Electric Fan Conversion AC Issue

[wstefan21]

I'll try to keep this post short, and yes, I promise I used the "search" button...

Specs:

I recently swapped a 99 Gen 3 LM7 into my 1988 Chevy C1500 pickup. I'll attach the tune, but it was taken from a stock 2002 12212156 OS with VATS turned off and the necessary CELs as well.

The PCM is a P01 4.3l, and the wiring is completely custom (I'm an Electrical Engineer by trade).

A/C is wired like an old chevy van with a cycling switch and high pressure switch (12V ac on signal from the hvac module).

Fans are two one-speed fans with a relay for each, ground switched to pins 33 and 42.

Fan settings were copied from 05 5.3 stock tune, with mode to "fan 2" and type to "analog cycling".

Writing full tune not partial.

Issue:

Fans work fine with temperature and manual switching in VCM scanner, but do not come on with A/C (which does switch compressor on/off).

All the forum posts I found either indicate take the settings from an 05 to "patch", or if using the older OS upgrade to 02 (which I have done).

Hopefully this is just something simple I have missed! Thanks in advance for any assistance.

88chevy1500afterUpgrade.hpt

[2xLS1]

You should wire the fans like GM does with 3 relays so they both run low speed when command fan 1 on and both run high speed when command fan 2 on. You have to do some modification to the A/C system to get fans to run based on A/C pressures. Here is a good write up on how to do it. And the GM fan wiring.

https://www.performancetrucks.net/fo...ontrol-553385/

[HATEHPTUNERS]

What that diagram fails to make clear is that the F-body uses an in-line pressure sensor on one of the A/C lines that triggers the Fan 2, or hi speed fan to come on when there is sufficient pressure in the a/c line... So... If you want it to work like stock, you can plumb in the pressure sensor and then the high speed fans will come on with A/C head pressure... Or, you will have to figure out some work-around. Specifically, I have the compressor clutch signal wire coming out of the PCM also trigger the High speed fan relay so that anytime the A/C compressor clutch is turned "on", the high speed fan is also turned on. The only problem with running it this way is why would you want the high speed fan running while you are doing 65+MPH down the freeway? You don't. So... I have had to come up with other ways to make sure the high speed fans do not run while the vehicle is moving at speeds greater than 40MPH.

[Fast4.7]

What that diagram fails to make clear is that the F-body uses an in-line pressure sensor on one of the A/C lines that triggers the Fan 2, or hi speed fan to come on when there is sufficient pressure in the a/c line... So... If you want it to work like stock, you can plumb in the pressure sensor and then the high speed fans will come on with A/C head pressure... Or, you will have to figure out some work-around. Specifically, I have the compressor clutch signal wire coming out of the PCM also trigger the High speed fan relay so that anytime the A/C compressor clutch is turned "on", the high speed fan is also turned on. The only problem with running it this way is why would you want the high speed fan running while you are doing 65+MPH down the freeway? You don't. So... I have had to come up with other ways to make sure the high speed fans do not run while the vehicle is moving at speeds greater than 40MPH.

A trinary switch on the high side is the easy way to run fan 2 without a pressure sensor. It is what I used for electric fans with a black box that had a single fan output. When you are rolling down the road the condenser has enough air flow that the fan turns off.

[wstefan21]

You should wire the fans like GM does with 3 relays so they both run low speed when command fan 1 on and both run high speed when command fan 2 on. You have to do some modification to the A/C system to get fans to run based on A/C pressures. Here is a good write up on how to do it. And the GM fan wiring.

Wait... is this implying the only way to get fans to work off A/C is to have an A/C pressure sensor?? Is there no other way to get this to work?

[wstefan21]

A trinary switch on the high side is the easy way to run fan 2 without a pressure sensor. It is what I used for electric fans with a black box that had a single fan output. When you are rolling down the road the condenser has enough air flow that the fan turns off.

Hm... I've seen these before. Definitely an option. I'd really hoped that there was a "more stock" alternative, but sounds like it's either this or switch to analog setup like f-body...

