actually, thats what the guy suggested. hes like 'i know the old ones used to go backwards when you drive in reverse, i didnt think these did but maybe thats what happened.' i ended the discussion at that point

 

My odometer adds miles if I drive in reverse

I found this when I drove over a block in reverse.

 

Just had my starter, battery and plugs replaced Thursday, both odometers (a&b) were reset, picked car up with 2 .1 miles on them, strangely enough my radio FM1 memory was erased but FM2 was not? Results from disconnecting battery. The new starter sounds much different, definately turns the engine over at higher rpm. Engine is running in 1 second or less of cranking.

 

They're supposed to record your radio presets before disconnecting the battery (so they can reprogram once they're done). Maybe they forgot to check/reprogram FM1?

 

cant really offer any info on the odo thing, but the last time i dropped off my car at a shop for a dent in the bumper (some bitch backed into me in a parking lot) when i picked up my car, i dont know if they were ******* with my radio or what, but all my bass and treble were turned to max, everything else was saved/set its just like someone must have been cranking my bose system while they worked on it. Which makes no sense because you DONT have to disconnect the battery to replace a bumper.... who knows


Jeff

 

^^ You don't have too...but if you have worked on a lot of cars...you will.

It can save a lot of potenial problems. One car in particular...if you tried to replace the interior dome light it would short out the ECU

Always remove the -ve battery cable and save yourself a lot of grief

 

It's possible that I may be able to help shed some light on this situation. I am not exactly sure how the odometer functions in the RX8, but I can tell you how it works on a Volvo. Most likely, its something very similar in the 8.

On a Volvo, (all newer ones anyway) the odometer reading is stored in two seperate locations. One in the CEM or Central Electronic Module (This is the main computer in the car), the other in the DIM or drivers information module (This is the instrument cluster). The reason for this is that in order to change the odometer to fewer miles both computers would have to be replaced. If one or the other is replaced, for example the instrument cluster, it automatically updates the milage from the CEM into the odometer. In fact, it will always take the reading from the computer with the higher milage and record that as the new milage in the other.

Example, odometer replaced and CEM has 43,000 miles stored. The odometer automatically changes to 43,000 miles.

Example, if the odometer from a car with 20,000 miles was installed in a car that originally had 43,000 miles on it, the CEM from the car that the odometer was installed in would write the 43,000 miles over the odometer with 20,000 miles and update it to reflect 43,000 miles. In other words, you can't take an odometer out of a car with fewer miles than yours and put it in your car....it simply won't work...your odometer reading would remain the same.

The only way to change the odometer reading in a Volvo would be to replace both the CEM and the DIM at the same time. In this case, the technician would be able to enter the proper milage. This is the ONLY way a technician can change the milage.

In knowing how the system works, I also know that the CEM milage isn't updated at every mile on the odometer. I'm not exactly sure WHEN it is updated but I know it is not every mile. For example, if the odometer reads 40 miles, as you drive it will go up mile by mile. This milage is recorded to the CEM at a certain interval, (lets say every 5 miles). So at 45 miles, both the odometer and the CEM would agree. If you drive an additional 3 miles, the odometer would now read 48 miles, but the CEM dosn't get updated for another two miles so it still reads 40. Now here's the good part. If the battery is disconnected, usually for an extended period of time such as two or more hours, the odometer FORGETS its 48 miles and as soon as the battery is connected, the CEM automatically writes the 45 miles into the odometer. So when the battery was disconnected, you see 48 miles on the odometer. When reconnected you see 45.

That's basically it is a nutshell. I VERY SERIOUSLY doubt that anyone turned back your odometer as it is basically almost impossible to do without replacing multiple parts. Most technicians aren't even aware of how this system works and usually after most repairs the cars are test driven at least 4-5 miles depending on the type of repair and thus usually this goes un-noticed.

DISCLAIMER: This is how the odometer works in late model Volvo's and may or may not be similar to the RX8. The reason for storing the odometer in two seperate locations is to alleviate odometer tampering, or at least make it VERY HARD to do, even for a technician.

If you have any quetsions, please let me know.

Enjoy your ride......no one at the dealer is trying to get one over on you!

 

thats for the explination, thats really all i was looking for all along. while this makes things a lot clearer, it seems odd that disconnecting the battery would cause the cem to lose data. i understand that radio presets are one thing, but id think that there would be a place in the ram to store information such as odo readings

 

now THAT actually makes sense!! thanks for the info!

