MX-5 OC Forum

Seemed like a good idea at the time. Have a BMW clock that I wanted to fit but since doing so my car hasn't been charging. Removed the wiring to the clock and the car still isn't charging. Everything was working fine before and thought this to be a nice, simple job. I know I've done something wrong but as I've returned the car to how it was, the problem is still there and I don't know why.

At idle I'm getting 13.4v at the battery but if I put lights and heater on it goes down to 12.4v. I have a 10 mile commute and I'm barely making it home. I know I'm on borrowed time. Cany anyone help?

 How old is the battery, and is it a wet cell or gel?

M-m

Looks like the battery is about 3yrs old and its a halfords job. Just says it's a maintenance free battery. Guessing it's a wet cell. I have it on trickle right now thinking that it's two nights in a row now it's been run virtually flat! It might take a charge. Have a new alternator belt on order from Autolink too. Get the occasional squeak/slip out of mine but that has just become more pronounced also.
Any idea of what sort of voltage the battery should be putting out and at what RPM? I was expecting about 14/14.5v?

 Hi,

Did you make the clock connections directly at the battery? Check/clean the batt terminals and check the alt sensing wire connection and any others on the pos terminal are good.

Does the warning light work as expected, on with the key, then goes out when running? Does it glow when it shouldn't with extra load switched in etc, have a careful look.

Check/clean the alternator connections.

Clock connections were made at the dash. Live feed came from the radio, earthed to the chassis bracket, clock light off the ashtray light. All battery connections clean as a whistle. Thinking about it, I'm not getting a battery light on dash at all. Not at accessory, not while the battery is draining (lights getting dimmer/wipers slower). Nada!

 Answered this in Mm

M-m

Ok, so the battery has been on trickle all night and is making 12.17. More interesting though, is the bottom length of belt from the alternator to crank (presumably this is the longest length) I probably have about 20mm of deflection. I can actually turn the alternator pulley by hand slipping under the belt! I'm presuming this isn't right.

 no it is not right, you need to adjust the tension , and see what happens on the charge side.

M-m

Mazda-mender:

 no it is not right, you need to adjust the tension , and see what happens on the charge side.

M-m

  Yep, you got 13V at idle which means the alternator is charging at low speed and low electrical load. When you put the lights on the extra load makes the alternator harder to turn and the loose belt may simply be slipping.

Wel, with the belt cinched up a little I'm getting 14v at idle but lights and heater put this down to 13.1-ish. Still a little bit of squeal but it's always done that on startup.
Under load (3000rpm) I was reading +/- .5 of a volt down on idle figures. Figures sound low to me. Was hoping to be about the 14's. Saying that, it did manage to get me to work and back AND still had some juice left to turn it over once I'd switched off so that is an improvement....

Put a new drivebelt on and tighten properly