[wstefan21]

What that diagram fails to make clear is that the F-body uses an in-line pressure sensor on one of the A/C lines that triggers the Fan 2, or hi speed fan to come on when there is sufficient pressure in the a/c line... So... If you want it to work like stock, you can plumb in the pressure sensor and then the high speed fans will come on with A/C head pressure... Or, you will have to figure out some work-around. Specifically, I have the compressor clutch signal wire coming out of the PCM also trigger the High speed fan relay so that anytime the A/C compressor clutch is turned "on", the high speed fan is also turned on. The only problem with running it this way is why would you want the high speed fan running while you are doing 65+MPH down the freeway? You don't. So... I have had to come up with other ways to make sure the high speed fans do not run while the vehicle is moving at speeds greater than 40MPH.

Interesting. I was thinking about tying the relay to the clutch, but you bring up a good point of not wanting the fan on at high speeds.

Looks like with the post linked above that hptuners with the "analog" function grabbed the chunk of code from the 05+ pcm to allow for the a/c pressure transducer to work (even uses the same pins as 05).

My guess is gm never programmed a/c fan control logic with "analog cycling" which is why it doesn't work, and the reason analog works is that it's basically the code for 05+ which uses the pressure transducer.

[Fast4.7]

Interesting. I was thinking about tying the relay to the clutch, but you bring up a good point of not wanting the fan on at high speeds.

Looks like with the post linked above that hptuners with the "analog" function grabbed the chunk of code from the 05+ pcm to allow for the a/c pressure transducer to work (even uses the same pins as 05).

My guess is gm never programmed a/c fan control logic with "analog cycling" which is why it doesn't work, and the reason analog works is that it's basically the code for 05+ which uses the pressure transducer.

Analog will not cycle the clutch from memory.

[wstefan21]

Analog will not cycle the clutch from memory.

Ok now I'm confused. I can't use serial cycling because I don't have class 2 data to an hvac module, and I can't use analog cycling because I will be using a pressure transducer instead of the low cycling switch.

Might make sense if you were talking analog on f-body OS since they have a variable compressor, but the code is from 05+ truck OS which doesn't have that, so that wouldn't make any sense.

If the compressor is just always on with analog mode my compressor will likely die very quick if it's ever running on low low-side pressure.

Or I could be misinterpreting your reply? Just want to make sure I don't blow my compressor up!

[Life is an Enigma]

What that diagram fails to make clear is that the F-body uses an in-line pressure sensor on one of the A/C lines that triggers the Fan 2, or hi speed fan to come on when there is sufficient pressure in the a/c line... So... If you want it to work like stock, you can plumb in the pressure sensor and then the high speed fans will come on with A/C head pressure... Or, you will have to figure out some work-around. Specifically, I have the compressor clutch signal wire coming out of the PCM also trigger the High speed fan relay so that anytime the A/C compressor clutch is turned "on", the high speed fan is also turned on. The only problem with running it this way is why would you want the high speed fan running while you are doing 65+MPH down the freeway? You don't. So... I have had to come up with other ways to make sure the high speed fans do not run while the vehicle is moving at speeds greater than 40MPH.

I may have a workaround for that pressure switch. You can use a variable resistor AKA a potentiometer in place of the pressure switch to trick the PCM and thinking it's seeing the A/C head pressure that it needs to turn on the fans. Just need to know the resistance between the positive and negative on the three wire pressure switch then find the right variable resistor and wired in. Some background I'm retired car audio security video etc installer 38 years. I was doing work around on the vats system since it came about for doing remote start.
t93.jpg

[Life is an Enigma]

I did find this diagram for bypassing the pressure switch it uses a regular resistor but I think a variable resistor would be better because you able to fine tune it.

Screenshot (751).png

[HATEHPTUNERS]

Life-correct me if I am wrong, but going that route with the resistor is going to have the high speed fan running anytime the A/C is turned on correct? It does not seem to me like it can "read" A/C pressure, or heat, and has no "sensing" abilities. Please correct me if i am wrong.

[wstefan21]

I did find this diagram for bypassing the pressure switch it uses a regular resistor but I think a variable resistor would be better because you able to fine tune it.

Screenshot (751).png

Interesting! That does tell me that the transducer still functions as a low pressure/high pressure switch and should cut off if low pressure is observed to protect the compressor. It just does so by measuring high-side pressure instead of low-side for cycling, but it does sound like it does cycle.

The transducer is just in itself a variable resistor controlled by pressure. Thereby it changes voltage based on the pressure between 0 and 5V, so picking that resistance places it's voltage drop within the acceptable range.

