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?

[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.

[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.

[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

[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.

[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.