One for auto electricians

[PositronicRay]

Hi I have a 1994 MB SL R129 fitted from new with an aftermarket Diavia AC system.

It uses 2 Spal pusher fans in front of the condenser. One of the fans has never worked in my ownership (A/C works fine), looking @ old invoices (and date on the fan) it was replaced about about 10yrs ago.

I've checked the fan, there is 12v at the connector so assume the fan has burned out. Now the odd thing poking around I've found the fans wired in series, the dead fan is the 1st one in series if I unplug the dead one fan 2 stops
Is this usual or would I be better off rewiring them in parallel?

[powerstroke]

PositronicRay said:

Hi I have a 1994 MB SL R129 fitted from new with an aftermarket Diavia AC system.

It uses 2 Spal pusher fans in front of the condenser. One of the fans has never worked in my ownership (A/C works fine), looking @ old invoices (and date on the fan) it was replaced about about 10yrs ago.

I've checked the fan, there is 12v at the connector so assume the fan has burned out. Now the odd thing poking around I've found the fans wired in series, the dead fan is the 1st one in series if I unplug the dead one fan 2 stops
Is this usual or would I be better off rewiring them in parallel?

I'm almost certain they would have been wired in parallel originally and operated by the trinary pressure switch ,so in normal running and cooler conditions the mecanincal fan was OK but if you were stuck in slow moving traffic and the system pressure hit 15 bar the electric fans would cut in .
Maybe someone has rewired them so they run at half speed all the time insead of intermittently through the pressure switch???

[PositronicRay]

The MB original equip was a 2 speed affair, high speed cutting in when engine temp goes over 100c

I think the after market system is just one speed operated from a temp sensor on the A/C, mine doesn't cut in with engine temp.

The main fan is engine driven I replaced this earlier in the yr before a summer trip down to the med.

Engine stays cool, A/C is okayish (kept us comfortable on hotter days)

I think I'll replace the burnt out fan and rewire in parallel, could wiring in series be cause of one fan burning out? Before I buy a new fan I'll try to dismantle the old one in case I can fix it.

[powerstroke]

[Doctor Volt]

It has always been a problem when wiring any electro mechanical parts in series, the problem arises because although these parts are of identical build they are not working in the exact same environment, one fan over time will rotate faster than its partner fan and the the problem arises that one fan is consuming more current than the other and eventually one fan will cease to work before the other

There is also the problem that one fan may accumulate more dust than the other and will then consume more current, also temperatures of fans may differ and also bearing efficiency may differ and these differences give the same result as above

If you are able to wire the fans in parallel they will operate much better and for much longer

This Is Important - Should you wire the fans in parallel they will rotate much faster than the original ones and you may need to add a speed control (Variable Resistor) in series with the fans supply so as to be able to slow the fans down should you have too much draft

I hope this helps

Cheers. Doctor Volt

[PositronicRay]

Doctor Volt said:

It has always been a problem when wiring any electro mechanical parts in series, the problem arises because although these parts are of identical build they are not working in the exact same environment, one fan over time will rotate faster than its partner fan and the the problem arises that one fan is consuming more current than the other and eventually one fan will cease to work before the other

There is also the problem that one fan may accumulate more dust than the other and will then consume more current, also temperatures of fans may differ and also bearing efficiency may differ and these differences give the same result as above

If you are able to wire the fans in parallel they will operate much better and for much longer

This Is Important - Should you wire the fans in parallel they will rotate much faster than the original ones and you may need to add a speed control (Variable Resistor) in series with the fans supply so as to be able to slow the fans down should you have too much draft

I hope this helps

Cheers. Doctor Volt

Yes thanks that helps. I suspect they should have been in parallel all along, maybe an error on the part of the installer. I will check the spec for the MB fit fans against the spal jobbies.

[Doctor Volt]

PositronicRay posted - I suspect they should have been in parallel all along, maybe an error on the part of the installer.

I agree with your suspicions, good luck

[Texpis]

Well if the fans are wired in series the first one cannot be burnt out because the second one wouldn't work with an open circuit in the first fan.

Mick

[PositronicRay]

Texpis said:

Well if the fans are wired in series the first one cannot be burnt out because the second one wouldn't work with an open circuit in the first fan.

Mick

Quite right, not burnt out, an internal short.
I stripped it and found the wires connecting the bushes was shorting (very poor design one wire crossing another with no insulation)
Being clumsy and a design not intended for maintenance I managed to break 2 of the 4 magnets.
Still I've re assembled it with just 2 magnets and it's working okay. Just need to refit.
I think I'll rewire them in parallel, what sort resistor should I use?

Edited by PositronicRay on Wednesday 24th December 13:58

[Doctor Volt]

Texpis said:

Well if the fans are wired in series the first one cannot be burnt out because the second one wouldn't work with an open circuit in the first fan.

