Saab 96 - Replacing Old Flasher Unit with Modern Relay


Follow a number of issues with several thermal flasher units, the Saab was updated with a more modern, transistorised equivalent part.

The page below details the wiring changes that were needed to do this.

     

The original 3 terminal thermal unit had 3 connections:
X  = Power in.
L  = Indicator switch.
P  = Pilot dashboard bulb.

The modern replacement part also had 3 terminals but with the following connections:
Pin 49 = 12V power.
Pin 31 = Ground.
Pin 49a = Indicator switch. 


Old thermal Mechanical unit
 
To make the correct connections, the green and grey wire terminals were swapped over within the plastic connector block.

Additionally, the grey wire was converted into an earth connection by cutting it and soldering on an extended wire connected to the chassis behind the dashboard.
 
After these changes the indicators were working but there was still a need to connect the pilot light on the dashboard. The pilot light was connected to the remaining end of the grey wire, previously cut to earth the relay. It was necessary to use two Diodes to make this connection. Failure to do this would mean that all 4 indicators would flash when turning left or right because of the current flowing through the pilot bulb. The diagram below shows how and why the diodes were needed.
 

Without the diodes in the circuit current would flow through the pilot bulb to light the indicators on the other circuit.
 
2 Diodes capable of taking at least 1 Amp were selected and joined as showed, noting that their polarity was correct.
The end with the band indicated the direction the current would flow to (Current flow as arrowed).

The parts were finished in heat shrink to produce a robust assembly.
 
The diodes (2) were connected as shown on the right. The single output wire was connected to the grey pilot light wire (1). This was the wire previously cut when the flasher unit earth connection was made.

Then each input was connected to an indicator circuit; one diode to each circuit. On this car, these were yellow and white wires under the dashboard. Scotchlok connectors were used for these connections (3).

Diode Connections
 
A check on the finished circuit showed a steady and reliable flash and confirmed that if a bulb was removed, the remaining one would flash faster to indicate a fault.
   
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