Re-solder the fuel injection and radio suppression relays. The fuel injection relay is white relay at the left end of the middle row of relays. It is almost certain to have bad solder joints which need to be re-soldered. We have had two bad ones in our fleet of three 700 series. The radio suppression relays are more robust, but there is no Bosch manufactered relay that does not have suspect solder joints. Remove the fuel injection relay and carefully remove the plastic cap. Use a magnifying glass to carefully inspect the solder joints on the printed circuit board. You will see bad solder joints with cracks around them. Use a low heat (15 to 25 watt) soldering iron to re-solder all the solder joints being carefull not to get things too hot. Use electronic solder. Or, mail me the relay(s) and I will solder them for you.

Your 1989 turbo car does not have an RPM sensor if you are in the US. The 1989 US turbo cars still used a Hall sensor in the distributor. The 1989 normally aspirated cars do use an RPM sensor which is prone to failure in the original (yellow mark) are prone to failure.

The sensor just forward and below the coolant temperature sensor is the knock sensor. The temperature sender for the temperature gauge is under intake manifold runner number two in the head.