P38A Rear screen heater

Sonnycbr · Nov 3, 2022

Hello again folks, my latest headache is that the rear screen heater is on permanently. I have no idea how long it's been like that but I need to fix it. Where do I start? What could make it stick on? The switch lights are not on for either screen heater so it could well be the case that both heaters are on permanently. Do they share a fuse? If so what number is it? Tia.

kermit_rr · Nov 3, 2022

The button has 2 bulbs, one for illumination and one for on/off status. They are make/break switches to send a signal to the ecu, so they should turn off and stay off with the ignition.
I'd start by swapping the relay

brianp38dse · Nov 3, 2022

As Kermit says relays would be first port of call.

Rubberknees · Nov 3, 2022

Plus1 as above. Once had the same on a SD1 rover relay had stuck on giving permanent live to the heater screen

Datatek · Nov 3, 2022

I agree with the others, the relay is most likely the problem.

Sonnycbr · Nov 3, 2022

kermit_rr said:
The button has 2 bulbs, one for illumination and one for on/off status. They are make/break switches to send a signal to the ecu, so they should turn off and stay off with the ignition.
I'd start by swapping the relay

Cheers Kermit, I'll do just that. However, here is where I really show my ignorance of all things electrical. Where are the relays housed, and how do I know which one to change?

kermit_rr · Nov 3, 2022

They are next to the battery, there's a sticker on the lid to say which is which

Sonnycbr · Nov 3, 2022

kermit_rr said:
They are next to the battery, there's a sticker on the lid to say which is which

Thank you so much, I'm going to look now.

Sonnycbr · Nov 3, 2022

kermit_rr said:
They are next to the battery, there's a sticker on the lid to say which is which

Had a look and sure enough they're exactly where you said they'd be. I took Nos 4 and 5 out and give them a good looking at. Not knowing what they are or what they do was a slight impediment to my inspection. I'll order 2 new ones and, fingers crossed my problems are all behind me.

Datatek · Nov 3, 2022

Sonnycbr said:
Had a look and sure enough they're exactly where you said they'd be. I took Nos 4 and 5 out and give them a good looking at. Not knowing what they are or what they do was a slight impediment to my inspection. I'll order 2 new ones and, fingers crossed my problems are all behind me.

Before you order new relays, you can use a relay of the same colour from another position and see if that fixes the problem.
It seems unlikely that 2 relays would fail at the same time.

Sonnycbr · Nov 3, 2022

Datatek said:
Before you order new relays, you can use a relay of the same colour from another position and see if that fixes the problem.
It seems unlikely that 2 relays would fail at the same time.

Oh right, I'll do that. I love this site, it saves me a fortune.

kermit_rr · Nov 3, 2022

Sonnycbr said:
Oh right, I'll do that. I love this site, it saves me a fortune.

It costs your sanity

Sonnycbr · Nov 3, 2022

kermit_rr said:
It costs your sanity

Long gone!

Sonnycbr · Nov 4, 2022

Here's what's happened today: took the battery off charge this morning and checked voltage, it started at 14.7 but dropped rapidly to 9.5, just started the engine. Did a bit shopping, drove the car about 3 miles and checked voltage when I got back. It started off at 10.4 but it was dropping slowly as I watched it. I then took both screen heater relays out and replaced them with two relays same colour. I then took car out again, did about 3 miles again and checked battery when I got back, same as before, started off at around 10.8V but slowly dropped as I watched it. I won't even begin to guess what all this means.....over to you guys.

kermit_rr · Nov 4, 2022

That doesn't sound right. A battery showing less than 12v shouldn't be able to start a car of any kind

Grrrrrr · Nov 4, 2022

Sonnycbr said:
Here's what's happened today: took the battery off charge this morning and checked voltage, it started at 14.7 but dropped rapidly to 9.5, just started the engine. Did a bit shopping, drove the car about 3 miles and checked voltage when I got back. It started off at 10.4 but it was dropping slowly as I watched it. I then took both screen heater relays out and replaced them with two relays same colour. I then took car out again, did about 3 miles again and checked battery when I got back, same as before, started off at around 10.8V but slowly dropped as I watched it. I won't even begin to guess what all this means.....over to you guys.

Take all readings with engine off.

1) charge it with battery leads disconnected and leave it 24 hours and see how much the voltage drops.

2) charge it but this time remove those 2 relays before reconnecting the relays and then see where you are 24 hours later.

3) charge it but this time leave the 2 relays in for 24 hours and then see what the voktage is 24 hours later.

Eric does a good video on YouTube on how to chase battery drain issues. I use junp leads so I can shut the bonnet but you can wedge the bonnet switches open if you want to leave it open while you wait for it to go to sleep.

Jon Dor · Nov 4, 2022

kermit_rr said:
It costs your sanity

Buying a P38 is confirmation of insanity, irrespective of age! :~}

Datatek · Nov 4, 2022

Sonnycbr said:
Here's what's happened today: took the battery off charge this morning and checked voltage, it started at 14.7 but dropped rapidly to 9.5, just started the engine. Did a bit shopping, drove the car about 3 miles and checked voltage when I got back. It started off at 10.4 but it was dropping slowly as I watched it. I then took both screen heater relays out and replaced them with two relays same colour. I then took car out again, did about 3 miles again and checked battery when I got back, same as before, started off at around 10.8V but slowly dropped as I watched it. I won't even begin to guess what all this means.....over to you guys.

If those voltage readings are correct, the battery is knackered.

Sonnycbr · Nov 4, 2022

Datatek said:
If those voltage readings are correct, the battery is knackered.

I agree and I'm resigned to the fact but I need to sort the problem out before I fit a new one. I'll do what Grrrr suggests.

Sonnycbr · Nov 4, 2022

Grrrrrr said:
Take all readings with engine off.

1) charge it with battery leads disconnected and leave it 24 hours and see how much the voltage drops.

2) charge it but this time remove those 2 relays before reconnecting the relays and then see where you are 24 hours later.

3) charge it but this time leave the 2 relays in for 24 hours and then see what the voktage is 24 hours later.

Eric does a good video on YouTube on how to chase battery drain issues. I use junp leads so I can shut the bonnet but you can wedge the bonnet switches open if you want to leave it open while you wait for it to go to sleep.

That's my weekend project. I'll take notes and post the results asap. Any idea of the title of Erics video? Thanks for the help.

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