Renault Lagun not charging???????

[committed]

My father has a 1998 Renault Laguna RT petrol 8V (no aircon) which is discharging the battery when running until it eventually cuts out. The car is in the last year of its life so we don't want to waste money on it but it is hobsons choice at the moment.

The battery has been tested and is deffinately not at fault! The guy who tested it said that it could be the alternator as it was only pushing out 12.9V & it should be 13.6V. The windscreen wipers run slowly etc.

Renault want £50 to run tests but I can get a recon alternator for less than £50. So before I risk throwing good money after bad, could there be another fault that would make a good alternator throw out a low current, such as the engine management system or a leak to earth?

Any help would be greatly appreciated.

[The Excession]

Alternators do wear out, or at least the carbon brushes on the stator do, so it could be a poorly alternator.

First thing I'd check is the output directly from the alternator, this sounds like it has been 'sort of' done, but was it measured at the battery or at the alternator? There could be a bad connection between the two. When measureing the alternator output directly you need to get into the back of the plug on the alternator and measure its output with respect to the casing or engine block, this completely removes any of the wiring at the battery from the measurement and will give a true indication of the alternator output.

It could be a bad earth connection from the battery to chassis, that's easily checked with a multimeter, diconnect the negative/earth from the battery, stick one lead of the multimeter on the connector that was on the battery, stick the other on the engine block somewhere - read the resistence on the lowest ohm scale, it should be nigh on zero.

My guess would be the alternator needs servicing/replacing - especially as it's an 11 year old car.

Incidently, changing the brushes in an alternator is not too difficult if you are a dab hand with spanners and screw drivers, I certainly haven't ever bought a new/recon alternator, I've always ripped them to bits and just bought the replacement parts. I guess it boils down to how much you like tinkering and fixing stuff yourself verses £50 for a recon jobby.

[Pigeon]

Not entirely sure what's going on here. 12.9V is low but does indicate some charge. What do you get if you turn all the lights and the heater fan on? Does it diminish over time? When it cuts out, how do you get started again? Have you tried swapping the battery for a known good one off another car to make sure... how was it "tested"?

The Excession said:

First thing I'd check is the output directly from the alternator, this sounds like it has been 'sort of' done, but was it measured at the battery or at the alternator? There could be a bad connection between the two. When measureing the alternator output directly you need to get into the back of the plug on the alternator and measure its output with respect to the casing or engine block, this completely removes any of the wiring at the battery from the measurement and will give a true indication of the alternator output.

Measure both at the battery and at the alternator.

The Excession said:

It could be a bad earth connection from the battery to chassis, that's easily checked with a multimeter, diconnect the negative/earth from the battery, stick one lead of the multimeter on the connector that was on the battery, stick the other on the engine block somewhere - read the resistence on the lowest ohm scale, it should be nigh on zero.

Ah... a multimeter won't give you meaningful readings if you do that. You need Kelvin probes and at least one more decimal place than a boggo multimeter has.

Thing to do is check for voltage drops... run engine at about 1500rpm, all lights and heater fan on, multimeter on 20V range, measure the voltage between the bolt/tag of the alternator output terminal and the post of battery positive. Then measure between the alternator body and the post of battery negative. Anything much more than about 0.2V and there's a problem... keep one probe in the same place and move the other one from point to point along the circuit to meet it; when you get a big change in the reading you know the bad connection is between where you last measured and where you are measuring now.

Have seen a charging problem on an Astra caused by a bad connection between the alternator body and the engine block... isolated by this method.

Usual method for testing alternators in situ avoiding confusion from possible faults elsewhere: take wire off field terminal, connect field terminal to battery positive, measure to make sure you really are getting 12V on the field terminal, start engine, blip throttle, rev to 1500rpm, measure voltage between alternator output terminal and alternator body. Should be 14.4V or close to it. Turn on all lights and heater fan, measure it again, it shouldn't have dropped by more than a few tenths of a volt. Repeat both measurements at the battery terminals; shouldn't be more than a few tenths of a volt lower. A shagged battery will bugger up the readings so use one off a working car to be sure it's good.

Renault being Renault may have buggered this up though... Trying to get the charging working on a Renault 5 GT turbo with bits of the loom missing around the alternator, I found that the alternator has not one but two low-current connections. Further experimentation did discover that (a) it is possible to determine which is the field connection by trial and error without blowing anything up, and (b) the fourth wire doesn't appear to do anything anyway, everything still works with it not connected. I don't know if a Laguna has a standard alternator, a weird one like this, or a differently weird one.