MX5 - battery not charging

[PeterGadsby]

Hi,

My son has a mk2 MX5 it is not charging the battery. We have replaced the battery and the alternator, but it still keeps losing charge.

Any ideas what it might be ECU?

- Pete

[Gad-Westy]

PeterGadsby said:

Hi,

My son has a mk2 MX5 it is not charging the battery. We have replaced the battery and the alternator, but it still keeps losing charge.

Any ideas what it might be ECU?

- Pete

Have you checked voltage across the battery with engine running?

[PeterGadsby]

Gad-Westy said:

Have you checked voltage across the battery with engine running?

Yep if revving the voltage drops

- Pete

[Gad-Westy]

What sort of numbers are you seeing?

[Dave.]

Check the earth on the powerplant frame (and the others while you're at it).

[PeterGadsby]

Gad-Westy said:

What sort of numbers are you seeing?

At the beginning of checking it was around 13V then when we revved it dropped toward 10v... My son fully charged the battery, managed to drive to work, but as he got there the electrics gradually stopped working & when he went to go home battery flat, we bumped the car then it started. Drove it about 100 metres then it stopped!

He's started to lose faith in this car.

- Pete

[skeeterm5]

Are you sure the replacement alternator is good? If you hadn’t said you had replaced it that is where I would look first.

[SchillingTwo]

The voltage dropping when you rev the engine suggests that the alternator is not working.

[Gad-Westy]

Could be a ground issue too though. You could connect a jump lead across each of the ground routes on the car and see if any of them fixes the voltage drop when revving. If so you’ve found the problem without spending anything.

[Richard-D]

Traditional style alternators require connection to battery, a good earth and an excitation voltage (of 12V). My suggestion would be to ensure that a Mk2 MX5 does not have any type of 'smart charge' system and then check that these 3 things are present. Don't use a multimeter, use a test lamp.