Low volts with fans on

Technical advice Q&A

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david.coan
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#1 Low volts with fans on

Post by david.coan » Sun Mar 08, 2020 8:51 am

My 1968 series 1.5 has often been giving me concern about low volts once the car is hot during my ownership. The alternator and battery were new by the previous owner in the US about 4-5 years ago During its resto and not much used until i bought the car about 18 mths ago. The alternator is a Lucas stock one, the ac11, for the car.
Once hot and the fans are on the volts at idle on the gauge would drop well into the red, about 11v showing. Even at higher revs the volts read low at about 12v with the fans on. With the fans off And car at idle or higher revs the volts show healthy about 14v. I have replaced the alternator relay, no change, i replaced the 4TR regulator, no change. In heavy stop start traffic recently the battery had become drained and just managed to re start me after parking up. On longer runs at speed its not been an issue as any drain on battery has probably been brief and so never caused any worries for restarting

The manifolds have wrap, but i think the alternator performance is possibly worse once it too gets hot, would a S2 heat shield do anything to help? S1 heat shield looks useless, more a heat trap!

The battery is always on trickle charge and shows healthy on the charger digital display and healthy volts off charge and never fails to start the car from cold. So my thinking is its time to have the alternator replaced, but before i jump to this step i wondered if others have any similar experiences or Any thoughts.
David
1968 S1.5 OTS, ex USA converted to RHD

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mgcjag
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#2 Re: Low volts with fans on

Post by mgcjag » Sun Mar 08, 2020 9:01 am

Hi....what battery do you have...is it tbe correct or larger spec....you have 14 v out from the alternator so thats charging ok...the fans do draw a lot of power....i would get battery checked or replace it first if its 4/5 years old..Steve
Steve
1969 S2 2+2 & Building a C type replica

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johnetype
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#3 Re: Low volts with fans on

Post by johnetype » Sun Mar 08, 2020 10:16 am

Check what the voltages really are using a digital voltmeter across the battery terminals under the same circumstances you've described above. The voltmeter in the car isn't accurate enough to make the diagnosis until it's readings have been checked against a real voltmeter.
John

1969 Series 2 FHC

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christopher storey
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#4 Re: Low volts with fans on

Post by christopher storey » Sun Mar 08, 2020 10:54 am

This is fairly typical and really nothing to worry about ( although your battery doesn't sound to have much reserve capacity and it might be worth having it tested under load by a battery specialist ) . The twin fans are far, far more powerful than the original S1 fan and at a guess they take about 20 - 25 amps ( possibly more ) , and thus at idle the alternator will not be able to cope with a total demand including pump and ignition of some 25-30 amps and the demonstrated voltage will drop. Provided the voltage comes back up once the engine is revved up the system is behaving as normal

PS I think your analysis of excessive underbonnet temperatures is probably correct and is exacerbating the situation

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david.coan
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#5 Re: Low volts with fans on

Post by david.coan » Sun Mar 08, 2020 6:59 pm

Thanks for thoughts so far, i am going to have the battery and the alternator tested and i will post how i get on.
David
1968 S1.5 OTS, ex USA converted to RHD

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Topic author
david.coan
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Location: Reading, UK
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#6 Re: Low volts with fans on

Post by david.coan » Wed Mar 11, 2020 7:25 pm

My battery tested as good condition, we tested the alternator on the car and the test gizmo showed it was providing weak charge only. I ordered the aftermarket alternator from SNG for £98, which seems a bargain, and on arrival, appearance wise i think it looks good and a reasonable match to the Lucas one. On removing the old alternators pulley and fan to transfer them i discovered....no woodruff key! The pulley could therefore easily slip, so no wonder it struggled to meet any load. Gotta love some garages / restorers , all for the sake of a item costing a few pennies.
I decided to install the new alternator, fired the car up, volts rose swiftly to just above normal on the gauge, so i turned on the headlights, heater fan, and radio and saw no discernible change on the volts needle. Drove the car until the cooling fans cut in and even at idle the volt gauge sat steady with headlights etc on, only on switching on the hazard lights to increase load still further did the volt meter dip down but not too far. i think the car might also have idled better, especially when still warming but that could be my imagination. So my initial conclusions are a great result, but totally unnecessary if the job had been done right first time :cussing:
David
1968 S1.5 OTS, ex USA converted to RHD

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