Cagem Posted May 31, 2019 Share Posted May 31, 2019 What voltage should I be getting from the alternator? I have 14.8 V across the starter battery terminals at idle (9,000 rpm). Is this too high and might it have caused the fridge fuse to blow? The fridge (Thetford N3175) stopped working on 12 V. The 20 Amp fuse on the PCB had blown. Replacing it seems to have fixed the problem though I have not moved off the drive and not run the engine for more than a few minutes. While trying to track down the fridge problem I checked the fridge 12 V input (HC) and also the D+ input to the fridge, with the engine running. Both showed 14.8 V. Thetford's installation manual indicates that the HC voltage should be in the range 12-14.4V. Presumably the higher voltage is the maximum expected when the engine in running. Is 14.8V ok or is the fuse going to blow again? Thanks Andrew Link to comment Share on other sites More sharing options...
witzend Posted May 31, 2019 Share Posted May 31, 2019 Voltage isn't what blows a fuse it's the amps which is going thru it Link to comment Share on other sites More sharing options...
Derek Uzzell Posted May 31, 2019 Share Posted May 31, 2019 Cagem - 2019-05-31 12:48 PM ...Is 14.8V ok or is the fuse going to blow again? Thanks Andrew I assume you meant to write 900 rpm ;-) On-line comments suggest that 14.8V at the starter-battery terminals can be treated as ‘normal’ for a Transit. The difference between 14.8V at the starter-battery and Thetford’s 14.4V figure should not be problematical - besides which, it’s quite likely that voltage drop will have reduced the 14.8V figure to at least 14.4V at the fridge. Whatever caused the 20A fuse to fail, it won’t be the alternator’s 14.8V. Suggest you treat the problem as ‘fixed’ until it happens again. Link to comment Share on other sites More sharing options...
Cagem Posted May 31, 2019 Author Share Posted May 31, 2019 Thanks, both. And yes, 900 rpm... Link to comment Share on other sites More sharing options...
Alanb Posted May 31, 2019 Share Posted May 31, 2019 Before you worry to much, it may be worth checking your meter on another vehicle, or on one of your MH batteries which has been left off charge for several hours. However the relatively small excess voltage observed is unlikely to cause the fuse to blow. It would result an slightly less than 6% extra heating in both the fuse and fridge element. The big question is what happens above engine tickover speed. Has your alternator regulator failed? In my opinion worth investigating further. As regards Witzend's comment, may I remind him that Ohms Law will apply to the fridge element, which is resistive, as is a fuse element. Alan Link to comment Share on other sites More sharing options...
Cagem Posted May 31, 2019 Author Share Posted May 31, 2019 Thanks Alan Alanb - 2019-05-31 2:21 PM Before you worry to much, it may be worth checking your meter on another vehicle, or on one of your MH batteries which has been left off charge for several hours. The meter gives 12.7 V on the habitation battery, which seems consistent with 7/8 lights on the control panel. . The big question is what happens above engine tickover speed. Has your alternator regulator failed? In my opinion worth investigating further. At 2,500 rpm the meter is still showing 14.8 V across the engine battery. I am guessing this means the regulator is ok? Thanks Andrew Link to comment Share on other sites More sharing options...
Keithl Posted May 31, 2019 Share Posted May 31, 2019 Andrew, Does your 2015 Transit have a 'Smart' alternator? ie does the voltage rise and fall depending on load on engine. The other thing to check is if the starter battery is an AGM, then they usually require a higher charging voltage you as are measuring. Keith. Link to comment Share on other sites More sharing options...
Guest pelmetman Posted May 31, 2019 Share Posted May 31, 2019 My newish Thetford fridge has just stopped working on 12 volt AGAIN........it stopped working on gas last year *-) ........My conclusion is they're modern built to last until the warranty runs out Cr*p >:-( ....... Damned if I'm paying 700 quid again...... I'm going to start looking for a 30 year old fridge B-) ....... Link to comment Share on other sites More sharing options...
Cagem Posted May 31, 2019 Author Share Posted May 31, 2019 Thanks Keith Not sure if it's a smart alternator. The negative terminal of the battery has a terminal fuse attached. The battery is a silver calcium, which I think means it is wet but maintenance free. I read that calcium batteries require a higher voltage. I think I will follow Derek's advice and stop worrying. Andrew Link to comment Share on other sites More sharing options...
747 Posted May 31, 2019 Share Posted May 31, 2019 Check your connections as a poor one causes resistance (and heat). If the connections were made properly they will be fine but that is not always the case with motorhomes. Link to comment Share on other sites More sharing options...
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