Another "no spark" thread - Paging jlevie

[iamsam]

Quick backstory:

1987 325is parts car (with complete drivetrain), I have seen it run a year ago, but now under my ownership, I have no spark.

PO ripped out the dash to sell it, so is there any possibility in doing so, something could have been disconnected to prevent the car from starting? It cranks fine though. I am not sure if there is any spark-critical connections under the dash other than the DME itself and the ignition switch.

Here is a diagnostic procedure (from jlevie) that I started to follow, and I have a few questions on it, listed within the quote.

Here you go:


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.

I do have 12V at the positive coil terminal, I couldn't say anything conclusive about the negative terminal, I am not sure what it does. Does it "pulse" a ground connection on and off? Also, to eliminate the dist/wires/plugs, I have one spark plug crammed in the other end of the coil wire to look for spark. Is this an ok way to do it? I also have the plug grounded as best I can against the motor.

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.

I didn't touch anything fuel related, I am focusing on getting spark first, I am sure the fuel works.

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.

Done. We have continuity between disconnected negative lead on the coil and pin 1, and pin 1 is not continuous with ground.

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.

Resistance was nearly 500 Ohms (495 or so), so I assumed that was good.

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

Here is where I got stumped. Which relay is the Main relay? Also, what the heck is the "85" on that relay? The three relays mounted on the inner fender have only single digit pin labels.

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.

How do you verify power on a DME pin when it is plugged in?

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.

I do get 12V at coil positive. Haven't checked fuel for reasons mentioned above.

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.

[acolella76]

Next to the ECU connector on the harness there should be a little 3 pin connector... make sure that is plugged in.

I believe the main relay is among the trio of relays on the driver's side shock tower. Pretty sure it is the far right one... pull it out and look at the bottom of it and it should have numbers. You will then understand what the '85' means.

[iamsam]

Next to the ECU connector on the harness there should be a little 3 pin connector... make sure that is plugged in.

I believe the main relay is among the trio of relays on the driver's side shock tower. Pretty sure it is the far right one... pull it out and look at the bottom of it and it should have numbers. You will then understand what the '85' means.

Do you mean all those disconnected little plugs coming out of the main harness a few inches away from the ECU?

Also, according to this pic, the main relay is the far left one.

[jlevie]

I do have 12V at the positive coil terminal, I couldn't say anything conclusive about the negative terminal, I am not sure what it does. Does it "pulse" a ground connection on and off? Also, to eliminate the dist/wires/plugs, I have one spark plug crammed in the other end of the coil wire to look for spark. Is this an ok way to do it? I also have the plug grounded as best I can against the motor.

The DME grounds the negative side coil to produce a spark. The way you are testing for spark is okay.

How do you verify power on a DME pin when it is plugged in?

You remove the back shell from the DME connector and plug the connector into the DME.

acolella76 is correct about C104. If that isn't connected the DME won't get a START signal on pin 27 and thus will not power up.

[iamsam]

I found and plugged in the little 3-pin connector, to no avail. That is, if it was the right one, which I am assuming it was, since I couldn't find another connector that would work.

Also, can someone clarify whether my main relay is near the front or back of the car on that little row of three relays? The one near the front is white, the two near the back are orange. The orange relays both have an "85" terminal, the white relay only has single digit terminal numbers.

Also, is there an easy way to test continuity for that little white 3-prong connector by the ECU? Which pins does it correspond to? I am suspicious of its continuity because the PO removed the dash and made a mess of all the wiring underneath, and there are so many disconnected connectors and I have no idea where they go, and of course most of them are irrelevant to the car starting, but one of them may be very relevant...

Thanks guys for your replies so far.

[iamsam]

Solved!!!

So, following jlevie's diagnostic procedure, I was able to pinpoint a lack of 12V at pin 27 on the DME with the key on, so I chased the discontinuity all the way to a mystery connector that I could not for the life of me find where it needed to plug into, so I simply ran 12V straight to the correct pin of that connector, and bam! It started right up!!!

So obviously a huge thanks to jlevie for his great writeup, I would have never ever found the problem without this writeup.

And so then after shutting the car off after running it for a minute or two I summarily yanked out the entire drivetrain and flopped it on the floor, awaiting the transplant patient.