Thoughts and advice on the care and feeding of fine automobiles from Machine Aficionado and bestselling author John Elder Robison, owner of JE Robison Service in Springfield, Massachusetts

We are independent restoration, repair, sales and service for Audi, BMW, Bentley, Jaguar, Land Rover, Mercedes-Benz, Porsche and Rolls-Royce automobiles.

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Replacing the alarm batteries in a 1990s Rolls or Bentley

I get quite a few calls asking about battery replacement in the security and memory seat modules of Rolls Royce and Bentley cars.  In this article I'd like to show how we address that issue here at Robison Service.

The car shown in a 1996 Bentley Turbo but the job essentially the same in any Crewe Rolls Royce or Bentley car from 1990 to 1998

We start by unlocking the doors, rolling down the windows, and disconnecting the battery.  We open doors and windows to protect us from being locked out if the controller does anything funny.  Disconnecting the battery is important because the vehicle should be powered off for removal and refit.  Switching off the battery switch does not switch off the alarm in many later cars.

Now it's time for removing the dash top.  That exposes the metal understructure, the wiring, and the modules.  To remove the top, you first remove the wood fascia.  The lower dash panels also come off.  With those out of the way, you can access the screws holding the top cover in place.

The wiring is all neatly bundled together, and there's a lot of it!  There are many different modules under the dash in these cars.  Luckily, the folks at Crewe labeled each one with real human-readable tags and not just part numbers.

The alarm or security controller (labels vary) is usually located in the middle of the dash, above the radio.

The next view shows the area where it may be found > > >

Here is a security controller removed:

Unplug the controller from the car and take it out.  When the brackets are removed you can take the case apart and remove the circuit boards.  They unfold to reveal something like this.  The batteries are in the center and lower right in this image.  One is white; the other is blue.

In this closeup you can see the corrosion, but in the earlier photo you can also see that the corrosion is not so bad that the whole circuit board is eaten up.  That's why it's so important to change these now, before they fail.  If you get a failure there is a good chance your circuits are too eaten up to be fixed by a simple battery change.

A look at the battery give us the information we need to find a replacement.  I don't know where to find identical replacements; indeed I doubt they exist today.  But that's okay; we can get functional equivalents.  Reading the label, we see that this battery is 6 volts, 280 milliamp hours, and it's a nickel cadmium (ni-cd) type.  

In the USA we head for Radio Shack, where they sell a wide variety of replacement batteries for cordless phones and other devices.  We find a battery that has the same voltage, similar amp-hour rating, and the same technology.  Note that you need an exact match for technology and volts, but amp-hour rating is flexible.  

If you fit a lithium ion battery in place of a nickel cadmium unit, the charge rate will be wrong, and it will soon burn up.  If you install a battery of the wrong voltage, the system won't work, and may be damaged. We remove the old batteries and solder the new ones in place.  We tape them or glue them down, because they have a different shape from the originals.

The result should look something like this:

 Now we're ready for the final step.  We put the electronics back in the case, and plug the whole thing into the car.  We hook up the battery, and give it a try.  This one worked.

If you have a working module in your car now, and you change the batteries as a preventative step, you are virtually assured of success (provided you do it right.)

Battery replacement and circuit board cleanup as a repair procedure is a lot more chancy.  I don't know the success rate but I do know it's chancy enough that we don't do these repairs unless we have the car here, so we can test our work and perhaps even go in for further repairs if we do not get success the first time.

Luckily, this controller worked right off, and we put the car back together.

This Bentley should be all set for another ten years or so.

I'm sorry to say this


Simon Logan said...

Thank you for the valuable information. I am trying to find on your site some instructions on how to carefully remove the dashboard wood, but cant find any. Or instructions on how to access the climate control unit to diagnose it. As I understand that the batteries from the alarm system can damage the climate control unit. The fans are not running in mine, and the blend flaps seem to be stuck in a kind of constant cycle. The vents alternate between hot & cold air and open and close in turn. I would like to fit a test box and remove the unit. My car is a 1997 RT. Thanks. Simon.

John Elder Robison said...

Simon, you can buy the factory workshop manuals on CD. I think they cost $99 for a one year subscription. The dash wood is held by the visible screws and the nuts behind the pull knobs for the round AC vents.

Battery leakage would not be a likely issue with the climate control problem

Simon Logan said...

Thanks ever to much for taking the time to reply. I appreciate your help.

Anonymous said...

John, both of these battery articles are great! Thank you for sharing.

Any insight in regard to replacing the alarm system siren back-up battery?

Dpjr said...

Do the general principles in the article apply to 99 Arnage Green Label? Would love to give you the car to work on but I live in South Georgia!

Dpjr said...

Do the general principles in this article apply to 99 Arnage Green Label?

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