Today I swapped out the DME Relay in my '86 Targa. Because of the ongoing debate, I decided to investigate the device a little more thoroughly. I have included three pictures: one of the coils inside the DME Relay, one of the manufacture date of the device (note: my old one has a manufacture date of 4-29-1996, obviously NOT the original), one of the bottom of the circuit board with a few things highlighted.
I conducted this exploration under the auspices of my friend David, who, in his own words "I am not an electronics technician, I merely played one at NASA." We reviewed the schematics and the potential issues with my old DME Relay (to his experienced eye). The DME Relay receives signals from the DME and the Alarm System. Based on those signals the coils charge and complete the connections providing power to the fuel pump and the O2 sensor.
David pointed out that relays usually fail because of faults in the soldering or corroded connectors. The actual electronics in the relay aren't prone to failure. You can check a 'dead' DME relay by putting an ohm meter across the connectors I labeled in the picture. The readings of 73.2 ohms across one coil and 82 ohms across the other should be about correct. If your coils ohm out like this, then you can repair your relay.
To repair your relay you should check for corrosion on the external connectors. Mine are nice and shiny so we had to look at the bottom of the circuit board. David pointed out that the solder connection for one of the coils has been heated excessively - hence the striation in the solder - and cracked probably causing intermittent faults. To correct this, we simply touched a hot soldering iron to each solder joint long enough to melt the old solder and reseat the component leg. If you simply have corrosion on the connectors, use an emory board to polish the leads.
There is a potential for poor performance from a bad DME. Intermittent or bad solder connections can add reistance to the circuit. The relay is desinged to provide 12 volts of power to to the fuel pump and the O2 sensor. Since the O2 sensor is simply a variable resistor altering the voltage returned to the DME, increased resistance in the DME relay would result in incorrect O2 sensor readings and ultimately improper fuel mixtures.
Hopefully this will put to bed the entire DME Relay question. Yes, a relay should work or not, but there is the potential of generating incorrect voltages - and varied performance. However,
replacing the DME Relay completely is not necessary as 'weakened' relay can be easily refurbished. David and I discussed creating a solid-state transistor based relay but decided the cooling issues outweigh the reliabile gains.
