What's most convenient 12v source in engine compartment?

[shertex]

Immediate need is to test the ELR actuator on an 87 300TD. But, in general, what's the most convenient place in the engine compartment to tap into a 12v source?

[connerm]

This sounds like a trick question, but 'll bite. How about the battery?

[Diesel911]

Originally Posted by connerm

This sounds like a trick question, but 'll bite. How about the battery?

Yes, sometimes we over think stuff to the extent that we are oblivious to the obvious solution.

[shertex]

Originally Posted by connerm

This sounds like a trick question, but 'll bite. How about the battery?

No, not a trick question. I just thought there might be easier sources....or switchable sources, for that matter....that could come in handy.

[jay_bob]

I took a quick look at the schematic for a W124 and you are right, there is no easy way to grab a protected +12 V dc for testing under the hood. The load side of the fuses are all recessed on the 124 fuse box so that's out.

IMO it is way too dangerous to have an unfused test lead straight from the battery flopping around the engine compartment.

I would rig up a test lead with a small in-line fuse holder and go off the battery +.

If your test lead is made with small wire (like #18 or #20) then I would go no more than 3-5 amps max on the fuse. If you are testing electronic inputs then go with a 0.5 or 1 amp fast blow fuse max. That way if you miss the pin you have a fighting chance of not smoking something, and if it hits the frame it will be a little sparky sparky and crackle before the fuse opens.

If you are testing solenoid actuators you will need the 3-5 amps slow blow and maybe more. If so you will need to step up to #12 or #14 wire and up to a 10-15 A fuse maximum. But I would only do that if you need more current for your test. That size of fuse will allow a big bang before it clears if you hit the frame, and it will smoke electronics if it is hooked to the wrong point.

One last thing - some actuators do not like having 12 Vdc directly applied to them, they operate off a reduced voltage from the electronics. And sometimes the driving circuit in the electronics does not like having 12 V dc applied externally to the output from a foreign source. Just be careful, I would hate to see you smoke something.

[Jeremy5848]

Mercedes has used a number of add-on fuses over the years, such as the 30A strip fuse for the climate control blower. Another fuse was added sometime in the 1990s for the cellphone. The picture shows the cellphone fuse for my 1995 E300; since I have no cellphone, I adapted it for a cabin power outlet. In the picture you can also see a dual fuse holder (got it from a junkyard BMW) that I added for use of a battery trickle charger. These fuse holders are inexpensively available in auto junkyards. Look near the battery or near the regular fuse box or panel.

Jeremy

[sixto]

There's the downstream end of the 80-amp fuse in the glow relay.

Sixto
87 300D

[Jeremy5848]

Originally Posted by sixto

There's the downstream end of the 80-amp fuse in the glow relay.

Sixto
87 300D

Which is hot at all times, I believe.

[connerm]

You could make test lights out of flimsy wire and 12v leds from radio shack. Push a straight pin through the wire insulation and solder the wire to the pin. When you're done, pull out the pin and the insulation should close up.

[Zulfiqar]

If there is nothing around - a regular 5A spade fuse, two female spade connectors some electric tape and a test lamp is all you need to modify a test lead

[vstech]

Hmm I think the feed to the aux fan is a good source... and it's switched with the key off a relay.