Auxiliary Fan Preresisor Delete? - PeachParts Mercedes-Benz Forum

JamesDean · #1 05-27-2009, 01:21 PM

Car: w201.029 190E 3.0L

I spend a good amount of the weekend diagnosing an air/fuel mixture problem as well as a circuit breaker trip issue. I installed a 143amp alternator and at the suggestion of Hit Man X I installed a 200amp breaker. The breaker had been tripping well before the 200amp marker. I used a ammeter to verify this. During the final load test in which I turned everything possible on, the auxiliary fan preresistor caught something on fire.

As I understand it the resistor is only used for "low" speed which is only kicked on by the air conditioning pressure switch trigger.

The fans are set into high speed when the engine gets too hot.

My question is: If I just connected the wires that were originally connected to the resistor (this giving the fan full speed at "low") would it harm anything? Would running the fans at full speed just for condenser cause any abnormal behavior?

Presently my ac is not charged so this isnt really a huge issue but I am curious.

I'm not too concerned about power usage as I have that 143amp alternator.

I was just wondering what everyone thought about deleting the resistor.

Thanks,

Kris

Arthur Dalton · #2 05-27-2009, 01:41 PM

The problem with deleting the resistor by jumpering it is that will get you high fan, BUT, the wiring from the pre-resistor relay is not as heavy as the high speed circuit . So , the trick is to use the feed wire to the rsisitor [ single wire side] to a power relay and feed that relays output to the 2 wire side of the relay..that gives you high fan with a/c instead of low fan w/o a problem with the wiring for low fan.......... Fuse that new line w/25 A

If you do not use the low fan for a/c , you can also put a cabin sw jumper at the drier sw to activate it manually from the cabin..that is a popular mod for Benz guys ..great to have a manual over-ride for traffic, towing, etc..and that does not change any stock fan cicriuts, it simply gives you manual command over them. [ like puting the heater on to cure an overheat condition]

You can also lower the high fan circuit if you have the blu 2 wire sensor for high fan by bridging that sensor with a resistor for lower temp cut-in.

JamesDean · #3 05-27-2009, 03:16 PM

Quote:

Originally Posted by Arthur Dalton

The problem with deleting the resistor by jumpering it is that will get you high fan, BUT, the wiring from the pre-resistor relay is not as heavy as the high speed circuit . So , the trick is to use the feed wire to the rsisitor [ single wire side] to a power relay and feed that relays output to the 2 wire side of the relay..that gives you high fan with a/c instead of low fan w/o a problem with the wiring for low fan.......... Fuse that new line w/25 A

If you do not use the low fan for a/c , you can also put a cabin sw jumper at the drier sw to activate it manually from the cabin..that is a popular mod for Benz guys ..great to have a manual over-ride for traffic, towing, etc..and that does not change any stock fan cicriuts, it simply gives you manual command over them. [ like puting the heater on to cure an overheat condition]

You can also lower the high fan circuit if you have the blu 2 wire sensor for high fan by bridging that sensor with a resistor for lower temp cut-in.

You bring up a good point. I did not think of the wire thickness.

So, you're saying install a relay in place of the resistor using the input side of the resistor as the switching signal for the relay? I have a few 12v/40a relays laying around that sounds like a good idea. This seems like a good idea actually, I would maybe recommend this to others if they are comfortable with the fans at full speed and uncomfortable with that resistor.

I do plan on having my ac recharged eventually here so I will be using it for the AC. However I honestly don't see any problems with the fans running at full speed during AC operations.

I do in fact have a switch installed to manually kick on the aux fans (low speed) as you mentioned. Just a precautionary measure. I don't have any overheating issues currently.

If I were to jumper a resistor across the blue two pin sensor, what value would it need to be to kick on the fans at say 100C? What is the default kick on resistance value?

Thanks,

Kris

Just for reference, here is the 201 Aux Fan circuit for late model 201.029s

Arthur Dalton · #4 05-27-2009, 04:17 PM

Yes

Look at the schematic.

If you use that low fan feed at the resistor [ single wire] for the coil of a seperate relay, with a ground for the other side of relay coil , when you toggle your cabin sw you have for the a/c pressure sw, the coil will pull in the extra relay's contactor set ..so, on the new relay , you just grab a good heavy 12v from Bat+ someplace , stick a 30A fuse in the line , and bring it to the relay contact terminal... the other side of the relay contact goes to the 2 wire side of the resistor..that gets you high fan instead of low when the a/c high pressure sw closes for a/c operation.. or when you use the cabin sw you already have hooked to the pressure sw at drier. That takes the R15 out of the circuit... notice low fan uses 2.5 wire and high uses 4.0 gauge ...if they both used the heavier wiring, you would only have to change the fuse at the low relay they use and do some terminal changes. @ R15......but guys that do that have had trouble with that 2.5 wire burning up...
You can see that the weak link in the low fan circuit is that 2.5 Bk wire running from the relay to the R15...the Input wire to that low fan relay is heavy enough, so some guys dig in there and change that 2.5 to a 4 or run another 2/5 along side and then change the low circuit fuse from 15 to a 30. Then you can just jumper R15 with a solid heavy wire, or move the single wire over to the R15 output side [ 2 wire side]
I like seperate relay cuz you can up the load capacity of the relay contacts.
Notice the high fan relay has dual contacts to share the higher load of high fan draw.
On the blu sensor, a 1500ohm will work, but you can use a variable to get that anywhere you want it and then just measure where you had it set for ohms an then install a value that matches.

