Heated rear windscreen voltage drop

Technical MGB discussion
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Tom Brearley
Posts: 75
Joined: Sun Apr 01, 2018 6:54 pm
Forename: Tom
Surname: Brearley
Location: Devon

Re: Heated rear windscreen voltage drop

Post by Tom Brearley » Thu Dec 24, 2020 6:19 pm

Just to update this thread from March a little --

Now we are in the winter, the rear screen has started fogging up on most trips out which makes testing functionality easier. With the engine at 1500 rpm, I'm getting 10.3V at the positive side of the HRW - and it clears well. Maybe not quite as good as our modern car, but still perfectly OK.
1973 MGB GT
Mallard Green / Autumn Leaf

Vic Butler
Posts: 565
Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: Heated rear windscreen voltage drop

Post by Vic Butler » Sun Jan 03, 2021 6:22 pm

Try running the engine at about 3,000 rpm. It's possible that at 1,500 engine rpm the alternator is not on full charge,
What's the reading across the battery terminals? It should be around 14v.
The alternator runs twice engine rpm.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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