I hope someone can help me with this problem on my '90 300CE with a 104 engine. Based on a few recent posts, I have done the following in an attempt to diagnose my electric fan problem:

1) jumped wires that connect to the temperature switch on top of the thermostat housing - fans didn't work

2) used multimeter to test for power at these same wires - no power

3) used mulitmeter to test for continuity between resistor posts - I had continuity (just to make sure I'm testing the right component, this thing is bolted to the frame right beside the a/c receiver/drier, right?)

4) checked fuse - looks fine, and the other components on that circuit work fine

5) cleaned ground for fan harness

6) jumped wires that connect to the switch at the receiver/drier - fans didn't work

7) tested for power at these wires - no power


O.K., so what am I missing? I looked for relays, but couldn't find any in the engine compartment. I read that there are two relays, one for the a/c switch and one for the coolant temperature switch, but I couldn't find either. Where should I look?

Secondly, why isn't there any power at any of these switches when the engine is running? Would this be caused by bad relays?

Third, the a/c IS low. I know people say this could prevent the fans from coming on, but even if it is low, shouldn't the fans run when I jump the switch at the r/d?

Lastly, are there any other components tied into the fan wiring circuit that would cause me problems (like a bad coolant level sensor or something strange like that)?

I don't know what to check next!

GregS
'84 300D, 174k
'90 300CE, 164k

two additional comments:

- I didn't have the a/c on when I checked the switch at the r/d, so I guess I need to run that test again

-this link may be helpful: ac fans not engaging but I still need to know where the relays are!

Thanks,

GregS

And is the link that says the coolant level sensor can prevent fans from coming on:

Aux fan not coming on, relays look good, what to check next? Is this true? If it is true, then, as a test, would the fans run if I jump the connection at this switch?

Thanks,

GregS

The relays [ both] are under the fuse cover , behind the fuse box... 6 phillip screws to remove cover..
The a/c fan has a fuse on top of the relay..

Easiest access to back screws is with hood in Full open position

GregS

May as well apply voltage to the fans individually to make sure they're working OK while you're at it...

However, your tests are indicative of a relay failure.

Just be sure the fans are working so you don't end up in circles.

Good luck!

Arthur- thanks. I thought maybe they were under there.

Azhari - I would apply power to both, but I could only find one harness coming out of the fan area with one plug on it. I'm pretty competent with this stuff, but this coupe is really tight in the engine compartment, and its hard to even see things, let alone work on anything.

Keep the advice coming!

GregS
'84 300D, 174k
'90 300CE, 164k

Good idea to apply power directly to fans to see they are operational

Another (informal) fan test: spin the fans by hand to see if there is resistence...if the fans have resistence it is mostl likely from dirt/muck accumulation & the bearings may be toast= aux fan replacement(s) may be necessary

it is common for dirt/muck accumulation over the years to affect fan performance and eventually cause their demise

I've seen the aux fan wiring harness also melt due to the heat generated from this type of condition- - fix for this condition is to replace failed aux fan(s) or their bearings, wiring harness, fuse(s)

I agree with you that things are pretty tight around this area.

hope this helps and best regards
-fad

My fans do spin freely. I think my biggest question is why there is no power at any of the sensors. Hopefully the problem lies in a bad relay or blow relay fuse. I would think there would be power even if the relay is bad (which would just keep the fans from turning on - the wires at the sensors are just part of the low-voltage "on/off" part of the circuit). Consequently, I hope it is simply a blown fuse. I'll keep my fingers' crossed and check when I get home from work.

Thanks,

GregS

That is correct and a little info here might help some in understanding these 2 relay circuits.
The low voltage , on/off circuit you talk of is the primary side of the relay [any relay].. this is simply the relay coil/solinoid that pulls in the secondary contacts [ in this case , high amps vs volts, as both primary and secondary are 12v.]
So, the sensors are on this primary circuit and they simply complete the circuit [usually to ground]. The thing to look into on this side of the relay is that there is also a power feed [ 12 v pos] that is fused to the fuse box.. so if this feed fuse is open, no fan circuit..
The actual high amp contacts in the relay are the secondary section/circuit.. this circuit simply feeds power to the fans.. this side is also fused and called the LOAD side ..
So, what we have here is a dual fan circuit -low/ac and high/ eng temp activated with , get this, 4 fuses... feed fuses for both relay coils side and load fuses for fan amp draw on secondary contacts..
The most common fuse problem will be , of course, on the LOAD side as this is the high amp draw circuit. , But , it is possible to have a coil side power fuse blown [ these are usaully in the main fuse box and coupled up with other circuits]
One of the test I do when jumpering a high side a/c switch to
test the low fan circuit [ and this test checks the entire circuits integrity, except the pressure sw itself] , is to listen up near the relay locations for an audible Click from the relay coil.. it can be faint, but this will tell you that you have primary coil feed to the relay and now one knows that the problem has to be on the secondary part of the circuit.... ie.- load fuse , dropping resistor, fan motor, etc,,,