[verbryck]

Good morning , patient is a 90 300sel. The aux fans will not come on at all even at very high temp where high speed would click in. So far I have replaced fuse 1 and replaced both relays C and D. If I bridge the A/C pressure switch I get 10.5 volts a the fan connector. If I jumper the fan direct from the battery it comes right on. So I realize I must be missing a step. Voltage seems like it would be enough for the fans to come on so must be an amperage issue? Is there a resistor pack for the different speeds? Thanks in aqdvance, George

 

[MBL87560SEC]

Good morning , patient is a 90 300sel. The aux fans will not come on at all even at very high temp where high speed would click in. So far I have replaced fuse 1 and replaced both relays C and D. If I bridge the A/C pressure switch I get 10.5 volts a the fan connector. If I jumper the fan direct from the battery it comes right on. So I realize I must be missing a step. Voltage seems like it would be enough for the fans to come on so must be an amperage issue? Is there a resistor pack for the different speeds? Thanks in aqdvance, George

Hi there,

Fuse 1 provides the current to both the Low-Speed & High-Speed Aux Fan relays, then on to the Aux Fan Motor itself.

Fuse 5 provides current to actuate those relays, and should a related sensor provide the ground supply, the motor is then actuated.

One needs both fuses for either the Low or High-speed to function automatically.

Here's a diagram for you .. just in case.

As an aside here, 10.5 volts is kinda low I feel. Normally, the battery should be roughly speaking 13.8-14.2v when the car is running.
My own albeit un-scientific research has shown alternator brushes to be quite worn at roughly 80,000 mile / 129,000 Km of service.
I've been driving elderly Volvos since 1970, usually starting when they've been at least 10 years old when I/we get them.

Cheers,

MBL

 

[John350]

If I bridge the A/C pressure switch I get 10.5 volts a the fan connector.

Is that without the fan motor connected? If not, you have some high resistance somewhere, and when you plug the fan motor in that 10.5 volts will disappear. (A current problem, as you called it.) With the motor disconnected you should see battery voltage. It would be helpfull if you could ground the trigger lead for the other (full speed) fan relay to see if the behavior changes.

 

[verbryck]

Good morning , patient is a 90 300sel. The aux fans will not come on at all even at very high temp where high speed would click in. So far I have replaced fuse 1 and replaced both relays C and D. If I bridge the A/C pressure switch I get 10.5 volts a the fan connector. If I jumper the fan direct from the battery it comes right on. So I realize I must be missing a step. Voltage seems like it would be enough for the fans to come on so must be an amperage issue? Is there a resistor pack for the different speeds? Thanks in aqdvance, George

Is that without the fan motor connected? If not, you have some high resistance somewhere, and when you plug the fan motor in that 10.5 volts will disappear. (A current problem, as you called it.) With the motor disconnected you should see battery voltage. It would be helpfull if you could ground the trigger lead for the other (full speed) fan relay to see if the behavior changes.

Thanks for the replies and especially the diagram. The problem was a rotted wire at the resistor terminal . Agreed 10.5 volts is low I was suggesting it should make the fan turn but would expect to see 12+ volts and with the car running 13+. I guess Mercedes thought the resistor would never need replacement as it is located in a very inaccessable area under the master cylinder. My hands still hurt from shoving them into that tiny space. System seems to be working now and If I am reading the schematic correctly the bad connection could have affected both high and low circuits. Thanks for the help. George

 

[MBL87560SEC]

Oh good, you found it and were able to access it as well!

Must have been the black/white wire then...

You are correct thats where the power from the two relays join to feed the aux fan motor, must have been at or just 'downstream' of that junction.

 

[isthenew]

Have you checked the AC pressure? I understand the aux fans won’t work if that’s too low. They can be jumped, but won’t activate on their own.