Larger fuse is needed
 

The info is from another thread but has morphed into this question. There has been no response to my recent questions on the other thread so I hope I am not being impolite by re-asking the questions another way.

My car is a 1989 300E and a local shop recently installed euro headlights and wipers. To engage the newly installed euro wipers, the shop blew the fuse #3 fuse rated at 8A. They could not understand why the fuse blew but tried a 16A fuse instead and the headlights and headlight wiper motors all work correctly.

But the continued operation with a larger fuse has me worried. If the headlight wipers only work with a larger fuse - does this indicate a damaged relay? Or perhaps the wiper motors are too old/worn? I am worried that the larger fuse will not protect some vital and expensive electronics.

Your comments will be appreciated as I really have no idea what risks I have with this larger fuse in place.

Many thanks,
Paul

Of course the question IS...when does the fuse flow??

When you turn on the lights, wipers, OR the h/l wipers?

No relays involved.

16amp fuse is fine.. no problem

The fuse blew when...
 

The technician had the headlights on and when the right headlight wiper motor harness was connected to the circuit - that is when the fuse blew. And the wash circuit was not activated.

Another tech stated that an 8A fuse is pretty light and if the wiper motor draws a little more than normal it might be enough to trip the fuse.

In my wiring diagram for the 1989 300E there is a "headlamp washer relay" that is located in the fuse/relay box. The power flows from the #3 8A fuse to the washer relay and then to the headlight wiper motors. According the diagram, this circuit is hot when the light switch is in park, headlight, or standing lights which makes sense.

Does this help?

Thanks,
Paul

That system should NOT require a 16A fuse. And the .75mm wires aren't designed for that current.

The most likely cause is one of the h/l wiper motors. The relay donsn't control the power ONLY when they operate.

I would check each motor for current draw with ammeter.

Thanks, MB Doc. I appreciate the direction. I'll pass this on to the shop or I'll arrange myself.

Thanks,
Paul

...and it is was a faulty wiper motor.
 

After 6 months of procrastinating I finally sorted out the issue.

I tried to do a circuit test myself but I was not able to measure that current flow to the right wiper motor. The left seemed to draw about 3A but the right just kept blowing the fuse so I didn't spend more time trying. I did replace the older washer pump as it had run dry previously and was concerned that it may have overloaded the circuit (thanks to this forum to consider this).

I took both wiper motors to an automotive electrical shop where , I'm sure, they had better equipment for testing. In a few minutes the tech tested both and reported that both worked but the right motor was not running well and pulled a higher current.

With that info I returned the right motor to the supplier and was provided with another motor. This replacement was just installed and everything appears to be working well. The headlight wash pattern is quite messy and lots of spray goes to the sides. Is this normal?

Anyhow, I'm happier that it is working. This was a baffling problem but I hope that the problem is now solved. Another lesson for me.

Thanks for the help and interest.

Cheers,
Paul