72 3.0 cs 25a fuse blows on ignition

[Hummerbeats]

Hi Guys,

Need your help, this has been driving me nuts. The no8 fuse is 25amp fuse and keeps blowing whenever i turn the ignition key, before starting the engine.
The left side windows are connected to this fuse and dont work. The sunroof and right side windows are separate and work fine.
I tried connecting the windows to another fuse, the windows didnt work but the fuse didnt blow.

The windows did work before so i do not know what is causing this.

Any ideas?

 

[HB Chris]

If the fuse is blowing it has a short to ground somewhere in the circuit.

 

[Hummerbeats]

If the fuse is blowing it has a short to ground somewhere in the circuit.

Yh need to do some more trouble shooting, but it's really annoying as it was working before without any problems. So not sure what has changed.

 

[bluecoupe30!]

old BMWs....stuff changes hourly, corrosion, leaks, electrical gremlins, investigations revealing a host of horrors, but this is all normal. All part of the "experience". Please share what you find. Mike

 

[arnie]

I'm actually working on my E30 power windows and I really don't know if the windows in the E9 are working like this, but if so, the 2 lines to the window motor should have ground when you don't use it. The switch gives power to one of the lines, which one depends if you'd like to close or open. So that means when you push the switch, it cuts off the ground and guides power to that line.

If your system works like this, there's nothing of a battery plus, which could cause a short circuit, because both lines to the motor are supposed to have just a ground connection. This can't blow any fuse. Did you try other switches ? These are not that reliable and failing very often.

It's maybe damaged and has a connection between power and ground internally, which would explain the effect.

If the Motor works differently for some reason, I would inspect the the wires, where they are mechanically stressed, and therefor the area at the A-pillar, where the wires are bended when you open or close the door.

Good luck

Cheers

Ingo

 

[adawil2002]

Advise buying either a simple circuit tester or an inexpensive voltmeter to help trouble shoot and trace the circuit.

As HB Chris wrote there is a short somewhere in that circuit.

I'd check the switch connections

 

[Hummerbeats]

Cheers guys going to try and tackle this today. Will keep you posted on progress

 

[Hummerbeats]

So far all the left windows are shorting, every wire seems to complete the circuit including the earth.

Further fault finding has shown that the rear windscreen heater is also shorting out somewhere with the window wiring. May need to replace these wires.

Anyone know where i csn find a wiring diagram?

 

[HB Chris]

They are in this thread as pdf.

https://www.e9coupe.com/forum/threads/wiring-diagram-late-73-75-e9s.22639/#post-165719

 

[teahead]

these are helpful too.

https://www.ebay.com/itm/BMW-3-0csi...ash=item4a95eef288:g:PFoAAOxy7RBRYnVv&vxp=mtr

 

[teahead]

I actually ordered this one:

https://colorwiringdiagrams.com/products/bmw-30csi-1971-1972-1973-e9

 

[jmackro]

every wire seems to complete the circuit including the earth.

I don't understand what that means. Are you saying that if you connect a multimeter between earth and the power lead to the rear window defroster, it indicates a connection? Keep in mind that a high-power device, like a window motor or rear window defroster, will look like a dead short to a high-impedance tester like a multimeter. When wired properly, these high-power loads will only have a few ohms of resistance between + and earth.

Further fault finding has shown that the rear windscreen heater is also shorting out somewhere with the window wiring. May need to replace these wires. Anyone know where i can find a wiring diagram?

Obtaining a wiring diagram is a good first step. Note that BMW changed e9 wiring over time, so be sure to use a diagram that corresponds to the year of your car.

I doubt you will need to replace any wires; generally when a car develops a short, it is at a connector, or internal to a device (switch, motor, etc). Unless a wire gets pinched, or its insulation becomes worn through, it will last forever. So inspect the wiring, sure; but you probably won't need to replace it

 

[Wladek]

So far all the left windows are shorting, every wire seems to complete the circuit including the earth.

Further fault finding has shown that the rear windscreen heater is also shorting out somewhere with the window wiring. May need to replace these wires.

Definitely, probably you have fried wire insulation under the tape.

 

[Hummerbeats]

I don't understand what that means. Are you saying that if you connect a multimeter between earth and the power lead to the rear window defroster, it indicates a connection?Keep in mind that a high-power device, like a window motor or rear window defroster, will look like a dead short to a high-impedance tester like a multimeter. When wired properly, these high-power loads will only have a few ohms of resistance between + and earth.

That is correct. When using a voltmeter between any of these wires shows a connection. So there seems to be touching wires somewhere. As soon as I turn the key the fuse blows.

 

[adawil2002]

Keyed power narrows it down at least.

 

[jmackro]

That is correct. When using a voltmeter between any of these wires shows a connection.

Hmm, we don't seem to be communicating here. Yes, your voltmeter will show such connections, even when things are OK.

Ohm's law says that resistance = voltage/current. So with a 25 amp circuit, 12 volts / 25 amps = .48 ohms of resistance. Are you measuring resistances lower than 1/2 ohm? Is your voltmeter accurate enough to distinguish between 5 ohms and .5 ohms?

I'm not saying that you don't have a short somewhere - obviously you do. I'm just saying the fact that current passes through a wire or device doesn't necessarily mean that it is shorted.

 

[Hummerbeats]

Hmm, we don't seem to be communicating here. Yes, your voltmeter will show such connections, even when things are OK.

Ohm's law says that resistance = voltage/current. So with a 25 amp circuit, 12 volts / 25 amps = .48 ohms of resistance. Are you measuring resistances lower than 1/2 ohm? Is your voltmeter accurate enough to distinguish between 5 ohms and .5 ohms?

I'm not saying that you don't have a short somewhere - obviously you do. I'm just saying the fact that current passes through a wire or device doesn't necessarily mean that it is shorted.

Yes the voltmeter is accurate to .00.
What we checked was voltmeter connected to fuse box and touched each end of the window wire switches and also the rear windscreen heater switch. All show a positive connection. I don't believe that should be the case.
Even when the voltmeter was connected between any of the window switch wires and the rear windscreen heater switch there was a positive connection, again I don't think that should be the case.

All these cables seem to be touching at some point. Just trying to find where that point is.

Will keep you posted with more findings need to trace if there is a break somewhere in the cables or loose connection.

 

[mulberryworks]

Sounds like you need some assistance troubleshooting, Perhaps a copy of Rob Siegel's book. I have it, it's very good.

 

[Hummerbeats]

Cheers guys, it's the first time I'm tackling a car restoration project. So a big learning curve is being experienced.

Luckily I have my dad to help me here so hopefully find this fault soon so we can get the car to the interiors.

Thanks for all the information and links. Very helpful indeed.

Also doesn't help when I live in Switzerland and the car is in London. Lol

 

[adawil2002]

The windows did work before so i do not know what is causing this.

What was the last thing done to the car? Worked before what?

Had a similar issue a year ago, a screw nicked a wire and kept blowing the turn signal fuse, 2 hours to diagnose and fix.

You mention you are in Switzerland & car in London. Who is doing the trouble shooting?