Wits end...
 

1988 Carrera. Diagram shows DME relay armatures connected directly to battery positive, through connector "NR173". No voltage on either DME relay armature, where does the power wire come through the "firewall"...trunk wall...?

"..no voltage.."

I was wrong, had voltage to the primary DME relay armature, poor/cracked solder joint to second relay armature where by pure happenstance I was checking for battery voltage. Battery to DME terminal 30 circuit was good all along.

Primary DME relay closes with ignition switch, secondary (fuel pump, O2 sensor heater) does not. Fuel pump runs if I depress 2nd relay armature manually.

Energizing the fuel pump manually is of no help since ignition coil isn't being "energized" while cranking, constant full battery voltage on both coil terminals.

So now I have to consider the speed sensor or the DME ECU.

Both sensors, speed and reference, measure ~960 ohms at the ECU connector.

While cranking I get 2.8-3.5 volts AC across ECU pins 8-27 on ECU circuit board.

Replace speed sensor anyway, first, or go for broke, EXPENSIVE DME ECU...??

Wishing I hadn't loaned the 78 Targa to nephew, much simpler troubleshooting.

Are you sure the factory anti theft is not keeping it from starting? It seems like the speed sensors is working. Do you get a reading on the pins for the reference sensor, pins 25 annd 26?

I am of the understanding that the anti-theft prevents the DME primary relay from picking up.

Plus there is no key tumbler in the back of the driver's door.

Does the reference sensor prevent the secondary DME relay from picking up?

Damn...double DAMN...:mad::mad::mad:

DME relay that I purchased was defective, poor/cold solder joint to terminal 30....

Thought I had found and repaired original relay but missed a second cold/cracked solder joint.

$100 tow fee to Squires only to find that substituting working DME ECU still wouldn't start the engine.

Only took a few minutes to check and find that the new DME relay was not supplying power to terminal 87....jumper bypassed, 30-87, started engine.

:):)SmileWavy

Glad you got it running.

You are big on best value. Where did you source the part from?

Good to hear.
To answer your question, the dme needs the signal from the reference sensor so it knows when it is at tdc, along with the signal from the speed sensor so it can adjust timing . The dme controls the second relay coil using pin 20 when it sees all the correct signals.

Pelican...Kaehler..same brand as the one I took out.

But at the rate I see "here" that these fail I suspect the source doesn't much matter.

The old was failing intermittently due to a cracked solder joint that finally went completely TU. The new one was the result of a cold solder joint as shipped.

Powering the fuel pump (25A fuse) is quite enough of an electrical load but then have a look at the number of loads on the primary relay.

Why not a fuse on the terminal 30 input..??

There will be one now.

I was more curious about which of the 2 is used to determine that the engine is turning over above 200 RPM...I suspected the speed sensor.

Interesting that you found the root cause of the relay failure. Most (including me) swap in a new relay and keep going.

I know the contacts are robust as hell and the coils should not fail easily so it leaves the solder joints to be the weak link.

You've inspired me to dig out my old DME and see if I can refresh it.

You are right. The speed sensor is the main one. I was just generalizing since with no spark due to the reference sensor, the engine will not turn once the starter is off anyway. At initial start up, when the signal on the yellow wire from the start circuit goes to the dme, it engages the relay for the fuel pump so there is fuel supplied as the starter spins, and when the key goes to run the speed sensor takes over.
The first thing I did when I put the engine in my car was to open the relay and reflow the solder connections, and bought a spare relay.

If anyone knows solder joints it's wwest.