I have pulled out the fan motor assy. It was so tired it would hardly turn at all. So I have a new fan. Here is the problem the wire harness that attaches to the fan motor pluges into the firewall behind the master cylinder. It has a black a red and a yellow wire at the plug. How do I check these wires for the correct voltages? On top of doing that someone has cut the red line and has jumped it straight from the fuse box with a key activated circut, thus not using the power sorce that comes thru the firewall plug. Why would this been done? I would like to nkow what the voltages sould be at the firewall and see if there are corect. Then re-hook up the original wires and harness before putting the new fan completely back in the car.
Thanks

The firewall 3 wire plug contains fan motor/regulator feed wires.
The Blk is ground [ -], the red is 12v [+], and the yellow is variable trigger voltage for the regulator..[ 2-9V. + ]... this feed is from the ACC control panel and the variable voltage is supplied to the regulators trigger circuit..the amount of voltage here effects the blower speed.

Why would someone jump the red wire cicrut to come out of the fuse block and not the firewall three plug conection?

B/C there is a circuit malfunction they could not diagnoses and they are Hacks.

..or, they added an external fuse b/c Benz has an aux fuse modification for bad wiring at the original fuse set-up.

If I dont have 12 volts at the firewall terminal where should I look next, all fuses are good and and controll unit works fine. I have key-on 12 volts at the jumped wire red wire but not at the firewakk post?

If you have no 12v at plug [ red], then they have eliminated that and gone over to fuse box to grab it ..so, it can be anything form a bad wire to CC, to who knows what they did.. ..the trick is to see if you still have varable V at yellow ..the blower can get 12v from anywhere b/c the fan motor is ALWAYS hot..the switching for the blower is done on the ground side of the circuit
[ Switched Ground] through the regulator.
If you ground the blu wire at the regulator, you should have full fan b/c that test by-passes the reg/cc panel circuit,... that is the test for full fan/regulator,,but you want to know if you have trigger at yellow first or you are wagging the dog by the tail. No trigger at yellow= no fan , no matter what you have at fan feed...the reg needs trigger to ground the neg side of fan motor to complete 12v to fan..

Thanks,
You been a big help. Time to button this thing up and get driving.

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Yeah..not the best way , but someone has been there before you and they have done a short-cut [ probably to save a few $$$ ], but as long as they have just grabbed the Positive supply side and it is still fused , I would just go for it.
As long as you still have variable trigger , then you still have CC speed control..so that is the important part.
If you really want to get to the cause of their jumper remedy, you will have to trace the circuit with a schematic for refernce..that is the only way to truely track it down..[ unless you luck out and see some burnt wires/connecors somewhere.]
Go for it and keep an eye on the pos feed fuse/wires. You may also want to remove the fuse and throw an amp meter across those terminal just to make sure that full blower is not exceding 25/30A, just to verify any bad wiring in what they did...............................................

Cool,(I hope) Thanks, I will let you know how it goes.
Brian

Very Good......................