalling, especially on deceleration, accompanied by flooded engine and difficulty restarting, that can be a bad MAP sensor causing the O2 sensor to try to compensate. If you get poor cold driveability, stumbling and bucking, and acceptable warm driving with poor gas mileage (a drop of 10 mpg or more), that is usually the O2 sensor.
OR
51 Internal logic module fault ('84 turbo only).
52 Oxygen sensor stuck at rich position
OR
52 Internal logic module fault ('84 turbo only)
53 Logic module internal problem
54 No sync pickup signal during engine rotation (turbo only)
OR
54 Internal logic module fault ('84 turbo only) - or camshaft sensor/distributor timing
55 End of codes
61 "Baro" sensor open or shorted
62 EMR mileage cannot be stored in EEPROM
62 PCM failure SRI mile not stored
63 Controller cannot write to EEPROM
64 Catalytic converter efficiency failure
65 Power steering switch failure
66 No CCD Message From TCM and/or No CCD Message From BCM

Has the vehicle been in a crash or minor accident recently? Have you installed any amps or anything under the seats? Are seatbelt tensioners working properly? These are typical issues.
Here is a list of airbag codes:
Air Bag Fault Code Table
AIR BAG DIAGNOSTIC MONITOR FAULT CODE PRIORITY TABLE
Priority Code Component/Fault Description
Highest -- No Air Bag Lamp - Inoperative Lamp Circuit or No Ignition Voltage to Diagnostic Monitor
' -- Continuous Air Bag Lamp - Diagnostic Monitor Disconnected or Inoperative
' 12 Low Battery Voltage
' 13 Air Bag Circuit or Crash Sensor Circuit - Shorted to Ground
' 21 Safing Sensor - Not Mounted on Vehicle Properly
' 22 Safing Sensor Output Circuit - Shorted to Battery Voltage
' 23 Safing Sensor Input Feed/Return Circuit Open
' 24 Open in Circuit 944B or Low Resistance in Crash Sensor(s)
' 32 Driver Side Air Bag/Safing Sensor Circuit - High Resistance or Open
' 33 Pin 7 Not Grounded at Diagnostic Monitor
' 34 Driver Side Air Bag/Safing Sensor Circuit - Low Resistance or Shorted
' 35 Low Resistance Across Pins 8 and 9 at Diagnostic Monitor
' 41 Crash Sensor Circuit - High Resistance or Open
' 44 RH Crash Sensor - Not Mounted to Vehicle Properly
' 45 Center Radiator Crash Sensor - Not Mounted to Vehicle Properly
' 46 LH Crash Sensor - Not Mounted to Vehicle Properly
' 51 Diagnostic Monitor Internal Thermal Fuse - Blown and Short to Ground No Longer Exists
' 52 Backup Power Supply - Voltage Boost Fault
' 53 Internal Diagnostic Monitor Fault
Lowest -- Rapid Continuous Flashing of Air Bag Lamp - All Crash Sensors Disconnected

Hello! This is a common fault on Neon Instrument Clusters...Cold solder joints on the Cluster Board are causing the fault...See link below to repair video...You can DIY or take the Cluster Board to "Geeks@ Best Buy" or an "Auto Stereo Shop" and have them solder those joints pointed out in the video..Guru..Saailer..

http://www.youtube.com/watch?v=CtwnsPmkX1M

I’m happy to help further over the phone at https://www.6ya.com/expert/david_6df67de3b14de867

There is a back up on the clock and the memory to the audio system which can casue this drain, So if there is malfunction on this circuit it can cause draining of the battery. Ideally there is only a trickle drain for the back up.
Also check the remote locking system and the alarm circuit as they are always on circuit.

DTC P0650 represents an issue with the actual check engine light (malfunction indicator lamp). Ignition voltage is supplied directly to the malfunction indicator lamp (MIL). The powertrain control module (PCM) controls the lamp by grounding the control circuit via an internal switch called a driver. The primary function of the driver is to supply the ground for the component being controlled. The driver has a fault line which is monitored by the PCM. When the PCM is commanding a component ON, the voltage of the control circuit should be low, near 0 volts. When the PCM is commanding the component control circuit OFF, the voltage potential of the circuit should be high, near battery voltage. If the fault detection circuit senses a voltage other than what is expected, the fault line status will change causing the diagnostic trouble code (DTC) to set.

A DTC P0650 indicates a problem with the circuit to the actual check engine light itself. If the fault is current, check if the check engine (service engine soon) lamp comes on when you turn the ignition on, If not, we will need to pull the engine control computer and ground the wire that goes to the light. If the light comes on, it is a failed pcm which must be replaced and reprogrammed. If not, it is either the circuit to the cluster or the cluster itself.

Good luck.