Now the AC compresser will not turn on

I was troubleshooting the low vacuum problem on my Passat 96 TDI and put it all back together.
Now the AC compressor will not turn on. AC switch light is on,fuse is good,the clutch will not engage.
What could I have done? Any hidden connectors I may have pulled apart in the engine compartment,or behind the dash?

I did some basic troubleshooting today and what I have found is there is no voltage going to the compressor clutch. The light is on in the dash for AC on.
It may be a sensor?
I have no idea where to look.
Please help

Check the white coolant sensor in the upper radiator hose. Bypass the coolant sensor with a paper clip temporarily. The white sensor is a 2 pin sensor where the engine coolant temperature sensor is black and yellow and has 4 pins and is closer the head.

I don't seem to have that particular plug. There is a THREE wire plug just under the top radiator hose. Heavy RED,Light Red, and a Heavy Red/white wire.
The other temp sensor is by the engine block/head.
I have a photo I can email you. I shutdown my Photobucket account due to the 300 buck charge they want to hit me with.

Do your fans run? If they don't, the system won't work. Check the fuses at the FCM. This is close to the battery, so something may have happened when you got behind it.

Otherwise, there'll be several safeties in the system:
Ambient temp sensor at the raintray
Trinary switch in the high side line, by the firewall
Coolant temp sensor at the head flange
There's also that stupid 135 relay, but that's behind the center console.

-Todd

Basic troubleshooting kept taking me back to the 135 relay.
The AC was working before I took the consul out looking for the vacuum leak. Now it's not.
Does the 135 relay turn on when the ignition is turned on? Yes.
I removed the consul again and the 135 relay. I opened the relay and contacts looked like new. 0 Ohms through the contacts.*
Put the relay back in and turned on the AC,no go. No power to pin #2 on the high pressure side sensor ( passenger side fire wall just under the AC refrigerant line). I found this tip on a paid site from ten years ago.
Back to the 135 relay.
The plug wires free float in the plastic receptacle. Only four of the six are used. Power was there obviously since the relay cycled. However,one of the female load contact connectors had backed out just enough to not make a good contact with the spade of the relay. Who'd-a-thunk?
I would recommend that anyone who removes the 135 relay for any reason use a pair of needle nose pliers and make sure all four wires are over the relay's spade connectors after you re-seat the relay.
My AC is now working again after chasing ghost all over the car.
* this is a Normally Closed relay. For you uninitiated, this means the relay is always ON until told to turn off. If you measure through the contacts out of the car,the contacts are ON. If you hook it to a 12 volt battery the contacts Open (off) and you have no continuity or "OL" on your meter.
Thanks to those who helped out.

There's also a 'how to' somewhere to eliminate the relay completely. Oddly enough both gasser versions (2.0L ABA and 2.8L AAA) use a different relay for the AC system. That's one reason used replacement relays are few and far between and new ones cost a lot more than they should.

If I had time I'd go through the wiring diagram to see exactly how the gas and diesel versions differ in wiring and relay.

Steve

I would think any SPST 12 volt relay with normally Closed contacts would work as long as it was rated for the amps.
There are only four wires being used. Two for the 12 VDC and two for the clutch.
Since on the Diesel,the AC clutch is dependent on the engine RPM ( must be 900 or more) I would not want to remove it. It prevents stalling from the stop light. Turns on/off as needed. I read it also senses the throttle.When passing at high RPM it drops out. I have noticed that as well.
I'm no expert on the wiring or how everything works..I just fumble around like the rest of us.

It was that stupid 135 relay! What do I win?

Pull it and toss it in a box. Make a jumper wire and bridge the connectors at 30 & 87. Both of my B4s have had the 135ectonomy and it's great. Who wants their AC turning on and off?

-Todd