While this certainly will work, there's no protection for when the pressure is low. Generally not a huge deal unless you're low on refrigerant, but still potentially something that can kill your compressor.

Fortunately for me, I've got a service port with a Schrader valve and correct pitch, so this is about as easy to install the correct sensor than a resistor anyhow, so I'll probably go this route.

[wstefan21]

Life-correct me if I am wrong, but going that route with the resistor is going to have the high speed fan running anytime the A/C is turned on correct? It does not seem to me like it can "read" A/C pressure, or heat, and has no "sensing" abilities. Please correct me if i am wrong.

From what I've read, yes, it will turn on high anytime A/C is commanded. Like my above post the issue is that with this setup there's no cut-off for low pressure so there's a potential risk to the compressor.

Looks like it completely ignores the low pressure cycling switch and fully relies on the transducer reading for cycling so the only protection you'd have is the high pressure switch on the command signal assuming you've got that wired.

[blindsquirrel]

High side is not a useful indicator for cycling. It's the low side that needs to be in control of that. Yeah, high side cutout is needed too, but that's not the pressure that a cycling clutch system is based on.

[Life is an Enigma]

Life-correct me if I am wrong, but going that route with the resistor is going to have the high speed fan running anytime the A/C is turned on correct? It does not seem to me like it can "read" A/C pressure, or heat, and has no "sensing" abilities. Please correct me if i am wrong.

You are correct it's just tricking the PCM thinking it's seeing the head pressure it needs to trigger the fans. Basically turning it from a smart system to a dummy system just like what's in the 98 -05 S10. But the system will still cycle because of the low pressure switch. This is a quick dirty way to get your fans working if you don't have the option of adding the three wire high pressure switch to your AC systems. But at least your fans will turn on when you first turn on your AC. Without the fans running your head pressure is going to go through the roof and destroy your pump or blow a hose or both either way that gets expensive.

[Life is an Enigma]

Interesting! That does tell me that the transducer still functions as a low pressure/high pressure switch and should cut off if low pressure is observed to protect the compressor. It just does so by measuring high-side pressure instead of low-side for cycling, but it does sound like it does cycle.

The transducer is just in itself a variable resistor controlled by pressure. Thereby it changes voltage based on the pressure between 0 and 5V, so picking that resistance places it's voltage drop within the acceptable range.

While this certainly will work, there's no protection for when the pressure is low. Generally not a huge deal unless you're low on refrigerant, but still potentially something that can kill your compressor.

Fortunately for me, I've got a service port with a Schrader valve and correct pitch, so this is about as easy to install the correct sensor than a resistor anyhow, so I'll probably go this route.

If your AC system still has the two wire high pressure switch in combination with the low pressure cycling switch you will be safe if head pressure gets too high. Example S10 uses a two wire high pressure switch which is in the back of the compressor and the low pressure cycling switch which is on the dryer. In theory the fan should turn off when you hit your set speed that you set in your tune file if you use the bypass resistor instead of the three wire high pressure switch.

[Life is an Enigma]

From what I've read, yes, it will turn on high anytime A/C is commanded. Like my above post the issue is that with this setup there's no cut-off for low pressure so there's a potential risk to the compressor.

Looks like it completely ignores the low pressure cycling switch and fully relies on the transducer reading for cycling so the only protection you'd have is the high pressure switch on the command signal assuming you've got that wired.

The easy way to test to see if your low pressure cycling switch is working is to turn on your system and unplug the cycling switch if the pump stops running it's working. And if your AC pump is constantly running and not cycling you may be low on Freon or it could be a bad cycling switch.

[Life is an Enigma]

High side is not a useful indicator for cycling. It's the low side that needs to be in control of that. Yeah, high side cutout is needed too, but that's not the pressure that a cycling clutch system is based on.

Yes I believe you're correct. If the system already has the dummy two wire high pressure switch in combination with the low pressure cycling and you're just using the variable resistor to bypass to get the fans to come on the system will be safe.

[wstefan21]

The easy way to test to see if your low pressure cycling switch is working is to turn on your system and unplug the cycling switch if the pump stops running it's working. And if your AC pump is constantly running and not cycling you may be low on Freon or it could be a bad cycling switch.

I'll have to check it out, but I believe that it completely ignores the low pressure cycling switch. I'll post a schematic and more details below.