Cars / e30 / Air Conditioning & Heat / r134a Pressure Switch Wiring

r134a Pressure Switch Wiring

Article by: Chrisca70

Article applies to: all e30 models.

This info is relevant for cars that came originally with two pressure switches (I think is pre 1989)

Alright, so I just did my r134a conversion on my ‘89 325i following the BMW TIS.

I got the new high pressure switch (64538390971) with the new dryer (64538391025), replaced o-rings here and there, and changed the oil/lubricant in the compressor.

However the pressure switch splice deal is not clearly explained in the TIS.

The TIS says you should cut and join the cables from one of the pressure switches but it does not specify which or how.

Here is what I did:

Please see the attachment which are the electrical diagrams and explanation of the circuit for the original set-up.

The cables that go to the high pressure switch are three:
1h. Black with white stripe
2h. Black with yellow stripe
3h. Black
2h and 3h go together at the connector

The cables that go to the low pressure switch are three:
1L. Black with Red stripe
2L. Black with Red stripe
3L. Black
1L and 2L go together at the connector


  • 1h, 2h, 3h are ALL soldered or crimped together
  • The new high pressure switch has two cables, one connects to 1L and the other to 2L+3L (through a connector).

The rationale is that by doing this the old high pressure switched is eliminated and the new high pressure switch replaces the old low pressure switch.

In this way the A/C Clutch Monitor Input and the Compressor Control remain connected to the rest of the system via the new high pressure switch.

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