Where to get constant 12V
 

Hi,

1980 240D here.

I am looking for a constant 12V (not an ignition switch ON position 12V).

I have a yellow wire behind my radio with a constant 8V but it is not enough to run the radio. The radio needs switched 12V (which I have taken care of) and a constant 12V.

I thought about running 12V from the battery, but I can't see anywhere on the firewall to bring the wire through to the cabin of the car?

Anyone know of a way to bring a 12V wire from the battery into the car?

Thanks.

There's a few different places to go through the firewall. Look for the big round rubber grommets below the brake booster and battery.

However, that may not be necessary. Most radio installs I have seen, have pulled constant 12v from the ignition switch. I don't have the diagram in front of me to tell you which pin that is, but it can be easily found.

8V doesn't sound normal, I'm going to guess that is coming from the connection at the ignition switch and has a poor connection causing low voltage but that is TOUGH to access. Run a fused wire from the feed to the fuse box and then run it through the firewall along with the other wires from the fusebox. The wire MUST be fused and protected from chafing where it penetrates the firewall.

Yes, I am using the red switched ignition wire for the radio. But, I need a constant 12V source to connect to my radio to get it to work and hold the presets.

I can make the radio work perfectly if I run a wire (constant 12V) from the battery, I just can't figure out how to get it into my cabin from the battery.

What size fuse would you use for the 12V from the fuse box?

Is there an obvious connection to the feed in the fuse box to run my constant 12V
I will have a good look and try to find where the wires go through the firewall from the fuse box. Any help would be appreciated. Thanks

The ignition switch has 12 V constant to its plug, when the key is turned the ignition switch closes and provides "switched power" but that connection point is a bear to reach.

What size fuse? That should be in the radio literature, if you don't have literature a search for it should help going to be similar for most radios. I'll guess at 2.5 amps.

To get power from the fuse box the feeds are usually under the fuse box but you'd probably be fine connecting to anything with constant 12V just don't use any fuse safety related e.g., brake lamps etc. in case it blows the fuse.

I can access the fuse panel from inside the car, driver’s side footwell? I take it that it is under the panels?

Fuse panel is under the hood under a brown plastic cover. Follow and use the path used by the factory to route wires from the fuse box to inside the cabin. The entry point through the firewall will be above your feet when driving a left hand drive car.

I removed the black cover concealing the fuse box. I see all of my fuses.

Unless I am missing something, there are no wires going to those fuses from inside the engine compartment to the interior of the car.

It looks like the fuse panel is bolted to the firewall and all of the wires connect to the fuse panel from inside of the car. Is that what you are talking about?

I am thinking maybe tie into the hazard switch?

There is a hole that the vacuum lines go through.

In post number 1 the picture on the left shows the glow plug relay. To the left and on the bottom of the relay is a flat copper wire with a Nut holding it down. That flat copper wire always has direct voltage from the battery.

Run a wire from there through the vacuum line hole in the fire wall and into the inside of the car. That keeps the routing of the wire on one side of the car. Note that it is a good idea to include a fuse somewhere in that wire.

http://www.peachparts.com/shopforum/diesel-discussion/254600-cracked-strip-fuse-pics-glow-plug-relay.html

Thanks, Guys.

X2 on Diesel911 suggestion...fuse the wire as close to the source as possible and protect it from chafing.

The other fuse is in-line from the 12v ignition switch, 12 volts only when key is turned. I take it a 15 amp fuse inline to the radio for constant 12 volts is good?

15 amps for a radio sounds high, can you find manufacturer specs for the radio online? I'd guess closer to 5 and if it is only for the memory even lower.

Thanks.

My radio is up and running now.

I grabbed the 12V from the glow plug relay as you suggested.

I also fished the wire from the engine compartment through the firewall to the interior of the car via the vacuum lines harness. I cut one of the ends off of
one of channels (the little tubes that the vacuum lines go through) that wasn't being used. After fishing the line through the small opening I used some black permatex to seal it off.

The radio is working fine now.

Thanks for all of the help, everyone.