'85 635csi crank no start

[burgundy635]

First time poster here and relativly new owner!

Currently my 635 wont start at all, and I have narrowed it down to the spark. I went through the recommended "no spark" procedure that is scattered throughout this forum and had a couple of questions regarding it. The coil seems to be in working order and i get about 11.7 volts at terminal 15 (+) with the key on, but no spark coming from the coil tower. When I checked the two sensors, being the speed sensor and reference sensor, the readings were a bit off. The speed sensor seemed to be working because it read 4.5 ac volts (crank no start guide says max 4v but I am assuming that 4.5 is close enough). The reference sensor is supposed to have a max of 0.4v, but mine read between 1.0-1.5v when cranking. Is this indicative of a bad sensor? if so, why would the outputs be too high instead of just plain 0? any suggestions are welcome.

[TimU]

Is this the guide you followed?
viewtopic.php?t=33245
If not, I suggest using this one.
I re-wrote this guide to consolidate several "Crank, No-Start" posts into a single "roadside friendly" reference article.

Is this indicative of a bad sensor? if so, why would the outputs be too high instead of just plain 0? any suggestions are welcome.

The CPS/FRS cables are both identical, so you can switch them around (at both ends!) and re-test for a faulty cable.

What else have you checked/tested before you came to the reference sensors?
From: viewtopic.php?t=33245

Symptoms:
No spark observed during spark tests

Possible Causes and Corrective Actions:
1. Low Battery Voltage
a. Check battery voltage. Min 10V needed during cranking

2. Faulty Main Relay
a. Bypass with a jumper wire, or replace as needed

3. Faulty On-Board Computer (OBC) Relay
a. Bypass with a jumper wire, or replace as needed

4. Poor Earth/Grounds
a. Inspect, clean and tighten all (5?) ground points in the engine bay

5. Faulty Ignition Switch
a. Test, bypass and replace as needed

6. Failed Ignition Coil
a. Test and replace as needed

7. Faulty Reference or Pulse sensors
a. Test and replace as needed

8. Missing Reference Pin on flywheel.
a. Test, inspect and replace flywheel (sorry)

9. Faulty Motronic DME Unit
a. Replace or repair

[burgundy635]

Thank you for the reply! The guide I was using was very similar, but the one you linked to was definitely more up-to-date. Here is a brief list of everything that I tested in order.

-Checked for spark using extra spark plug (negative)
-Checked for spark coming from the coil tower (negative)
-Checked resistance of different terminals on the coil, came back ok
-Checked for voltage at terminal 15 (+) of the coil, read 11.7 volts with the key on
-Checked for voltage at pin 86 of main relay, came back with full 12 volts
-Checked ac voltage of sensors, results in my previous post

So if I understand the procedure, my next steps should be:

-Bypass main relay (just in case)
-Bypass OBC relay (didn't even check this the first time)
-Test for faulty sensors, replace if the voltages are still above the suggested maximum

If I do those three things and still get no spark, is the last step just to replace the ECU? Also, am I correct for assuming that if I get voltage at terminal 15 of the coil but no voltage from the tower, my ignition switch is not the problem?

Thanks
Charlie