Alternator amps..
Discussion
Hi peeps..Would any of you kind souls know what the alternator on my 280 should be kicking out as its only around 3-5 amps and im contemplating buying a new battery or new alternator..My battery is only a 45amp and when looking at the spec for mine it says 60amp..Is my alternator shagged or is it the battery not cranking enough..Cheers..Ziga
I have had similar issues on my 280i, could be either or both TBH but the alternator should be certainly more than 10amps, (either 45 or 75 rings a bell) might also be (as often on TVRs?) a wiring issue?
I have a new alternatoor on my wedge and it still barely (but enough to keep the car going/starting etc) charges the battery properly when driving at night. We're gonna have a bash at the charging wiring later this year. Esp as the car has 2 batteries fitted (one in the boot and a switch to change between either/both batteries) where I believe it was used for providing power to a trailer tent in the past.
Hope this has been of some help.
Norman
I have a new alternatoor on my wedge and it still barely (but enough to keep the car going/starting etc) charges the battery properly when driving at night. We're gonna have a bash at the charging wiring later this year. Esp as the car has 2 batteries fitted (one in the boot and a switch to change between either/both batteries) where I believe it was used for providing power to a trailer tent in the past.
Hope this has been of some help.
Norman
My car has a 55amp alt and 525 CCA battery.
Of course a 55amp alt only pushes out 55 amps when you are doing 120mph
Usual test is just to put your headlights on and see a decent change from idle to a couple of thousand revs.
People often underesitmate the load of electric cooling fans, if they run too long after shut off they can easily empty a battery.
When I bought my car the clutch fan was shot so I put in an electric fan until I could locate a replacement clutch fan.
Summer 95F, black car....the thing ran for hours after turning it off
I quickly put in a battery cut off.
A "made in China" thermostat was the real problem.
Much prefer the clutch fan.....it free wheels most of the time, locks in on idle.
Auto part stores in the States will do a Battery /Starter/Alternator check for free in a few minutes. They use a DC clamp on for the alt current.
For the home diy person, just look at the voltage. Car off 12.15 volts says your battery is good and accepting charge. 13.5v + with 2000rpm says charging is good.
Switch meter to AC you should get zero.
Of course a 55amp alt only pushes out 55 amps when you are doing 120mph
Usual test is just to put your headlights on and see a decent change from idle to a couple of thousand revs.
People often underesitmate the load of electric cooling fans, if they run too long after shut off they can easily empty a battery.
When I bought my car the clutch fan was shot so I put in an electric fan until I could locate a replacement clutch fan.
Summer 95F, black car....the thing ran for hours after turning it off
I quickly put in a battery cut off.
A "made in China" thermostat was the real problem.
Much prefer the clutch fan.....it free wheels most of the time, locks in on idle.
Auto part stores in the States will do a Battery /Starter/Alternator check for free in a few minutes. They use a DC clamp on for the alt current.
For the home diy person, just look at the voltage. Car off 12.15 volts says your battery is good and accepting charge. 13.5v + with 2000rpm says charging is good.
Switch meter to AC you should get zero.
Edited by jeff m on Wednesday 6th April 14:39
mad4amanda said:
Hmmm interesting how do you test an alternator for amp output ? I have a digital multimeter if thats any use?
My Mx5 doesnt seam to be charging properly when lights etc are on .
readings on the voltmeter are max of 13.8v when engine on ?
It is possible, but slightly tricky, to test the alternator current output with a DMM (as long as it has 10A range, anything less will probably just catch fire My Mx5 doesnt seam to be charging properly when lights etc are on .
readings on the voltmeter are max of 13.8v when engine on ?
)- best option is, as Jeff says, with a clamp ammeter. You can get one for about £22 now that would work fine. Don't buy the B&Q £17.50 special though; it's only useful for domestic mains as it only has AC measuring capability, you need DC for car electrics.As you mention 13.8v with engine running, that's usually a good sign that the battery is charging. If it wasn't you'd only measure the nominal battery voltage (about 12v of course, usually less a bit because the car's electrics will be trying to drain it).
Alternator output current shouldn't really be related directly to engine revs: usually the alternator needs to be spinning at more than a couple of thousand RPM for it to generate properly but it should be as capable of making 40Amps (or 50A, 70A, whatever) at 2000 rpm as at 5000rpm.
To use a DMM you really need a way of leaving the alternator connected while you start the engine, then disconnect the charge output wire whilst keeping the ammeter leads across the ring terminal and the threaded post - not easy unassisted. What this does is to avoid having the alternator trying to drive into an open-circuit (ie no load). As you then disconnect the charging wire the multimeter takes its place and you'll see either charge current going into the battery and electrical system or nothing at all. Assuming you get an Amp or two of charge current plus say another Amp for the ignition coil etc., switch-on the lights and observe the alternator output current going up - or not, as the case may be. As stated above, items like the cooling fan can draw a sight more than 10A so stick to lights or other relatively low-drain circuits (wipers, heater fan).
Of course the foregoing presumes some ability with a DMM, don't come running to me if your car ends up as a pile of melted plastic and charred leather
You'll only draw the amps if there is sufficient load - low load = low amps.
Turn on all headlights, fogs, heater blower on max, cig lighter, wipers on full (don't burn out the motor if the window is dry by leaving them on too long), hazards on, you get the idea - as many electrical things as you can.
Measure the voltage acorss the battery or alternator output - it should be about 13.5 to 14 volts with the engine running, it may be a bit lower at idle but get up to this figure with slight revs. If you are getting low voltage the alternator isn't coping (and headlights will probably look a bit dim).
Turn on all headlights, fogs, heater blower on max, cig lighter, wipers on full (don't burn out the motor if the window is dry by leaving them on too long), hazards on, you get the idea - as many electrical things as you can.
Measure the voltage acorss the battery or alternator output - it should be about 13.5 to 14 volts with the engine running, it may be a bit lower at idle but get up to this figure with slight revs. If you are getting low voltage the alternator isn't coping (and headlights will probably look a bit dim).
mrzigazaga said:
Thanks for that..I understand now..Its probably where i only do about 20-30 miles a week and the rad fan kicks in early..Maybe i should rig up a spare battery just for the fan..without blowing myself up that is..Cheers..Ziga
Just grab one of these from your Auto store Dollar bin.(Pound bin in your case)Better than adjusting your fan thermostat and getting it wrong.
Good for low use cars that don't have modern lectricals
Gassing Station | Wedges | Top of Page | What's New | My Stuff