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A/C Relay failure theory (long)
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Posted by Gene N [Email] (#1094) [Profile/Gallery] (more from Gene N) on Wed, 25 Aug 2004 05:19:51 Share Post by Email
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I had no A/C on my 1990, so I started the normal diagnostic chain. Static freon pressure was good, the pressure switch on the receiver/dryer seemed to be working, compressor fuse was OK. I also noticed that the engine idle increased after 10 seconds of starting the engine, which meant that the A/C time-delay relay was getting power. I pulled the A/C relay and jumpered the main feed pins (31 and 86b) and the compressor kicked in and started cooling. Next I pulled the cover off of the relay and inspected the relay. The contacts looked OK, but I cleaned them and plugged the relay back in with the cover off. Started the car up and, after 10 seconds, the relay armature was moving but still no power to the compressor. The circuit for the relay contact path is simple, but using an Ohmmeter, it looked like there was an open between the relay pin 31 and the relay armature. Tracing the path on the circuit board showed that the path was open between circuit board and the relay frame, which is how the relay is mounted to the board. Looking at the frame pin solder joint under a magnifier didn't show any opening, but after resoldering the connection (with a 250 watt soldering gun and lots of flux) the problem was solved.

Where am I going with this? Well, the thought that this would work for 14 years and 180,000 miles and then quit got me to thinking about this and the similar problem with the headlight relays. I think that the manufacturing technique used by Bosch for these relays is to put the components on the board and run them over a flow soldering machine. This works fine for normal components on the board, but on something like a relay with a high thermal mass, the relay frame never heats to the minimum temperature required to allow the solder to bond to the frame. The result is that the solder flows around the frame pin and makes a surface contact that allows electricity to flow, but the two materials are just touching. Another factor is that the relay frame is nickel plated steel, which requires a higher temperature for bonding. After 14 years of vibration, the pin pushes the solder slowly away so that the pin ends up centered in hole with a very small gap between the board and the pin. This gap causes the open connection.

Maybe some one in the electronics industry has some better insight?

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