Aircon low pressure switch

[Chris_Von_Bron]

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

[Farrar]

Yes.

[David T]

Yes.


The failure rate of the switch is low. If the A/C isn't running the most likely cause is that the pressure is actually low, too low to run and that is why the pressure switch is not allowing the system to run. What is the pressure in the system when it isn't running? If it is not near 60 psi or more don't waste time replacing the switch. There is a valve under the switch so Yes, you can replace the switch and you will not lose refrigerant.

[PJ Grady Inc.]

In total agreement with David on this one Chris FWIW. All except "waste time" replacing the switch as it's 60 second job but as he said probably not the issue.
Rob


The failure rate of the switch is low. If the A/C isn't running the most likely cause is that the pressure is actually low, too low to run and that is why the pressure switch is not allowing the system to run. What is the pressure in the system when it isn't running? If it is not near 60 psi or more don't waste time replacing the switch. There is a valve under the switch so Yes, you can replace the switch and you will not lose refrigerant.

[Chris_Von_Bron]

Thanks for the input guys.
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.

Does the low pressure switch need any form of calibration?

Any additional thoughts as to what could be causing this?

[DMCMW Dave]

I've actually seen quite a few of them leaking between the wire terminals at the adjuster screw. Given the cost (under $10) we often pre-emptively replace them, and I'm not usually one to toss parts on a car "just because".

99% of the time you can just unscrew it and replace it. 1% of the time the shrader valve under it will be stuck open and you'll let the refrigerant out. So I'd rather change it as part of the job on an empty system.

And they do fail, but your pressure gauges should tell the story.

They are adjustable but it's not recommended or often necessary. And you can do an immense amount of harm by overadjusting it or jumping it out. As in blowing up the system or destroying the compressor.

[David T]

I've actually seen quite a few of them leaking between the wire terminals at the adjuster screw. Given the cost (under $10) we often pre-emptively replace them, and I'm not usually one to toss parts on a car "just because".

99% of the time you can just unscrew it and replace it. 1% of the time the shrader valve under it will be stuck open and you'll let the refrigerant out. So I'd rather change it as part of the job on an empty system.

And they do fail, but your pressure gauges should tell the story.

They are adjustable but it's not recommended or often necessary. And you can do an immense amount of harm by overadjusting it or jumping it out. As in blowing up the system or destroying the compressor.

If you use a switch in a converted system (not R-12) it should be adjusted. You should never be doing things on an A/C system without having a manifold gauge set-up so you can monitor system pressures. The switch could be leaking but a leak test will find that. You are hearing the cooling fan relay clicking, turning the rad fans on and off when the compressor cycles. Your symptoms sound like the system is cycling JUST LIKE IT SHOULD! Put a thermometer into the center vent. Put the A/C in NORMAL and the fan on 4. Hold the RPM's at 1200 or drive around a little. You should see at least a 20 degree difference between inside and outside temps.

[dustybarn]

If you use a switch in a converted system (not R-12) it should be adjusted. You should never be doing things on an A/C system without having a manifold gauge set-up so you can monitor system pressures. The switch could be leaking but a leak test will find that. You are hearing the cooling fan relay clicking, turning the rad fans on and off when the compressor cycles. Your symptoms sound like the system is cycling JUST LIKE IT SHOULD! Put a thermometer into the center vent. Put the A/C in NORMAL and the fan on 4. Hold the RPM's at 1200 or drive around a little. You should see at least a 20 degree difference between inside and outside temps.

..depending on how fast the compressor is cycling. I agree, it sounds normal to me.

[Chris_Von_Bron]

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

[Kozmatic]

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

It's all speculation without gauges on the system to see what the pressure is when the compressor cycles. Your symptoms are exactly what would happen with low refrigerent.

[David T]

It's all speculation without gauges on the system to see what the pressure is when the compressor cycles. Your symptoms are exactly what would happen with low refrigerent.

Depending on how rapidly it cycles, it certainly could be low on refrigerant. As mentioned previously, you really can't know how the system is working without a manifold gauge set hooked up and a thermometer. I would not think the low pressure switch is the cause of the problem. It is probably reacting to the low amount of refrigerant because of a leak.

