No spark after E28 M20B25 swap, help?

[nickskater09]

So long story short, I got a B25 from an 89 325is and swapped it into my 83 528e. We made the wire harness adapter with the male fuse box connector from a 92 b25 vert, and connected it to the E28 fuse box connector. Now all the car isn't getting spark, we checked over the fuse box adapter and all seems correct and checked for power at the coil and it's there, just no trigger to send a spark.

We checked the CPS and it seems to be operable, and bypassed the fuel relay so we know it's getting fuel too. Tried swapping in known good 173 ECU and nothing. We were checking pins on the DME connector and apparently the harness doesn't have a pin for the engine speed reference? I don't really know what's up with that.

At this point I'm at a loss for what it might be. Can anyone help me figure this out.

[Golfer_705]

So long story short, I got a B25 from an 89 325is and swapped it into my 83 528e. We made the wire harness adapter with the male fuse box connector from a 92 b25 vert, and connected it to the E28 fuse box connector. Now all the car isn't getting spark, we checked over the fuse box adapter and all seems correct and checked for power at the coil and it's there, just no trigger to send a spark.

We checked the CPS and it seems to be operable, and bypassed the fuel relay so we know it's getting fuel too. Tried swapping in known good 173 ECU and nothing. We were checking pins on the DME connector and apparently the harness doesn't have a pin for the engine speed reference? I don't really know what's up with that.

At this point I'm at a loss for what it might be. Can anyone help me figure this out.

try swapping the connections where your CPS and #1 spark plug wire pick-up connect

[nickskater09]

Already tried that, along with a whole new crank trigger too. No dice.

[jlevie]

For the engine to run the following conditions must be met:

Power on DME pins:
27 Start Input
18 Un-switched Power input
37 Power Input from Main Relay

Ground on DME pins 2, 14, 19, 24

Timing data from the CPS on DME pins 47 & 48 from a rotating engine

To have spark power must be present at the coil positive and ground pulses
from the DME's pin 1 must reach the coil negative. Power to the coil is
controlled by the ignition switch via C101. When checking for spark, use the
output lead from the coil to eliminate the distributor, rotor and plug wires.

To have injector firing power must be present at each injector and ground
pulses from the DME's pin 16 (Bank1) and pin 17 (Bank2) must reach the
respective injector bank. Note that the injectors are wired as two banks of
three. With cylinder 1,3,5 being bank 1 and 2,4,6 being bank 2. Power to the
injectors is controlled by the main relay. Injector firing is best checked
with a noid light.

The fuel pump relay must have power on pin 86 (relay coil) from the main relay
output (pin 87) and power on pin 30. The DME will ground pin 85 to turn on the
relay and power the pump(s) via pin 87. Of the above, only the fuel pump power
is fused. So if the there's power at pin 87, but not at the pump, check fuse
11.

The main relay and DME pin 18 receive power from the smaller of the two wires
that connect to the battery's positive terminal. That wire incorporates an
in-line fuse. When the DME is presented with a start signal, it grounds the
main relay pin 85 and furnishes power to the fuel pump relay, injectors, and
DME.

Troubleshooting:

Disconnect the battery and the DME cable. Then:

1) Disconnect the coil negative and check continuity from that connector to
DME pin 1. Also verify that from DME pin 1 to ground is an open circuit.

2) Check the resistance across DME 47 & 48, which should be 500-560 ohms. If
the CPS is dismounted, the resistance can be seen to change from about 500 to
540-560 ohms when a ferrous object is brought to the face of the
sensor. Neither pin should be grounded.

3) Check for continuity from DME 36 to main relay 85 and from DME 3 to fuel
pump relay 85.

Reconnect the coil, remount the CPS (air gap should be 1mm), plug the
relays back in, reconnect the DME, and connect the battery. Then do the
following checks:

1) With the key off, verify that power is present at DME pin 18 and main relay
86 & 30.

2) With the key on, verify that power is present at DME pin 27 and pin
18. Power to pin 18 is from the main relay and there should be power to the
injectors and fuel pump relay.

3) With the key on, verify that no voltage is present at the DME grounds (2,
14, 19, 24).

4) Verify that power is present at the coil positive and at fuel pump relay
pin 30. Those get switched power from the ignition switch via C101.

The engine will start and run (if poorly) with only those connections to the
DME in place. The other signals from Cylinder ID, AFM, temp sensor, etc., are
necessary for proper operation. But they won't prevent the engine from firing.

IMPORTANT:

A power check means seeing a voltage within about a tenth of a volt of what
you measure across the battery terminals, which should be at least 12.6v on a
charged battery.

A continuity check means seeing less that 1 ohm of resistance.

An open circuit means seeing a resistance of at least 100k ohms.

A good quality auto-ranging digital multimeter will make these tests much
easier.