what exactly is this code. I know it has to do with the charging system but any specifics?

 

Not much help but here is the description if you don't already have it.

P0621 Generator L-Terminal Circuit

 

Here are a few things to look at:

Service Charging System Message on DIC and DTC P0621


Models: 2005-2008 Chevrolet Corvette

2004-2008 XLR and XLR-V Series




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This PI was superceded to add information about aftermarket spark plugs. Please discard PIC4536A.


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The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Condition/Concern:
Some customers may comment that the Service Charging System message displays on the DIC. Using the Tech2, DTC P0621 is stored. This may be a result of aftermarket spark plugs or an open in circuit number 2540.

Recommendation/Instructions:
1. Check for aftermarket spark plugs. Plugs that are designed with a "Pulse" technology may interfere with this circuit and cause the Service Charging System message to illuminate on the IP and/or P0621 DTC store in the ECM. If aftermarket plugs are installed, replace plugs with OEM parts. This is not considered a warranty repair and all cost associated with this repair will be the customers responsibility.

2. Circuit 2540 is supplied voltage from the Horn fuse, position #2 in the UBEC. Apply the horn to see if it works. If the horn does not work, suspect an open fuse. This fuse may open if the horn is shorted due to water intrusion. If the horn is functional, check the integrity of circuit 2540 between the UBEC and the generator. Refer to SI document 1481125 for the schematic. Repair circuit 2540 as needed and re-evaluate the customers concern.







DTC Descriptor
DTC P0621: Generator L-Terminal Circuit


Diagnostic Fault Information
Circuit
Short to Ground
Open/High Resistance
Short to Voltage
Signal Performance

Generator Turn ON Signal
P0621
P0621
P0621
P0621


Circuit/System Description
The engine control module (ECM) uses the generator turn on signal circuit to control the load of the generator on the engine. A high side driver in the ECM applies a voltage to the voltage regulator. This signals the voltage regulator to turn the field circuit ON and OFF. The ECM monitors the state of the generator turn on signal circuit. The ECM should detect low voltage on generator turn on signal circuit when the ignition is ON and the engine is OFF, or when the charging system malfunctions. With the engine running, the ECM should detect high voltage on the generator turn on signal circuit. The ECM performs Ignition ON and RUN tests to determine the status of the generator turn on signal circuit.

Conditions for Running the DTC
Ignition ON Test
• No generator, crankshaft position (CKP) sensors, or camshaft position (CMP) sensor DTCs are set.

• The ignition is in RUN position.

• The engine is not running.

Run Test
• No generator, CKP sensors, CMP sensor DTCs are set.

• The engine is running.

Conditions for Setting the DTC
• During the Ignition ON test, the ECM detects high voltage on the generator turn on signal circuit for 5 seconds.

• During the RUN test, the ECM detects low voltage on the generator turn on signal circuit for 15 seconds.

Action Taken When the DTC Sets
• The ECM will command the charge indicator and or warning message to be illuminated on the instrument panel cluster (IPC) and the driver information center (DIC), if equipped.

• The ECM will not illuminate the malfunction indicator lamp (MIL).

• The ECM will store conditions, which were present when the DTC set as Failure Records data only.

Conditions for Clearing the DTC
• The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.

• Using the scan tool Clear DTC Information function can clear the DTC.

Diagnostic Aids
On vehicles equipped with a sense circuit connected to the voltage regulator.This circuit must have voltage for the charging system to operate properly.This circuit is used as a reference to control system voltage. All circuits must be tested at the generator before replacing any components. refer to system schematic to confirm if the vehicle being serviced uses

 

Thanks ChevTech!

The horn works, OEM plugs, the alt was replaced and I tried a new battery for grins but servive charging system comes on once in a while while idling at a light. It goes off right away though. I could have a short in circuit 2540 but i would expect this message while driving also not only at idle.

Dealer wants to replace the PCM but I am thinking of having my tuner shut off this code instead.

Your thoughts on this?

Rich

 

How repeatable is it? Can you put a meter on it and drive to confirm the voltage on the wire when the light comes on? If you can, try and follow the rd/wh 2540 wire from the fuse to the alt., that would be a good start, we have had some issues with the under hood elec. center as well. The terminal connection is very important, if at all possible see if you can confirm the drag on the terminals is good.

If all that is ok, and the voltage on the line is good but still sets the code, a PCM would seem the next step.

 

sporadic --cannot reproduce and only at idle

 

That sucks, pretty much what I stated before is where you or the dealer is going to have to go. How many miles, any thing been done just before this came up? Wash the engine comp. down, etc.? Any aftermarket acc. installed?

 

nothing new
315's
cai
FAST
headers
tune
23K miles
only wash the exterior never the engine

 

Only thing I can suggest is check the wiring around the back of the right cylinder, there have been some that rubbed to ground in that area. Also, any ground wire you can see or feel, make sure it is tight.
May want to talk to your tuner, I don't think it would have caused any issues, but if it did he hopefully would know. Good luck