S70 A/C problem (Mike F?)

Mike, fellow Volvo experts.....

Car is a 98 S70 with manual air conditioning, which today was re-gassed as
it was working but not very well with the compressor cycling every 15-20
seconds,- probably upon instructions from the low pressure switch.

470 grammes was sucked out and the correct 750grammes (sticker on wing) was
put back in with some dye incase of future leaks. I now have chilly air and
the compressor is behaving as it should. (on the move)

However, I still have a problem with the radiator fan which is not running
with the a/c engaged. It does run on slow speed about every 40 seconds for
less than 1 second at a time. This was present before the re-gas- I assumed
it was because it was short of gas.

The condensor is obviously roasting hot when the car is stationary- not
good.

I havent done any investigation upon, what appears to be the high pressure
switch down by the charge point by the PAS pump, which I assume instructs
the fan to run.

I am aware that the ECU controls the fan, via the 2 speed relay, and not the
HP switch directly, but my Haynes BOL wiring diagrams do not show how the
a/c system interacts with the ECU, apart from their being 4 connections from
the ''air-cond.'' to the ECU. (possibly 2 from H and L pressure switches??)

Any pointers??

Tim..

 

The high pressure switch is a pressure sensitive potentiometer that
reports the A/C high pressure to the fuel ECU. Based on that signal (as
well as engine temperature information and other things) the ECU
"decides" whether the fan should be off, on low speed or on high speed.
Also the ECU actually turns the A/C on as well - the climate control
panel sends a "request" to the ECU for A/C when you turn the system on.
If the engine temperature is too high, the A/C won't come on. The ECU
grounds the control wires at the relay to turn the fan on high or low
speed.

Pull the 2 pin connector with the small wires off the fan relay. Ground
one of the relay pins, and see what the fan does. Ground the other, the
fan should start on the "other" speed. High speed sends power down the
red wire to the fan, low speed uses the green wire. In any case the
black wire is grounded. I believe the relay is wired in such a way as
only one of the red or green wires gets power at any one time.

--
Mike F.
Thornhill (near Toronto), Ont.

Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)

 

Mike,
Cheers for all of that.

Investigation has revealled all you say above- the a/c pressure sensor is
registering about 1v with the a/c idle, rising to about 4v after running for
abit. All good.

Grounding pins on the fan controller results in low speed fan activity or
full speed. All good.

Leaving the engine at idle for a while eventually brings on the full speed,
by which time things are pretty cooking.

However, the ECU does not seem capable of calling for low speed fan at all
(a/c running or not) it is not grounding pin 1 of the fan controller to run
the fan on slow.

I am thinking- either i have a broken wire, or the ECU is faulty.

I have not yet investigated continuity along the wire due to the need to cut
the ECU cover off in order to remove it to gain access to the multiplug at
the base of the ecu box.

Perhaps the best next stage would be to have a Scanner plugged in to OBD and
request the ecu runs the fan- this will check the ecu and the wiring at the
same time???

Hope I am not missing anything??

Many thanks
Tim..

 

Well to cut a long story short, the problem was two fold.

Firstly the low-speed fan resistor was open circuit- this became apparent
when I bypassed the relay and tried the fan direct from the battery.
Nothing.

However the ECU was still not switching the low speed side of the relay,
working fan or not, so I thought I had an ECU fault. ATP tested it and
pronounced it fine. Hmmmm I thought..

Anyhow it turns out that the ECU doesnt / can't switch the relay without a
load on the circuit it is switching- somehow the relay feeds a signal back
to the ecu, and with the resistor o/s it wasnt getting this signal...

For anyone with the same problem a 1 ohm 200w resistor is the baby you want
to replace the factory one, which is part and parcel of the fan- Volvo want
to sell you a complete fan and motor and resistor. This gives the fan about
7v and can be zip tied to the fan cowl in place of the broken one.

Tim..