[Chris_Von_Bron]

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

New switch has resolved 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.

[Bitsyncmaster]

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.

I hadn't thought about installing a relay to drive the AC compressor clutch. The current is not to bad. My clutch measures 3.0 ohms cold so max current would be 4.8 amps (my compressor is not stock). But it is that inductive spike that eats away at the contacts when they open. That would be the contacts in the low pressure switch and the contacts in the mode selector switch. I may install that relay some day.

I have helped a few owners that have had problems with that compressor clutch noise causing problems. I sent them an 18 volt 5 watt zener diode to clamp that spike and it fixed their problems. A lot depends on each cars wiring harness. When one wire runs beside another wire for a long distance that controls a critical circuit, the voltage spikes in one wire can feed through to the other and induce noise in that other circuit.

[David T]

I hadn't thought about installing a relay to drive the AC compressor clutch. The current is not to bad. My clutch measures 3.0 ohms cold so max current would be 4.8 amps (my compressor is not stock). But it is that inductive spike that eats away at the contacts when they open. That would be the contacts in the low pressure switch and the contacts in the mode selector switch. I may install that relay some day.

I have helped a few owners that have had problems with that compressor clutch noise causing problems. I sent them an 18 volt 5 watt zener diode to clamp that spike and it fixed their problems. A lot depends on each cars wiring harness. When one wire runs beside another wire for a long distance that controls a critical circuit, the voltage spikes in one wire can feed through to the other and induce noise in that other circuit.

It is all too true that the electrical system in the Delorean could stand a LOT of improvement. Besides the fact that it is a Lucas system, it had to fit a price point too. Every extra part costs. The cycling of the A/C using the pressure switch was a BIG cost cutter saving a bunch of valves and other controls to prevent the system from frosting up. Rather unconventional and I have not seen such a system in any other car. I agree a relay would be an improvement here AND for the horn.

[Bitsyncmaster]

It is all too true that the electrical system in the Delorean could stand a LOT of improvement. Besides the fact that it is a Lucas system, it had to fit a price point too. Every extra part costs. The cycling of the A/C using the pressure switch was a BIG cost cutter saving a bunch of valves and other controls to prevent the system from frosting up. Rather unconventional and I have not seen such a system in any other car. I agree a relay would be an improvement here AND for the horn.

All the GM cars I have owned ran the same AC system with orifice and cycling compressor.

[David T]

All the GM cars I have owned ran the same AC system with orifice and cycling compressor.

The GM systems used all kinds of valves to prevent the evaporator from freezing up. Unlike any other A/C system, the compressor in a car has different flow rates depending on the speed of the motor. To keep the evap coil above atmospheric and above about 35 degrees, the valving would maintain a constant pressure in the evap coil. By cycling the compressor you lose a lot of cooling capacity and essentially are running in a defacto defrost mode when the compressor is not running. By doing that and constantly blowing air over the evap coil you keep it from frosting over. Under just the right conditions the evap coil could freeze up anyway, the system is not smart enough to actually know if the coil is freezing up. At that point the high side would trip the high pressure switch hopefully. Another problem with cycling the compressor is that the clutch was not made for that kind of duty. It will eventually wear out. I have seen cars where I had to adjust the gap on the clutch by removing washers so it would engage. If the gap gets too big the clutch won't engage. A real Rube Goldberg kind of way to do it. Most A/C shops will refill till the clutch stops cycling. Standard practice if you don't fill by weight and there is no sight glass. Not good for the Delorean though!

[Bitsyncmaster]

I hadn't thought about installing a relay to drive the AC compressor clutch. The current is not to bad. My clutch measures 3.0 ohms cold so max current would be 4.8 amps (my compressor is not stock). But it is that inductive spike that eats away at the contacts when they open. That would be the contacts in the low pressure switch and the contacts in the mode selector switch. I may install that relay some day.

I have helped a few owners that have had problems with that compressor clutch noise causing problems. I sent them an 18 volt 5 watt zener diode to clamp that spike and it fixed their problems. A lot depends on each cars wiring harness. When one wire runs beside another wire for a long distance that controls a critical circuit, the voltage spikes in one wire can feed through to the other and induce noise in that other circuit.

