Possibly a 2 part question:

I went for an inspection sticker yesterday. The inspection system found 2 tests "not ready": catalyst heater monitor and evap system monitor. I was surprised, and left. Checked readiness with scangauge. The above 2 tests read "not ready". 1 mile later they indicated "ready". On the way back to the inspection station they went back to not ready, then ready again. At the station they confirmed "ready" then performed the odb-2 inspection test and again read "not ready" on the above 2 tests. Baffled, I left again.


Perhaps related, my 2.5 year old 12v battery died last week. After reading hillbilly's posts here related to the 12v battery I have been monitoring the voltage on the 12v system via scangauge. I have noted the value varies from about 14.1v to as low as 12.1v, with no known cause (not related to speed, load, or ICE vs HV).


My questions are these:
1) has anyone experienced or have info / ideas of the emissions readiness?
2) what values on the 12v system would be "within normal limits" (ie 13.8v with an alternator, varying with RPM)?
3) does my readings of 12.1v - 14.1v indicate an issue with the dc-dc converter? Are there additional tests I can perform? Is this device adjustable as with dc-dc converters not used for HV's?
4) is it likely that the low voltage on the 12v system is causing the ECM to repeatedly "reset" and subsequently need to complete another drive cycle?

Any help is greatly appreciated

FWIW, I recently had the electronic steering assist motor replaced by the dealership. They indicated that GM Tech Services diagnosed the problem and that there was no diagnostic print out, and in fact that there was no such thing (though they clearly emailed some diagnostic information to GM Tech Services).

 

Low 12V will reset and those diagnostics may take time to run and pass.


Normal 12V should be between 12.8 and 14.8 depending on many conditions, NONE of them related to engine speed. The voltage is computer controlled.


Use of headlights, high blower speeds on HVAC, wipers will cause a boost in 12V.


In auto-Stop it should be 13 or a little more.

 

Thanks HH!

After spending 2 days reading through every thread on here (got back to the beginning of 2011. LOL)
I learned that there are 4 (?) modes related to the dc-dc converter, including, as you indicated, use of wipers, headlights, etc. It does seem my converter is putting out low voltage. I also learned that this device is adjustable, but through a config file (as opposed to the adjustment of variable caps, resistors, and inductors with a scope).

I agree that the repeated test reset is due to low voltage. Thus, the question would be:

is the dc-dc converter bad? I have no error codes indicating that, but I don't know for sure if the 08 Tahoe has such a code.

The alternative would likely be that there is a hybrid battery problem (if the dc-dc converter works at one of 4 preconfigure step down levels, then low input would equal low output). I am going to check the calculated pack voltage vs SOC. As I recall, 50% should correspond to > 302v at < ~107° F. I have never found the scangauge codes for min / max voltages, so I would have to go through each module individually if this was a problem, and perform the prescribed soc reset if warranted.

I also saw a reference in one post to a dealership diagnostic that indicated a "replacement of the 12v battery and subsequent reset procedure" was necessary. I am not familiar with any reset procedure associated with changing the 12v battery, other than the natural reset caused by disconnecting the battery. Are you aware of any other procedure?


Thanks again...

 

Not aware of any other procedures. If you replace a 12V battery, the change should be similar to a non-hybrid regarding the whole system. The 12V system voltage should run at the programmed level.

If you were to leave the lights on for 15 minutes or more and then start it, the voltage will spike up to re-charge the battery. Same in other computer controlled alternator vehicles.

All newer vehicles reduce 12V system voltage once they think the 12V battery is charged.

A vehicle is not a Power Conserving device. It is much more of a big resistor. Since Power = Voltage Squared divided by R... these systems set the voltage lower after some time following start up.

So with a new healthy 12V battery, the 12V should begin the key cycle maybe 13.5V. Then within 15 minutes of operation drop to 12.8 ish. 12.8 won't charge it but it will maintain it. Sometimes the reduction below 13V may take an hour.

If the converter is bad, they are more of an all-or-nothing device. There is always the "Kitchen Sink" alternator test.

Dry windows front and back wipers ON
Rear Defog grid ON
AC/Heat blowers front and rear full blast
High beam headlights ON

How is voltage looking here?

Now for the "over-the-top" pile on. This should droop the system a little.

With all that above pull up on ALL 4 power windows at the same time when they are closed. Don't keep them there long, bad for power windows.

That should be well over 150 amps of 12V loads there. A non-hybrid will not keep up at idle. You will have to rev it up.

 

Thank you for the help, as always.

I have not done the load test yet, but figured how to get through inspection. I will do the load test next to see what might be faulty.

Using the info I learned reading through all the threads here, I understood there are 4 modes for the dc-dc converter. By simply turning on the headlights while driving, the computer set the converter voltage to 13.5v output. That was sufficient to keep the computer from resetting. When parked at home I connected a 2A charger to the 12v battery to keep the voltage up until the next time I drove. Passed with no problems.

I will let you know the results of the load test you described when complete.