..
Guys do both these mods in real hot climates to help the a/c output and get a lower coolant temped sensed high fan protection.
The advantage to my set-up is you can get High Fan anytime you deem it needed..you are the sensor and you can over-ride the system at will...but at the same time, the stock set-up still will kick in by itself..
I know a guy I did this for and the idiot would toggle the high fan anytime he pulled into a place with other motorheads just to watch them look when the noise from the fans came on.....unreal

babymog · #5 05-27-2009, 09:39 PM

I believe that the '95 E320 has an updated resistor, a metal coil instead of the ceramic resistor, better design IMO and probably a direct replacement.

JamesDean · #6 05-27-2009, 11:37 PM

******** doesnt have the ceramic resistor. They have the metal coil one.

I'm pretty set on the relay idea unless it god awfully loud at full speed. I cant imagine it being able to over power the exhaust system I have though.

I recently upgraded the wiring on my alternator, you think the wire that used to run from the stock 80a alternator to the dist. block would cut it for the relay wiring?

Hit Man X · #7 05-28-2009, 01:39 AM

Maybe you just caught some leaves/debris on fire? My SDL did this but I caught it before anything occurred.

Arthur Dalton · #8 05-28-2009, 02:22 PM

If you want to test it for noise level, simply pull the blu sensor plug for high fan default and take it for a rdie.

That wire is fine....and dist block is a good source. Any heavy copper /stranded will do.

ke6dcj · #9 05-23-2015, 10:15 PM

Quote:

Originally Posted by Arthur Dalton

The problem with deleting the resistor by jumpering it is that will get you high fan, BUT, the wiring from the pre-resistor relay is not as heavy as the high speed circuit . So , the trick is to use the feed wire to the rsisitor [ single wire side] to a power relay and feed that relays output to the 2 wire side of the relay..that gives you high fan with a/c instead of low fan w/o a problem with the wiring for low fan.......... Fuse that new line w/25 A

If you do not use the low fan for a/c , you can also put a cabin sw jumper at the drier sw to activate it manually from the cabin..that is a popular mod for Benz guys ..great to have a manual over-ride for traffic, towing, etc..and that does not change any stock fan cicriuts, it simply gives you manual command over them. [ like puting the heater on to cure an overheat condition]

You can also lower the high fan circuit if you have the blu 2 wire sensor for high fan by bridging that sensor with a resistor for lower temp cut-in.

FWIW- on an early W124 gasoline that's being upgraded to dual-fans OR running the low-speed fans in "high mode", you must either use a separate relay as Arthur suggested OR upgrade all the wiring, fuse holder, and preferably the "ice cube" relays with the newer style relays with fuses, to prevent melting items, and a potential fire.

I updated my 300TE to the late-model dual fans, and even when run in "low-speed" mode, I was melting fuse holders, and wiring, all the time.

I finally ran a 10-gauge stranded copper wire for relays feeds, as well as from the low-speed relay to the aux. fan resistor.

I measured 11-amps on the "ice cube" relays, and 23-amps on the high-speed circuit. These "ice cube" relays were replaced with the newer relays with ATC fuses on their side (15-amp and 30-amps).

By doing so, I basically upgraded everything to the 1990+ circuit, although with 10-ga. wire which has reserve capacity, instead of 12-ga.

No more melted wires or fuse holders.

:-) neil

pmckechnie · #10 05-24-2015, 09:43 AM

Kris
I would like to get a few things straight in my mind.

First you said

"fan preresistor caught something on fire.
As I understand it the resistor is only used for "low" speed which is only kicked on by the air conditioning pressure switch trigger.
Presently my ac is not charged so this isn't really a huge issue but I am curious."

Why would the preresistor get to hot if it wasn't being used? Did you have the A/C pressure switch jumpered?

Did your car come with the duel fans?
If it did, then you may have other problems like a bad fan (Drawing too many ams) or something.

Now look at the diagram you supplied. Look at the wire from the preresistor to the fans. It is a 2.5mm wire that splits off to 2- 1.5mm wires. That makes me think the MB thought that a 2.5mm wire was big enough to run both fans at low or high speed. I know the length of the wire comes into play but generally wires are rated for up to 10 feet in the automotive field. Larger wires if more than 10 ft.

Second
I am having a problem with the 200A CB tripping. Where in the circuit is this CB installed? Unless you have added a lot of power consuming devices, you should never see a 200A load. The only time that much would be used is while the starter is engaged and the engine not running and the starter usually is never fused. If your car is drawing 200A when not starting, then you have a fire looking for a place to happen. In over 50 years of working on cars I have never seen anything like that.

Could you set my mind at ease and help me understand how these things are happening?

Thanks
PaulM