nandos_z28

iTrader: (5)

Hello everyone,

I'm looking to see if you guys can help me figure out the problem I am having. First off it started the other day while driving on the highway (3.5 hour drive). I turned the A/C on and it was working fine. I turned it off for awhile and went to turn it back on and it wasn't working. I kept trying to turn it on and finally it began to work again.

When I finally got home I checked the codes that were thrown they consisted of P0452 & P1639. I went to gearchatter and followed the diagnostics for the codes. As of right now I believe there is a short or something on the Gray 5 Volt reference wire on the A/C pressure sensor.

The wierd thing is when testing the Gray wire for "hot" using a test light nothing comes up but when I test it for ground using the test light it shows ground. I compared by testing the same way on my dads Vette which has the same sensor and color of wires. His has "hot" power when testing with a test light. Also there is only about .20 Volts according to my Digital Multi Meter. The vette has as expected Near 5 Volts.

I found a 5 Volt reference wire on the plug for the something to do with the smog I think. Anyways I cut the gray 5 Volt reference wire that goes into the A/C pressure sensor and spliced in the new 5 volt reference wire from the smog plug and the compressor clutch kicked in and A/C worked fine.

So obviously the issue is with the Gray 5 Volt reference wire that goes into the A/c pressure sensor plug. If anyone has an idea of what the issue can be or how I can fix the wire I'd appreciate your help!

Thanks in advance.

BOA279

iTrader: (1)

The reference voltage comes straight from the PCM. find that wire on the PCM plug and take a reading. If its still reading ground then the PCM is not supplying the voltage and the ground your reading is probably not from a short but from the ground on the other side of the variable resistor inside the inside the sensor.

Basically its one of only a few things. You've eliminated the sensor as the problem by splicing in another 5vdc reference so there's only two other possibilities....the PCM is not supplying the voltage (some thing internally wrong with PCM) or you have a broken wire between the PCM and sensor. There is also another possibility of something telling the PCM not to supply the voltage but without schematics I'm only guessing.

nandos_z28

iTrader: (5)

Here is a link to the schematic just to be able to see things a little more clearly.
Tried to upload it to photobucket but it wouldn let me.

https://ls1tech.com/forums/attachmen...estion-ac1.gif

fleetmgr

If you measure the reference coming out of the PCM and have your 5v, the only other possibility is the wire. If you don't have it at the PCM, then the PCM is bad.

nandos_z28

iTrader: (5)

Ok thanks guys, I'll try and take a look at it tomorrow sometime.

Any idea what could of cause the PCM to go bad all of a sudden? Cuz Like I said the A/C was working fine, I turned it off for awhile, went to turn it back on and it wasn't working then about an hour later it started working again. All in the same drive and ignition cycle.

fleetmgr

It has a finite life span like any other part. Some obscure little piece in the PCM decided it was time to let the smoke out.

nandos_z28

iTrader: (5)

*Update*

Ok I finally got around to borrowing a friends PCM yesterday and plugged it in and still no difference. The same symptoms occur.

So it must not be the PCM because the PCM I tried is a known working one.

According to gearchatter.com diagnostics for DTC P1639 it says to monitor the 5V on the AC pressure sensor while disconnecting each sensor connected to the 5V reference circuit one at a time.

If the voltage changes while disconnecting any of the sensors then that sensor must be replaced.

fleetmgr do you or anybody else, know what all sensors are on the 5V circuit? I believe the MAP, TPS, FTP, and of course the A/C pressure sensor are but don't know what else may be.

Here is the page from gearchatter I am using
http://www.gearchatter.com/viewtopic9327.php

Any other advice or suggestions on what could be my problem?

Thanks

fleetmgr

Not without looking at a schematic, I don't.

nandos_z28

iTrader: (5)

FTP sensor connector also is not putting out 5V. Everything else is though, (TPS, MAP, and the smog plug).

What could be causing the computers to not be giving 5V to these two sensors? (FTP and A/C Pressure sensor)

fleetmgr

Are they on a common bus, as in there's a broken wire?

nandos_z28

iTrader: (5)

Not that I found. I traced them all the way back to the PCM and there are no breaks in the wires.

eseibel67

iTrader: (10)

On the blue C1 connector at the PCM, pin #45 is the 5V ref for A/C pressure sensor. Same C1 connector, pin #46 is the 5V ref for the FTP sensor. Both wires are gray.

Check for your 5V right at the PCM. If you're not getting it, you need a new PCM. But if this is your finding, I would open the PCM and take a look before I buy a new one since the problem is with 2 adjacent pins.

nandos_z28

iTrader: (5)

I have checked both wires coming directly out of the PCM and all I get is around 0.15 Volts.

Also I have tried another PCM in plac eof my original one and nothing changes. (This new PCM was working perfectly when removed from a friends car.)

fleetmgr

Been looking around in Alldata, and ran across this:

This is for my '96, but I'd bet there's no difference. A shot in the dark, but you never know.

eseibel67

iTrader: (10)

If 2 different PCM's give the same .15V on those wires, my bet is that you have corrosion on the connectors of C1 at those 2 pins.

luigiandme

iTrader: (2)

Sorry to bump this thread after 2 years. This is only for people looking to figure out this issue. Try checking your wiring harness behind the ECM. A smaller group of wires in black conduit will be coming out of the firewall on the passenger side. Remove the ECU to gain access. There's a self tapping screw that this harness may have rubbed through. Open the conduit and repair wires if necessary and secure harness away from screw!

Mike96z

iTrader: (78)

Just curious what the resolution was for this, I have the same exact symptoms.

CamaroDr

I also have this code P1639 some days ago.
I just replaced the fuel pump some weeks ago, and it is a Fuel tank pressure sensor (FTP) to be found some where that i havent heard of. And that sensor can trig this P1639 .
So I may check that sensor now

radiopeople

Hey bud did you ever find a fix for chevy 5volt reference issues with ECM, A/C, FPS and Tractioin Control issues? The local Chevy dealer here in Monroe, La have replaced MAP, Throttle Body, ECM and FPS sensor.....$2000 bill fixed it for one day....very nex day ir fell back into failsafe mode. Any help woud be awesome!! radiopeople1@gmail.com

nandos_z28

iTrader: (5)

For some reason I was not getting notifications for this thread. I don't quite remember exactly what the fix was as it has been quite some time since. I remember using the 5V reference from the smog wire temporarily. That worked for awhile, but now the car is wired back to the way it should be from the factory and everything is working as it should. I would follow the advice listed in the thread and work through it.

Good luck!

**edit: After thinking about it more I think I ended up replacing the FTP sensor and that corrected the issue but I can't say for sure.**