I'm still thinking about installing a relay to drive the compressor clutch. I would use the blank relay socket that was for the AC panel lights.

[David T]

I'm still thinking about installing a relay to drive the compressor clutch. I would use the blank relay socket that was for the AC panel lights.

If you do that you may want to consider adding another WOT switch to shut off the compressor to give you a little extra oomph. And a relay for the horns?

[DMCMW Dave]

If you do that you may want to consider adding another WOT switch to shut off the compressor to give you a little extra oomph. And a relay for the horns?

Wouldn't need another WOT switch, just diode off the one that is there.

[Delorean Industries]

One of the many perks to an advanced EFI system.

TPS above 90%/ KPA above 65 cuts out compressor operation. Decel off full throttle has a half second delay prior to compressor engagement after 90%-WOT. Cooling fans are then staged 1 second apart and varies due to CLT/MAT input in a decel condition. IE no stalling or electrical system load when snapping throttle shut after WOT. (that annoying sub 750 rpm dip K jet offers). Also on decel/below 15% TPS and KPA under 30 the IAC kicks up between 20-30 percent initially with a rate of decay based on CLT input. (ever engine is different with the final parked rate hovering between 18-22% increase in IAC hold position with compressor engaged) This prevents fluctuations and stalling when coming to a stop. Also when starting out from a stop TPS input up to 18%/rpms up to 1350 and KPA below 35 prevents compressor engagement. The coincides with ecu controlled vac advance to the ignition distributor which is set to kick in at 1250 rpm/tps above 10%/ KPA value over X amount depending on engine temp.


EDIT:
I completely forgot why I initially posted. It was after reading the above regarding WOT cut out options.
On turbo applications primarily due to acceleration rate (but now available on all DPI EFI packages) TPS above 70% disables window switch/defrost control. Nothing worse than power shifting a DeLorean and your elbow puts the window down. I found it distracting as it is a knee jerk reaction to look over at the window when it happens.

[PJ Grady Inc.]

One of the many perks to an advanced EFI system.

TPS above 90%/ KPA above 65 cuts out compressor operation. Decel off full throttle has a half second delay prior to compressor engagement after 90%-WOT. Cooling fans are then staged 1 second apart and varies due to CLT/MAT input in a decel condition. IE no stalling or electrical system load when snapping throttle shut after WOT. (that annoying sub 750 rpm dip K jet offers). Also on decel/below 15% TPS and KPA under 30 the IAC kicks up between 20-30 percent initially with a rate of decay based on CLT input. (ever engine is different with the final parked rate hovering between 18-22% increase in IAC hold position with compressor engaged) This prevents fluctuations and stalling when coming to a stop. Also when starting out from a stop TPS input up to 18%/rpms up to 1350 and KPA below 35 prevents compressor engagement. The coincides with ecu controlled vac advance to the ignition distributor which is set to kick in at 1250 rpm/tps above 10%/ KPA value over X amount depending on engine temp.


EDIT:
I completely forgot why I initially posted. It was after reading the above regarding WOT cut out options.
On turbo applications primarily due to acceleration rate (but now available on all DPI EFI packages) TPS above 70% disables window switch/defrost control. Nothing worse than power shifting a DeLorean and your elbow puts the window down. I found it distracting as it is a knee jerk reaction to look over at the window when it happens.

"Nothing worse than power shifting a DeLorean and your elbow puts the window down. I found it distracting as it is a knee jerk reaction to look over at the window when it happens."

That can't happen with a "Mattaligner". Coming soon to a Delorean vendor near you!
Rob

[Delorean Industries]

Is it stainless? I'm not interested in anything that isn't stainless. It could cure cancer... Gotta be stainless.

Saw your message. Email you tracking shortly.

[PJ Grady Inc.]

[QUOTE=DPI JOSH;177143]Is it stainless? I'm not interested in anything that isn't stainless. It could cure cancer... Gotta be stainless.

Saw your message. Email you tracking shortly.

Of course it's stainless Josh! Here's the first prototype with the switch protector. The production part should be available in about three weeks.
Rob