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Author Topic: Battery light on at low revs  (Read 220 times)

Casper

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    • 93 XR2 and 91 Turbo Clubsprint (tribute)
Battery light on at low revs
« on: July 31, 2017, 04:33:55 AM »

Hi All,

Just trying to figure out a problem with an 93 N/A which has the battery light staying on after starting and volts low (about middle of gauge) when driving - if I rev hard (about 4000rpm) the charge picks up and battery light stays off.  I can't hear a belt slipping or any unusual noise.  I just drove home from work (10 min drive) and the battery light wouldn't turn off - but when I revved in the driveway (again about 4000rpm) it turned the battery light off and stayed off with dash gauge indicating charging as normal.  I let it idle for a while to recharge.  I know the battery is good as it is reasonably new - it is the original alternator.  Any one had similar issue?

Any thoughts appreciated before I start fumbling in the dark ...!!!!
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NomakeWan

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    • 93 Capri XR2
Re: Battery light on at low revs
« Reply #1 on: July 31, 2017, 09:41:13 AM »

Sounds like that alternator is shot. There are tests you can do to determine what exactly is wrong with it using a multimeter but to be honest, most auto parts stores will test your charging system for free so why not drop by your local store and get it tested?
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Casper

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    • 93 XR2 and 91 Turbo Clubsprint (tribute)
Re: Battery light on at low revs
« Reply #2 on: August 01, 2017, 06:05:15 AM »

thanks for the reply - I was figuring that is probably the issue.
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Casper

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    • 93 XR2 and 91 Turbo Clubsprint (tribute)
Re: Battery light on at low revs
« Reply #3 on: August 14, 2017, 06:02:18 AM »

Just had a chance to test the voltage - battery is fine at 12.5 volts but when engine is on the alternator (at the B terminal) is only 11.3 volts.  It seems like the alternator is producing just enough voltage to keep the battery alive (as it only seems to be draining slowly) but not enough to keep it in the sweet spot.
The belt seems fine and no noise from the alternator itself.  The battery lights stays on.

Voltage when engine off from B terminal is equal to battery voltage when engine off.

Research in the Ford manuals online diagnostics indicate replacement alternator required..... :(

Here we go....
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SHOwn

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    • 91 XR2
Re: Battery light on at low revs
« Reply #4 on: August 14, 2017, 11:59:18 PM »

It might be a bad diode trio - more accurately, one bad diode of the three. That often allows enough to keep the car going but you don't have enough voltage to keep up under certain loads. I had one go bad and I took my alternator to an alternator shop and they rebuilt it - using a good used diode trio from the 94 NA I stripped. If you can get a shop to rebuild yours, at least you will be sure to have the right pulley (since you already have it). Buying a replacement can often get you the wrong pulley and cause additional pain trying to stop the squealing belt.
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R. Powell
'91 XR2, Liquid Metal 16" Chrome Drifters, White Gauge Faces, Kuhmo 40's, 2 1/2" Mandrel Bent CATback, BMW318i Heated Seats, `94 console, '99 Dodge Neon Spoiler, '94 XR2 Sideskirts Bumpers and taillights, Fresh Basecoat-Clearcoat Cardinal Red (Jul2014), vj11/vj14 hybrid, manual boost CTL.

Casper

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    • 93 XR2 and 91 Turbo Clubsprint (tribute)
Re: Battery light on at low revs
« Reply #5 on: August 16, 2017, 03:52:40 AM »

New alternator installed today.... :)  I was going to have a go but on closer inspection there is no space to get the alternator out - so I took it to my local mechanic.  They had to remove the axle/drive shaft - on the oz RHD version the brake booster/steering fills up the space above next to the inlet manifold - whilst the axle is in the way down low.  No possible way to get the alternator out.  As it was the mechanics underquoted the time required....lucky for me this time..... :)

Now to look at the a/c compressor in the near future.....
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