I'm curious if anyone knows at what voltage level one might expect to start seeing electrical problems, particularly with a 1999 XK8?

I’ve done a search and found many problems attributed to low voltage but nothing about what the lowest reliable voltage level might be.

I'm getting a recurring P1722 code (transmission "Stall speed") and some intermittent generic codes (P0732 and P0734). After perusing some BMW websites, I found a few cases where drivers experienced similar problems that were not mechanical failures (there are a variety of possible problems attributed to these codes.)

So, since my battery is at 12.2 volts I'm wondering if that would be low enough to potentially cause a sensor or solenoid problem in the transmission.

I was planning to pull the transmission pan and see what I found in there first but with the cost of fluid being rather expensive I'm now thinking about trying to check the fluid level, fully charge the battery and try a hard reset and see if the problem recurs.

I realize I'm being wildly optimistic here but does anyone see any likely harm that could come from this plan?

When taking off from a stop, the transmission is clearly selecting a higher gear. I've got to rev the engine to about 3k before it engages and when it does, it does so with a very loud "clunk." I presumed that was indicative of a mechanical problem but I'm now wondering if that noise might just be the result of starting in too high of a gear?

Thanks in advance for any help - I'm just trying to get some ideas and formulate a plan for this weekend when I get some time to work on the car!

 

You ask about the battery and the answer is that a battery should have 12.6v at rest with the engine off and no less than about 11v under cranking. The electronics, which includes the PCM, in this car will fail if you get near 10.0v which can easily happen with a weak battery while cranking.

The transmission issues is another separate issue. Your car has a transmission that has a history of 'A' drum failures caused or coincident with valve body issues that have been documented by ZF. Perhaps you can just replace the A drum but by the time you have the transmission out you might as well go for a full rebuild.

 

Somewhat of a fatalistic read on things, heh?

 

+1 on it likely being two different issues.

Let the car sit overnight and test the battery before starting. If it is 12.3v or less, the battery is low. You can try a recharge, but if it is an older battery, I'd go for a replacement.

You can always try that first. Won't hurt and is certainly easier/cheaper than working on the transmission. Do a hard reset and see if the codes or transmission problem comes back.

 

 

Not to split hairs here, but the chart for a traditional unsealed battery is different enough from that of an AGM or other sealed battery to matter. I believe Cambo's chart is for the traditional; the one I'm posting here is for the sealed battery. The voltage at 100% charge is about .20 volts higher, which changes the interpretation of any particular reading quite a bit.

Also there's a temperature effect. In really round numbers ... if it's around freezing, add .05 volts to your reading; around 0 degrees F., add .10 volts. This applies to both charts.