87 300E Auxilliary Fan Switch

Can anyone help?

I seem to have a bad auxilliary fan pressure switch because the fan does not run when the ac is on. I've checked the circuitry to the fan and it does in fact work, so I believe it's the switch. Can anyone tell me the duty cycle for the fan when the ac is on? Should the fan only run when the engine is idling or all the time or what?

BenzGuy 124

[J.HIDALGO]

Is your AC working properly?Use the search feature on the right upper corner of this screen. I know it was been recently discussed. There are two things that will trigger your fans:
1. Temperature/pressure of freon (low speed) and,
2. Temperature of your engine 105 degrees, I think (high speed)
If your AC freon is low, the fan will not work until the engine reaches 105.

[Jim Anderson]

I was thinking the same thing on my 190 but I'm not sure where the AC fan switch is. I thought it was on the dryer but there are two, a red single connector and a two pin connection. Which switch is which?

[J.HIDALGO]

on the 300E. I am not sure on the 190's but, it should be the same. If you touch both leads, the fan should work.

Thanks for your inputs.

On my car, the air conditioning works fine during normal driving. I am wondering what the duty cycle should be for the auxilliary fan when an ac system is working as it should. Does the fan run all of the time, or only during engine idle?

[400E]

There is what I believe to be a misconception that the auxilliary cooling fans are supposed to come on whenever the AC is on, as they do on some other non-MB vehicles. MB took the approach that the aux fans come on only when actually needed. For newer vehicles (starting, I think, in the late '80s) the aux fans have two speeds. Low speed is triggered by high AC pressure and is designed to protect the compressor from excessive refrigerant pressure. High speed fan operation is triggered by high engine coolant temperature and is intended to protect the engine from overheating. In neither circuit is there any provision for the fans to operate just because the AC is on; the other condition must be met for them to come on.

If you do that search now, you can learn more about the two separate circuits and how to test them.

Steve,

Thanks for the input.

Is it possible that the aux fan will never run if the ac is working properly?

[pmizell]

Is it possible that the aux fan will never run if the ac is working properly?

Yes it is possible, BUT only under certain conditions. If you're running your AC in mild weather (let's say 60 f) and the AC isn't being taxed, then it's likely that the low speed fans won't come on. However, if you're running the AC in 100+ degree weather, it is almost certainly going to trigger the low speed fans because it takes more AC compression to keep your cabin cool.

In both instances the AC is working just fine.

~Paul

____________________
'91 300E, 209k miles

[Jim Anderson]

My fans don't run with the AC on, but it is a little cool out, I did it for a test. My question is what should I see in the sight glass? What I see is white, like the freon is well aeriated. I unforunatly never checked it when everything was working right

Jim,

You should NOT see any bubbles or signs of aerated freon in the sight glass...it's definitely a sign of being low on freon. BTW, this is one of the causes of the aux fan not turning on, as well!

[MikeTangas]

"You should NOT see any bubbles or signs of aerated freon in the sight glass..."

Not necessarily so. On some MB A/C systems it is perfectly normal to see a small amount of bubbles in the freon. In fact on my 108, with a fully charged system, the bubbles cycle with the compressor - you see clear for a few seconds, then some bubbles for a few seconds. I asked about that here on the board and learned it was normal.

If all you see is white bubbles all the time, then you are probably low on charge.

Hopefully, in the coming week I will be able to compare A/C systems between the 108 and the 126, just waiting for some R-12 to arrive.

[Jim Anderson]

I checked my electric fans by unplugging the pressure switch and shorting it out and the fans do work. So I thought I'd go have my AC "topped off" but I was wondering how often the AC typically needs recharging. I think I heard somewhere avary couple of years is normal.

[jcyuhn]

The degree to which refrigerant appears bubbly in the sight glass depends on which refrigerant is in the system. For R-12 it is normal to see only a few, if any, bubbles. Indeed, the sight glass exists to aid in properly charging an R-12 system.

If the system is charged with R-134a, then the sight glass is useless. With R-134a the state of charge cannot be determined via the sight glass - there are always bubbles present, even when the sytem is over charged. In later models, MB eliminated the sight glass - my 210 does not have one. In this case a set of refrigerant guages is required to check the high & low side pressures in the system.