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.

[PeterGadsby]

Thanks everyone, I've read that the ECU controls when the alternator charges, is that true?

If so I think that might be the culprit...

- Pete

[RandomCarChat]

I had the same issue on mine years back. I took the ground off the PPF at the back and cleaned it up, problem solved.

[Gad-Westy]

PeterGadsby said:

Thanks everyone, I've read that the ECU controls when the alternator charges, is that true?

If so I think that might be the culprit...

- Pete

I would think an ecu issue would lead to overcharging if anything.

[Richard-D]

Gad-Westy said:

PeterGadsby said:

Thanks everyone, I've read that the ECU controls when the alternator charges, is that true?

If so I think that might be the culprit...

- Pete

I would think an ecu issue would lead to overcharging if anything.

ECU controlled alternators are usually controlled via a LINbus or CANbus system and send a message to the alternator to control output voltage. Typically a lack of message results in zero output.

[PeterGadsby]

RandomCarChat said:

I had the same issue on mine years back. I took the ground off the PPF at the back and cleaned it up, problem solved.

Sorry stupid question, where is the ground attached to/from?

- Pete

[Gad-Westy]

Richard-D said:

Gad-Westy said:

PeterGadsby said:

Thanks everyone, I've read that the ECU controls when the alternator charges, is that true?

If so I think that might be the culprit...

- Pete

I would think an ecu issue would lead to overcharging if anything.

ECU controlled alternators are usually controlled via a LINbus or CANbus system and send a message to the alternator to control output voltage. Typically a lack of message results in zero output.

It’s just a 0-5v signal on an NB. But a bit of googling does suggest that a 0v signal would result in no charge. That is something that could be measured on the alternator connector or at the ecu end potentially. But because voltage is dropping with revs I’m still thinking it’s a grounding problem.

[Belle427]

Some tips here, its at the rear of the car underneath, check the one the boot too.
https://forum.mx5oc.co.uk/t/2001-nb-fl-ground-wire...

[Richard-D]

Going around the car checking ground points is a good way to introduce new faults. You should check the ground but the place to do so is at the alternator. Check is with something that draws a decent current, a headlamp in a holder is a good way to do this.

[PeterGadsby]

Richard-D said:

Going around the car checking ground points is a good way to introduce new faults. You should check the ground but the place to do so is at the alternator. Check is with something that draws a decent current, a headlamp in a holder is a good way to do this.

How do I check the ground at the alternator and what should I expect if it is fine?

- Pete

[Richard-D]

PeterGadsby said:

How do I check the ground at the alternator and what should I expect if it is fine?

- Pete

High power test lamp is the easy way. You can make your own with a bulb holder and a headlamp bulb. Attach a couple of decent lengths cables. Connect one to battery positive and touch the other against the alternator ground. It should be nice and bright, just as bright as if connected directly to battery ground.