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Topic: Aircon WOT relay problems (Read 1150 times) |
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TonyS
Junior Member
Scorpio Nutter - 12 years
Posts: 96
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Aircon WOT relay problems
« on: May 3rd, 2004, 7:57pm » |
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Summer is starting to arrive and to my horror I found my aircon would not engage.
Tracing back the power feed to the pump clutch showed no problems with the system (dual pressure switch showing there was gas in the system) I finally traced the power feed to the clutch back to the WOT relay and found that it was showing a closed resistance of about 50 ohms.
It's noted that reseating this relay can cure problems but I found they just reappeared after a short time. In the end removing the relay, taking the plastic cover off and cleaning the contact points is the correct answer (or replace the relay but more of that below).
Thinking about the operation of the WOT relay there is a flaw in the design. Under Wide Open Throttle conditions the PCM activates the relay to shut off the power drain of the aircon compressor. The compressor is engaged by powering a electronic clutch, where a electromagnet is used to pull the clutch plates together.
The WOT relay breaks the power feed, and there is a back EMF diode to shunt the amps generated by the solenoid field collapsing. This diode protects nothing in the case of WOT, as the relay has opened leaving the circuit O/C. In the case of the relay opening the holding current in the solenoid will be relatively low giving little of an arc as the contacts seperate.
On leaving WOT the PCM deactivates the relay with the resultant energisation of the clutch, but at a much higher initial amperage and hence increased chance of arcing and contact errosion. The light rating of the relay does not help as the contact area is small and in the one I cleaned the contacts did not align fully.
As part of my diagnosis I'd swapped the relay, R24 - blue, with one from my laid up Mk3 (which had inoperational aircon as well) and the problem had persisted for the same reason. From this I'd guess the relay is a weak link in the system and should be checked if erratic pump operation occurs.
Cleaning is only a partial answer as the contact plating will have been eroded. Replacement with new is the better option.
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1996 Mk4 2.9 24v Estate
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scorpio_man
Administrator
Ford Kuga awd
Posts: 5654
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Re: Aircon WOT relay problems
« Reply #1 on: May 3rd, 2004, 9:19pm » |
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hi tony
can you take some photos and do a write up for the main site? where the relay is, how you diagnosed the problem etc. this is good info and needs to be shared. send it to pete c. that's who did most of the a/c pages.
Pete c
pete@dg142fy.fsnet.co.uk
owner-scorpio@simplelists.com
regards
ps. i think that's the right email addresses for him.
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ianmc
Full Member
2.9 12v saloon 95
 
Posts: 102
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Re: Aircon WOT relay problems
« Reply #2 on: May 4th, 2004, 1:14pm » |
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hi tony what numbers are on top of your blue relay R24 as i had to update mine.
ianmc
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