Mick

Texpis - What if.............?

1) The faulty fan is wired into the return (Earth) side of the 2 x fan circuit, what I mean by this is the working fan has a supply to it and picks up its earth (Return) through the faulty fan.

There are two possible scenarios if the above is the case

Scenario 1) The faulty fan could be burnt to a cinder and its commutator being as good as a dead short to earth (Return) with the brushes still intact and touching its surface - The good fan will still work by picking up its return from the faulty burnt out fan

Scenario 2) The Armature windings of the faulty fan have an internal short (Armature wires connecting to eachother) due to being burnt out in one place, the Armature windings would not draw enough current to creare enough magnetism in all the correct places for the Armature to turn but it would allow the working fan to draw current through those Burnt/Shorted windings

Scenario 2 would allow the good fan to work even if the faulty fan was wired into the supply side and the good fan into the return side

Texpis - My conclusion is that you really havent given the subject enough thought before posting

I hope this helps

Cheers. Have a good Christmas

Doctor Volt

[Doctor Volt]

PositronicRay

Sorry, I was typing while you were posting

We have crossed posts above

Edited spelling

Edited by Doctor Volt on Wednesday 24th December 14:50

[Doctor Volt]

PositronicRay

Use a Variable Resistor - A Rheostat

A variable speed fan switch will work good if you can get hold of one, I dont mean a fan switch with positions for speed 1,2,3 and 4 or more - I mean a switch that as you turn it the fan speed alters and there are no set positions built into it. You will need to ask Google, thinking back I recall them being used on older vehicles before fan speed control units became the norm

You could use a fan speed control unit and a modern day fan switch with positions 1,2,3,and 4 or more but that would be creating a hell of alot of work for what you are planning to achieve

You mentioned two magnets being broken in the faulty fan

Are you sure that motor is working ok with only 2 magnets? I would expect that fan to be working at a much slower speed and even to possibly stick in a certain position. Have you tested the motor with the fan fitted as that is its load?

I would also be concerned about using an unmatched pair of fan motors, the fan motors are no longer a match since the magnets breaking

I hope this helps

Doctor Volt

[powerstroke]

All good fun but those old diava systems ran both the fans via a pressure switch so they only ran for a few minutes at most when the mechanical fan wasn't enough say in traffic and the head pressure went over 15 ish bar, not that I have a clue I only fitted hundreds of them!

[PositronicRay]

The motor powers up okay and doesn't stick with even the magnets missing.
I've tried it with the fan fitted, seems to create a good draft. It might run a bit slower than the other one, I won't really know until it's installed. Still it'll be better than not working at all, a new one is not expensive if it doesn't work too well.

I will try them in series 1st and maybe re-wire in parallel.

Powerstroke would a Diavia system originally be running @ full speed or 1/2 speed with a resistor? They do seem to run all the time the A/C is on but it's hard to know if they are cutting out when I'm driving.

I've not tried it in a low ambient temperature, would this effect the operation?

Edited by PositronicRay on Wednesday 24th December 17:40

[PositronicRay]

I've left them in series. Both seem to kick in okay and operate at a similar speed. They appear to run at the same speed albeit a little slower than when only one was working so that'll do me for now.

I've not tested them with the A/C yet I'll leave that till the weather is warmer.

Thanks chaps.

[powerstroke]

PositronicRay said:

The motor powers up okay and doesn't stick with even the magnets missing.
I've tried it with the fan fitted, seems to create a good draft. It might run a bit slower than the other one, I won't really know until it's installed. Still it'll be better than not working at all, a new one is not expensive if it doesn't work too well.

I will try them in series 1st and maybe re-wire in parallel.

Powerstroke would a Diavia system originally be running @ full speed or 1/2 speed with a resistor? They do seem to run all the time the A/C is on but it's hard to know if they are cutting out when I'm driving.

I've not tried it in a low ambient temperature, would this effect the operation?

Edited by PositronicRay on Wednesday 24th December 17:40

Sorry for the late reply
As before they were originaly fitted to suplment the mechanical fan so only ran when the ac was working hard you should have a round switch on the dryer with four wires, from memory two green black that control the compressor and two blue black that work the fan relay , sounds like someone has been messing with it and altered the wiring otherwise the only way you would get the fans to run is either running the ac in hot conditions or shorting the pressure switch!!

[PositronicRay]

Thank's. I've found the pressure switch but not the relay.

Last summer/autumn the single fan was kicking in okay. Warmish weather, switch the A/C on and the fan was kicking in a minute or two later. So I should hope that'll be the same now both fans are hooked up.

I'll suck it and see, I'm guessing that the fans wouldn't kick in in this weather anyway.