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  #1  
Old 05-12-2007, 09:13 AM
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300E Aux Fan Fuses? Strip fuse?

Can anyone tell me where the second fuse (strip fuse?) is in my 87 300E? I've read it's just in front of the main fuse box, but I can't find it...
Thanks.
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'87 300E 135K Miles
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  #2  
Old 05-12-2007, 09:25 AM
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There probably isn't one. The added strip fuse was installed on later models in production due to early failings. It is also often added where the fuse contacts have fried in the box. If it isn't around the shock tower or near the master cylinder compartment then you probably don't have one.
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  #3  
Old 05-12-2007, 10:43 AM
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aux fan strip fuse

Is it mounted on the strut tower, fairly heavy gauge wires on both ends, with a small, thin metal (i.e. strip fuse?) between them? If so, it was right in front of my face, just didn't realize what it was...

I have made some progress (hopefully I didn't do too many things at once leaving me unsure of the problem...) I cleaned and replaced the #7 and 10 fuses--they both seemed fine. I also rotated the relays (they're all silver Hella fuses--indication they're all replacements?) just to see if that made any difference...

With the car at running temp (i.e. 80c), I jumped the 105/115 switch and the FAN CAME ON (yipee!) at high speed. Then I jumped the AC switch w/ the ac running and engine temp somewhere between 80 and 100 and the fan came on at low speed. So, it looks like the ballast resister is ok since it ran on low speed...

With that, it looks like I need a new AC switch. I'm not so sure about the 105/115 switch. I'm not certain the engine got up to 105 just sitting in the driveway running (it did rise above 80 but I wouldn't say it was half way to 100). The only thing that confuses me, is yesterday when I was running it around lunch time, the engine temp was above the 100 mark and the fan still didn't turn on... That would make me think the 105/115 switch is bad too. Seems unlikely that both would go bad at same time though... I need to get it up to temp today (over the 100 mark) after messing w/ fuses and relays and see if it kicks on. If not, it would appear both switches are bad? Am I thinking clearly here?

BTW, this forum is fantastic--I know enough just to get myself in trouble, but w/ all the info on this board, I might be able to actually figure it out--with out having to run wires and a toggle switch up to the dash to turn it on manually (not at all becoming of a MB ).

Thanks in advance.
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  #4  
Old 05-12-2007, 10:59 AM
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You have verified the low and high fan circuits w/jumpering the sensors/switches.. . Now you need to know if the 105 trips at temp for high fan.
But, on low fan you have verified the circuit by jumpimg the a/c pressure sw,. but do not assume that sw to be bad. It has to see a high pressure from the a/c system before it trips and many do not trip b/c the system is slighly low on refrigerant [ meaning high side never gets to trip pressure] . Gauges are needed here , or you can monitor the refrigerant eye sight. [ if yu still have the original R12]. Also be aware that the high side of a fully charged system will not get to sw trigger pressure if the ambient temps are low. The aux fan is just that---an aux air-flow when either the coolant gets out of line or the a/c sees a high thermal load...they both work when either system is being taxed ..they do not work until these conditions are met.

The strip you see may be the heater blower fuse...which would have nothing to do w.aux fan circuits.
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  #5  
Old 05-12-2007, 11:05 AM
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Does the refrigerant eyesight look different at high or lower pressures?
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  #6  
Old 05-12-2007, 11:20 AM
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Quote:
Originally Posted by cmcdonnell View Post
Does the refrigerant eyesight look different at high or lower pressures?
Low refrig would be indicated by foam/bubbles.full charge would be clear ..
That is if the unit was not converted to R134...many were. R134 will show some bubbles even when fully charged,,that's why you would need gauges and sw pressure cut-in spec to make that sw diagnosis if R134.

Usually you can get the high pressure sw to trigger if you try the a/c at a high idle on a hot day. Can even block the radiator a little to getthngs up there. Just use your head.

On you coment of a manual toggle in the cabin..this is an excellent idea used by many on the older Benzs..you put a toggle in the cabin wired across the a/c pressure sw. that it gives you control of low fan anytime you see the temps creeping up..this is excellent in traffic or towing, etc...the niceity of it is that it does not interfer with the stock system, but lets you over-ride the sensors to get a jump on it...best of both worlds..I have used one on all my Benzs over many years and they are just a nice saftey feature to have when that one time comes into being..like stuck in traffic at the Beach, ya know??????
The newer units have fan default if a sensor goes, so they do not need it, but the older units don't , so you just add this and you are in control..
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  #7  
Old 05-12-2007, 11:26 AM
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Refrigerant is a colorless gas or liquid depending on temp or pressure. The sight glass will appear clear when the system is empty or when it full. When it is part full the liquid is foamed by the turbulence of flow with gaseous refrigerant. This will look milky.

To really get an idea of what you are looking at, have someone inside the car shut off the AC compressor while you watch. Even a clear/full system will slow down and show some foam and then the liquid surface level will appear and recede. The only true way to know the state of charge is by weighing in the proper amount. Arthur is sure right about that fan not coming on if the pressures are low.

I get a few cars every year that start to run hot because the AC has lost enough refrigerant that the fan stops coming on and the cooling system gets noticably hotter as a result.
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  #8  
Old 05-12-2007, 11:43 AM
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SB.
Thanks for the compliment......Do I get the job??? I will start at the bottom , sweeping the floors

TSH24.

