W210 Switched Accessory Power Locations? - PeachParts Mercedes-Benz Forum

KarTek · #1 05-12-2016, 10:07 AM

Guys,

Where is a good spot to pull a couple amps of switched accessory power under the dash?

In the old days, cars had a blade terminal on the fuse block and simply adding a fuse to the empty slot provided power at that point. I'm looking for something similar on my '98 for a relay and possibly USB charging.

Thanks.

97 SL320 · #2 05-12-2016, 08:26 PM

There might be a couple of screw type terminal strips under the dash. I seem to recall something like that on my SL so it might have carried over to others.

97 SL320 · #3 05-12-2016, 08:28 PM

Also, perhaps the factory cell phone wires in the center console and below radio could be used.

jay_bob · #4 05-12-2016, 09:50 PM

The lighter socket. Real easy to access the back side, pull the shifter wood off, 2 screws, lift out ash tray assembly.

I will have my dash totally apart next week to change the evaporator, if I see anything better down there I will let you know.

Cell phone power on these models is in the trunk on the DB25 connector. The cell display head gets its power from the 8P8C jack behind the radio, which connects to pins in the DB25.

Fuse block serving this area is under the hood. There is a fuse block on the left end cap of the dash but that is all exterior lighting circuits.

Hondas and GMs have the 1/4" tabs for switched and unswitched bus in the under dash block but not MB.

KarTek · #5 05-16-2016, 02:28 PM

Thanks for the replies guys.

Jay,

I had to look up "8P8C" connector, I just call it RJ45!

I am already using the cigarette lighter to power itself, a second USB/charging jack and the gauges. I'm also using the cigarette lighter lighting circuit to power the gauge lighting.

What I need is a good switched circuit to run the relay for a future electric fuel pump conversion. The circuit needs to be low amp and not tied to anything else non-critical so that a failure in a peripheral device doesn't knock out the fuel pump.

I haven't looked up under the dash yet, is the fuse panel and related circuits under the driver's side?

Thanks again.

Oh, and Jay, I'll be looking forward to hearing about your dash removal adventure. I'm going to have to do the same thing at some point when I'm sick of putting R134 in the system. I already have the evap, just need to grow the balls to start the job.

VW1300 · #6 05-16-2016, 08:47 PM

Why are you converting to an electric fuel pump?

Just wondering.

jay_bob · #7 05-16-2016, 09:29 PM

How about using the following circuits in the car already for this:

For the pump trigger use the fuel solenoid signal - pin 3 of the ECU
I would be careful and not put but a few tens of mA extra load on this pin - so use a small relay in parallel with the solenoid to drive a power rated relay for your pump, you sure don't want to overload whatever driver circuit is in the ECU for the fuel solenoid. This pin sources voltage, the valve coil is between pin 3 and ground. Definitely put a reverse diode across the coil on your pilot relay.
ECU pins 18 and 19 are ground.

For the pump power the gas models use fuse 6 (25 A) in the rear fuse box. It is fed from the battery bus.

KarTek · #8 05-23-2016, 11:20 AM

Quote:

Originally Posted by VW1300

Why are you converting to an electric fuel pump?

Just wondering.

I'm going to simplify the fuel system, eliminate some of the potential air leaks as well as the leaky shut off valve. An electric pump in the rear of the car will also aid in priming the system after service or a filter change as well as deliver a more consistent fuel pressure and flow.

Quote:

Originally Posted by jay_bob

How about using the following circuits in the car already for this:

For the pump trigger use the fuel solenoid signal - pin 3 of the ECU
I would be careful and not put but a few tens of mA extra load on this pin - so use a small relay in parallel with the solenoid to drive a power rated relay for your pump, you sure don't want to overload whatever driver circuit is in the ECU for the fuel solenoid. This pin sources voltage, the valve coil is between pin 3 and ground. Definitely put a reverse diode across the coil on your pilot relay.
ECU pins 18 and 19 are ground.

For the pump power the gas models use fuse 6 (25 A) in the rear fuse box. It is fed from the battery bus.

Using pin 3 sounds like a good idea as long as it will shut off if the engine stalls which it probably does not. My concern is having a signal that is hot only when the engine is running in case of an accident, the pump won't keep spewing fuel into a fire.

My plan is to use an oil pressure switch that closes when the engine builds enough pressure.

If it comes to it, I could probably use a transistor to drive the relay when it's activated by the pin 3 voltage.

jay_bob · #9 05-23-2016, 12:31 PM

Oh and by the way there is nothing behind the dash to tap.

The harness runs out from the left front under hood area (where the fuse box, relay module, and front SAM are located) and everything goes to connectors that directly plug into devices. No X30 multi purpose power tap block in the 210, like there is in the 124.