Aircon low pressure switch

[Chris Hawes]

The aircon system has a suspected low pressure switch fault. Luckily I have an unused spare. The question is can this be safely unscrewed from the accumulator/dryer whilst the system is full of gas?

Thanks in advance

[Chris Williams]

Yes, did it myself last year when I got mine working again. Mine had a schrader style valve on the dryer part. Though this was also new, an old one may be other!
Chris

[Chris Hawes]

Thanks Chris

For context the system was refilled with 950g of R134a (after being vacuumed and pressure tested) a few weeks backs (converted over years ago from R12). It has in the last two years had new orifice tube, dryer and compressor. All fine for a few weeks and then the weirdest symptoms started:-

Driving along you can hear the blower relay twitching on/off and with it the compressor clicking on/off in time with this. I have my Cooling Fan Fail light setup so that when the radiator fans/compressor come on it lights up and this is blinking too. If you lift off the revs so that the car is at tick over then the aircon/compressor runs without twitching and as soon as you apply throttle it starts twitching again.

Troubleshoot was done to bypass the low pressure switch (briefly) which solves the problem hence why I wanted to test the spare unused switch.

Any additional thoughts as to what could be causing this?

Does the low pressure switch require calibration?

[DMCTodayMichael]

Chris Hawes Wrote:Thanks Chris

For context the system was refilled with 950g of R134a (after being vacuumed and pressure tested) a few weeks backs (converted over years ago from R12). It has in the last two years had new orifice tube, dryer and compressor. All fine for a few weeks and then the weirdest symptoms started:-

Driving along you can hear the blower relay twitching on/off and with it the compressor clicking on/off in time with this. I have my Cooling Fan Fail light setup so that when the radiator fans/compressor come on it lights up and this is blinking too. If you lift off the revs so that the car is at tick over then the aircon/compressor runs without twitching and as soon as you apply throttle it starts twitching again.

Troubleshoot was done to bypass the low pressure switch (briefly) which solves the problem hence why I wanted to test the spare unused switch.

Any additional thoughts as to what could be causing this?

Does the low pressure switch require calibration?

The switch is there to protect your system so bypassing it is a very bad idea. Adjusting it can be just as bad if you do not know what you are doing. if your system recently just started doing this then something has changed, most likely you lost a bit of freon. This also means your switch was set incorrectly to begin with.

A perfectly functioning AC system will indeed cycle at higher rpms. Ideally it will click on for about 7 seconds, off for around 3 but that is up for debate among the "by the book types". The switch is there to protect your system but also to keep the evaporator from freezing up, which is why it cycles. At idle, the system will not cycle as much or not at all since the pressures are different. You are probably hearing the compressor relay, not the blower speed relays. Keep in mind this is all for r12, with most cars running 134 these days the amount of freon used is less and will no doubt affect pressures slightly, especially if the system has the incorrect amount of 134 freon(I believe it's supposed to be 10% less with 134)

I doubt your pressure switch is bad. People say they don't need adjustment but that's in a perfect world. In the past I had to adjust a brand new switch which was supposed to be factory set but was so far off I couldn't get the compressor to even turn on long enough to take freon. After the correct amount was in it still cut out way too often. (I was using freeze 12).

If you do monkey around with the setting It is very important to adjust with the gauges to make sure you are not going overboard.


As far as your fan fail light, I believe this was supposed to be unhooked at the relay box unless you have fixed it with Dave's relay or a proper replacement.

[Chris Hawes]

Sorry I wasn't clear it is twitching on and off at a rapid rate (way faster than normal). Morse code fast, up to few times a second fast. Think of it as an erratic, rapid blinking.

[DMCTodayMichael]

Chris Hawes Wrote:Sorry I wasn't clear it is twitching on and off at a rapid rate (way faster than normal). Morse code fast, up to few times a second fast. Think of it as an erratic, rapid blinking.

Put your gauges on it. You will most likely find you are low in freon. This is what the switch is there to protect, don't bypass it.

[Chris Hawes]

low pressure switch was the fault...
Luckily I had a spare that had from a previous vendor order.

New switch has re-solved the problem. Just been away for the weekend and had lovely normal cycling aircon for the full duration.

Had input from a friend who is a harness designer in the automotive industry. He has introduced a relay into the aircon circuit (I can't believe the DMC setup doesn't have this as some of the circuits have relays). It's 'shocking' to see the lightning storm through the plastic cover of the relay as the aircon is switched on and off as this means the stock setup has this instead spread across the contacts across the rest of the setup. When a part fails you otherwise think its because of age/DeLorean/Lucas etc....

In short, the new relay above handles the load and takes the strain that the existing components would otherwise carry on taking. This now means long term reliability increased for all associated 'contact' based components in that part of the circuit.

The horns as standard don't have a relay (this car does).

I'll sort some photos out.

[DMCTodayMichael]

Glad you fixed it. Very odd but not impossible. Those switches rarely go bad and 99% of the time if it is cycling too much then there is a low freon situation. You found the 1%.

[Chris Hawes]

It's only taken two years, but here are the photos mentioned from my last post.

Chosen location for aircon relays in an automotive rated enclosure



Cover removed


I suspect it was a lack of time/development as to why the cars didn't have relays incorporated into the aircon circuit.  This is a very much an upgrade that should be on any DeLorean to improve the longevity and reliability.  If not the various switches are all having to handle the loads that the relays can now handle!