Piston Slap: New tricks for an old car phone (Part III)

piston-slap-interior-phone-lead BMW

Mark writes:

Dear Sajeev,

I read an article that you wrote many years ago regarding analog-to-digital phone conversion. I am 86 years old, and want to auction my almost 25-year-old BMW M Roadster. When I purchased it, I had everything that was available loaded onto the car, including this BMW CPT 4500 Cellular Telephone System!

Mark the OP

This was pre-wired into the speaker system and could be used in the hands-free operation. Can you please help me find a proper tool to convert this phone to a 21st-century digital car phone? I want to submit the car for auction and need to be able to say everything in the car works.

Sajeev answers:

Thanks for reaching out! Sadly there’s no easy conversion to make these old phones work with the new digital network. While my previous Piston Slap suggested the cellular networks used to offer digital conversion modules for their customers, whatever kits they implemented are long gone. If someone from the industry would be so kind to post up a part number, I’d buy them all when they show up on eBay!

Or not, but I’d at least buy one to try on my Lincoln Mark VII or Mark VIII. It’s been said that you can take apart a car phone and successfully solder in the guts of a cheap, newer digital phone. But it still might be tough to get the phone to interface with the factory stereo on 1990s-era implementations.

Instead, adding a Bluetooth interface would be nice, and look what I found!

At some point, this creation might turn into an aftermarket part we can all purchase. You might wanna bookmark this page to see his latest progress on the Bluetooth adapter. But for now, you are better off submitting your M Roadster to auction in its factory state, unfortunately.

On the plus side, doing so won’t hurt the value of the vehicle, as vintage equipment doesn’t need to be fully functional to impress would-be buyers. People love old-school analog car phones: I leave the console door open in my Mark VIII when attending car shows, just so normal people (i.e. not Lincoln nerds) find a reason to point and exclaim as they pass by.

It’s worth having them installed, even if they do absolutely nothing … because they are still doing something!

Have a question you’d like answered on Piston Slap? Send your queries to pistonslap@hagerty.comgive us as much detail as possible so we can help! Keep in mind this is a weekly column, so if you need an expedited answer, please tell me in your email.


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    Sajeev, I have a Ma Bell coin-op phone booth phone (quarters/dimes/nickels) hanging on the wall in my shop office. If you can dig up a video on how I can convert it to cellular service, I’d love to install it in the cab of my Dodge pick-up. 😎

    Hmmm, that seems entirely too easy (and I suspect it will prove to be), but it IS intriguing and I’m gonna add it to the chalkboard “to do” list over my project bench (picture submitted long ago in a comment on Hagerty Community). Thanks, man!!! 😃

    You found my Bluetooth adapter project just a bit too soon. I now have it fully functioning and integrated with the original OEM hands-free system in my 1993 3000GT VR4: https://www.youtube.com/watch?v=52AuhS5Jq0Q

    I even have voice dialing/commands working through the car phone hands-free system, which would have blown everyone’s minds back in 1993.

    Unfortunately, my solution only works for this one specific model of car phone, so market potential is extremely limited. I don’t think I’ll ever develop this any further than a prototype.

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