Speedo default
Thread Starter
Intermediate
Joined: Aug 2012
Posts: 30
Likes: 0
From: Tandragee Co Armagh
Racer
Joined: Oct 2011
Posts: 265
Likes: 5
EDIT: True measurement should be with a volt meter between battery positive and chassis ground, rather than the DIC.
Pro
Joined: Aug 2010
Posts: 622
Likes: 169
From: Montgomery Texas
The cutoff voltage I'm talking about occurrs when your battery is already low and you begin to crank the engine. Because of the load of the starter the voltage will drop below a critical threshold and cause the memory functions to default. That number is generally around the low 11's or high 10's. The 12.5 volts is the point at which the electronics starts to work funky but some functions work and others do not.
The gauges never show you the true voltage of the battery and that's because with the key on and engine off there are always some circuits in the C5 loading the battery.(BCM, PCM and IPC to name a few). With engine running you are reading approximately the output voltage of the alternator. Nice to know the alternator works but for some functions that occur BEFORE you start the engine, the battery could be low enough to result in funky things. It's interesting to know that the car will start with a no load voltage of less than 12.2 but the electronics will not be happy.
True no load voltage is measured with the neg battery cable disconnected and the meter directly across the battery posts. Once the no load voltage is below 12.5, count on the electronics doing strange things in the C5.
The gauges never show you the true voltage of the battery and that's because with the key on and engine off there are always some circuits in the C5 loading the battery.(BCM, PCM and IPC to name a few). With engine running you are reading approximately the output voltage of the alternator. Nice to know the alternator works but for some functions that occur BEFORE you start the engine, the battery could be low enough to result in funky things. It's interesting to know that the car will start with a no load voltage of less than 12.2 but the electronics will not be happy.
True no load voltage is measured with the neg battery cable disconnected and the meter directly across the battery posts. Once the no load voltage is below 12.5, count on the electronics doing strange things in the C5.
Just because the PCM works the engine starts, is not a reliable indicator that the battery and associated connections are providing sufficient power to the other circuits so that they function as expected. The PCM may function at 10.5V, but every other electronic device will have it's own threshold of pain. You would be surprised how low the battery voltage will drop during cranking, with a marginal battery. This is the first, best place to start checking.
Race Director
Joined: Apr 2007
Posts: 11,150
Likes: 890
From: South Western Ontario
Is it a Canadian or Euro car with 300kph speedo?
Try setting the mph as well as the mirrors and HVAC and seats and setting the memory for whichever remote you use, or just set all 3 memories.
Try setting the mph as well as the mirrors and HVAC and seats and setting the memory for whichever remote you use, or just set all 3 memories.
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