Mod Details

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Get The Right Alternator

When I initially fitted a replacement alternator to my Roadster it didn’t actually work. I thought I had misdiagnosed the fault but actually I had been sold the wrong alternator for my car. On the smart there seems to be 6 different alternators but although they all fit, they won’t all work.

The big problem now is that Smart are no longer selling the alternator for the Roadster. People seem to having good luck with the following alternators.

Works (Apparently)

I haven’t personally tested any of these but…

Magneti Marelli MAN1009 (confirmed by S2Smarts)
Herth+Buss 32063341 (confirmed by Peter G)
Lucas LRA02000 (confirmed by Andy D)

May Work

Nigel C contacted me to say that he fitted the Lucas LRA02000 alternator and found it didn’t work but Lucas sent him a diode to fit inline between the ECU to alternator wire. Once fitted, it worked fine. I’ll get into the details later.

Denso DAN521 (confirmed by Karl S M). This alternator used to work fine but apparently Denso started selling 2 types with the same number. 1 has a round rectifier and 1 has a square rectifier. The round rectifier apparently works and the square rectifier doesn’t. Check the bottom of the page for more details on this.

The AutoElectro AEC1734 (confirmed by Daniel G) all worked fine but now there are reports of some not working, so it looks like something has changed. Probably best to avoid.

What If I Ignore You?

From the number of emails I get from people who have ignored the primary rule, there is a very good chance you’ll be sold the wrong one for your car. For most car parts retailers, there is 1 alternator for every smart. So, if you do get the wrong one, you’ll waste time and money and you might even damage your car.

What Faults Can Occur?

It can be something simple like the battery not charging all the way through to the car not starting, the engine not running correctly, engine check light on, numerous stored fault codes, the car not revving, revs hunting and even the electrical destruction of the ECU. You may get a P2041, P2042, P2047 or P2018 error code if you check with a generic error code reader.

Diode Fix

The Roadster alternator changes the amount it outputs depending on how much the car requires. If the car has just started and the battery is charging, the alternator outputs more voltage. If you have the AC on, more voltage, headlight on, more voltage. The amount that the alternator creates is governed by the ECU. If the battery is topped up and there is little electrical drain, the alternator will stop creating electricity and the car will run off the battery.

The ECU sends a DC voltage into the alternator to initiate a magnetic field within the rotor. This starts the generation of electricity.

I don’t know for sure but it looks like alternators that don’t work are feeding a current back into the ECU. This is causing the ECU to have a little panic. The conclusion to this panic is one or more of the faults previously mentioned.

So, to protect the ECU, Lucas (the alternator manufacturer, not a person) recommended that a diode is placed in-line of the wire between the ECU and the alternator. This allows the voltage to be sent to the alternator but nothing can be sent back to the ECU. A diode is an electrical check valve that only allows current to flow in 1 direction. Nigel C tested the diode that Lucas sent and the alternator worked properly. Thankfully he also made a note of the markings on the diode and they are easy and cheap to buy.

This worked on the Lucas alternator but it may also work on other brands that don’t function correctly when fitted. If you try it and it works, let me know.

10SQ050 Schottky Rectifier Diode

I crimped and soldered ring terminals to the diode but this is just a pain and unnecessary.

You are better off just finding the correct wire to the alternator (white wire from ECU Plug 1, Pin 47), cutting that wire, slide on heat shrink and solder the wires to either side of the diode.

Once it’s soldered in place, protect the soldered joints and diode with the heat shrink tubing and use fabric loom tape to tape it back to the original loom.

How Do I Know The Alternator Isn’t Working?

With a cheap multimeter (that you really should own as it’s an essential piece of kit) you can test the alternator from the battery as seen on this page.

Before You Start

Check the engine to body earth cable. Make sure it’s tight, undamaged and the connections are clean. Same with the body to battery negative connection.

Electricity needs a circuit and the alternator earths itself through the engine. If there’s a resistance in the earth cable, you can lose voltage.

Method 1

This is an update to the existing page. The old method that was found here and on similar sites just wasn’t/aren’t well thought out. After having a little alternator crisis with my Roadster which resulted in having to remove and refit the alternator 3 times, I worked out a better way.

Jack the car up, remove the rear wheels, disconnect the battery and remove the arch liners on both sides. Remove the 3 plastic nuts with a 10mm hexagon socket and pull the arch liner out.

Protecting the belts is a plastic cover. Pull out the front plug…

Denso DAN521 Issues

The original version of the Denso DAN521 had a round rectifier which apparently worked correctly.

Then at some point, Denso changed to a square rectifier version which didn’t work.

Credits

Thanks to Tim for pointing out the earth cable issue.