 

By its very nature, RAM can only remember stuff when there is power supplied, due to the principles of capacitor leaks. RAM is basically little chips with millions of tiny capacitors on it used to store data in digital form. This is a very efficient way to manipulate data, since a capacitor can release its charge in a split second, and since they are using so little power, recharge very quickly, allowing data to move and be manipulated at extreme speeds. Since this is an imperfect world, however, the capacitors loose a little bit of the stored energy every second, and without a source of energy to replentish, they will eventually loose it all.

If one of those minute capacitors is over 50% full, it registers as a 1, and less than 50% registers as a 0. If power is cut for a sufficient amount of time, all the capacitors will drop below 50% and the memory will be cleared. In contrast, in a magnetic or permanant memory space, the information is not dependent on capacitors, but on magnetic strips that once aligned stay aligned until disrupted by a sufficient magnetic field.

Think of it like autosave on Word or some other software program. The information being worked on is changed all the time, and it would not make sense to permanantly record it every milisecond. However, to prevent excessive loss of data, the program can write a copy to the permanant memory every 5 minutes or so. Since accessing and changing RAM bits and bytes happens hundreds of times faster and using less power than Permanant magnetic storage, it would be very inefficient if the data were constantly being written and rewritten to the permanant magnetic storage. Without high-speed RAM, the computer would be limited to a few thousand operations or less per second as opposed to millions.

 

Though that is an interesting way of outlining the function of RAM, it is not wholly accurate nor does it describe what is happening in the NVR in the RX-8 PCM and instrument cluster micro-controller.
Try this:
http://computer.howstuffworks.com/flash-memory.htm

 

Just chalk it up to them owing you a few miles due to the purposely inaccurate speedo . The car's smart enough that it might actually do it. Who knows.

 

Flash RAM like that used to store trip odometer settings and radio presets is as far as I am aware, a form of capacitor based memory storage like I described, which is why it does not retain your radio settings or the trip odometer setting if power from the battery is disconnected for too long. I was under the impression that the displayed vs permanant odometer reading was a flash RAM/ flash Memory setup, in which case what I said was essentially true: flash memory is still slower, and more expensive than flash RAM, but stores data in a way that does not require power to be retained.

Is this not the difference between what gets cleared when you do the brake pedal clear vs the odometer button clear? I know that one of them essentially is the same as disconnecting the battery, while the other clears the keep-alive memory which is flash memory. Obviously there has to be a third or more memory storage locations since the odometer is not cleared when the keep alive memory is cleared.

Anyone who thinks flash memory is fast has never used a digital camera or other flash memory device: while each individual bit is addressable much faster without seek times, but for large amounts of data transfer, without a fat highspeed bus connecting the flash memory to your processing unit, its about as slow as a floppy drive, which is why disk based storage still has the edge over flash memory.

 

The KAM is powered. There is a capacitive battery in the PCM that holds that data. Grounding the PCM for a few days will clear it also.
Flash RAM is not capacitive - it uses a tunneled transistor setup.

 

this entire thread for 1 mile missing on the odometer? Oy vey ...

 

I stand corrected

 

I can tell you that is not how Ford instrument clusters work. If your cluster needs to be replaced by Ford, the dealer orders a cluster from the Visteon service center. The dealer needs to advise the service center of the mileage so that it can be programmed into the cluster (Ford clusters have integrated modules). The cluster is then shipped with a core value. The dealer needs to return the OE cluster to the service center for inspection and repair. There is no CEM that stores the mileage as back-up for transfer to the cluster on Ford vehicles. I would not be suprised if the PCM stored mileage information based on that fact that the PCM reads the input from the various sensors used to measure speed, and then transfers that information to the cluster. If the mileage is stored on the PCM....then it can be transferred to a replacement PCM through the inhale/exhale process....but this information is not used to back-up the cluster.......if it were, then the service center would not need to program the mileage in. For older Ford vehicles that used mechanical odometers, the dealer would use a sticker to indicate the original mileage and the new odometer would be zero. Ahhh the good ole days

 

i just wanted to know how it could happen, but to be a conspiracy theorist perhaps they took my car home and out for a joyride overnight and just set it back wrong? if it was one mile over thats one thing, but one mile back is the issue