Here is a schematic that will help ..your chassis uses *footnote #1..
You can see that the #2 circuit went with the later temp trigger sensor using a thermistor type [ that had the default fan activation feature].

http://members.aol.com/ajdalton7/auxfan.jpg

PS . I see that schematic did not come out too clear, so S/32 a/c pressure sw specs. are: Closes/20 bar---open/15/bar...

Last edited by Arthur Dalton; 05-12-2007 at 11:50 AM.
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  #9  
Old 05-12-2007, 12:02 PM
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Thanks. Refrigerant levels...

Thanks to all of you for the insight. I appreciate the help.

The car was converted to 134A some time ago. It was low, so I added approx. one 12 oz can. I can see lots of bubbles in the sight glass. the system didn't seem to want to accept the whole 12 oz can (I did not evacuate the system before hand) and the pressure gauge was reading about 50-55 PSI (in the yellow area of my cheap little gauge). I assumed that was a sufficient amount. It does seem to blow fairly cool--not what I would describe as really cold, but ok. Maybe I should evacuate the system and recharge?

Thanks again for the info. I've just had the car 2 wks now, but it wasn't driven much the last few years (owned by an elderly man) so I'm planning on having to work out some bugs. It's a pretty nice car though--91k miles, nice paint/clear coat, smooth ride, runs well, etc. I'm liking it so far. I know it's cliche at this point, but it does seem really well built--pretty cool.
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Old 05-12-2007, 12:06 PM
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As we can see you are inclined , Go to ebay and get a 124 chassis service manual cd ..I see them for $5/$10 now ..worth it .... if not just for the schematics
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  #11  
Old 05-12-2007, 12:07 PM
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What actually causes high pressure in the AC system? thus triggering the sw? Do higher temps cause the pressure to rise? If this is the case, then the ac sw is actually triggered indirectly by heat (directly by pressure)?
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Old 05-12-2007, 12:10 PM
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I've seen the service manuals on cd on ebay. I wasn't sure if they would be legitimate and worth it or what. They are pretty cheap though, so probably worth it like you say. Thanks for the suggestion.
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  #13  
Old 05-12-2007, 12:31 PM
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Quote:
Originally Posted by TSH24 View Post
What actually causes high pressure in the AC system? thus triggering the sw? Do higher temps cause the pressure to rise? If this is the case, then the ac sw is actually triggered indirectly by heat (directly by pressure)?

Yes...Kind of.
The system has two sides .The Low side [evaporator] and the high side
[ condenser] . The main cause of high side pressure is the output of the compressor...but the proper refrigerant level effects this capacity.. the compressor can not compress something that is not there. And the higher the temp, the higher the pressure becomes..that is what High Termal Load refers to. So, if this high side presure gets too high b/c of high ambient temps, an added amout of air-flow across the condensor helps keep it in check. This air flow reduces the condensers temp and that directly reduces the high pressure..if the pressure get too high for even the aux fans to keep it in check, then you will see a compressor saftey cut-out sw ..These vary on different systems..
Your sw is pressure triggered, but some condenser fans are in fact temp sensed/triggered.
The refrigerant just has a higher pressure at higher temps, regardless if the a/c is on or not. Same as your tire pressure increases w/temp. It is a contained gas and a law of physics..
Just a SB mentions cars coming in that are running hot coolant temps b/c they are low on refrigerant [ low fan never coming on, but enough passive heat from condensor to effect the engines cooling ability], the same applies to the a/c fan where the complaint is the fan never comes on when a/c is on. You wind up with a hot running engine temp b/c you have a hot condenser in front of the cooling radiator, thereby restricting the radiators ability to tranfer coolant heat. .......BAD
Another reason for a no low fan can be [ and often is] simply b/c there is low ambient temps when the a/c system is on, so the condenser never needs assistance from the fan . [ never reaches high pressure cut-in]. This condition seldom cause high engine temps and is Normal.
That is why the correct diagnosis should be made with gauges to compare the sw specs to the actual high side readings...and evap temps.
Same problem as with a unit that is low on refrigerant, but different diagnosis b/c of ambient temp coming into the equation..that is why we referred you to eyesight..MOST common NO FAN on a/c system is low refrigerant, where you still have cooling, but not enough refrig to trip sw.
You were corrct in jumping the high side sw before anything else b/c you verified the circuits electrical integrity before getting into the refrigeration system itself. I see many guys just change the sw [ which means total evacuation. etc] when they really never needed one in the first place.
Clear as mud.........
In you newly aquired car, I would evacuate the system and recharge the syatem with the correct capacity charge ..that will probably do it...

Last edited by Arthur Dalton; 05-12-2007 at 12:57 PM.
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  #14  
Old 05-12-2007, 12:49 PM
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by the way, I wish I had the problem of being stuck in traffic at the beach...!
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  #15  
Old 05-12-2007, 12:55 PM
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Believe me, It is not all it is cracked up to be. In my 88 300E w 91K miles, no air conditioning (yet) and the temp gauge creeping past 100, it can be very aggravating! I think I may install a toggle switch some where on the dash.
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