coil ground problem

[BrandonG]

i finnaly got my m20b25 in my e and everything hooked up , i have fuel and power... but the coil has a bad ground wire it seems ??? i did a voltage drop test on the ground and it jumps from 12v to 10v wen cranking and i measured the resistence of the ground and its hella high ??? < but not OL ?? i have three coils and they are all good i did resistence checks ... ANY imput i wanna get it started so i can get it on road :) from wat i know a coil should jump from 12v to 0 wen cranking :) correct me if wrong :) to help im using the old m20b27 fuse box and ignition and engine is from an auto car converted to standard i got alot time in this fucker and i wanna get it started

[jlevie]

The coil is grounded to produce a spark by the DME. Sine the ground pulses are very narrow you aren't going to get meaningful data on meter. You'd need to use an oscilloscope to look at the ground signal.

I presume that you are looking at the coil wire because the engine doesn't have spark when cranking. That being the case, run through what is below and you should be able to find the problem.

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.

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 to 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 540-540 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 0.040"), 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 batter 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.

[BrandonG]

much apreciated ill print this and do it this weekend

[BrandonG]

kk i went through your list and did everything not including relays cuz i couldn't find wer they wer even with Bentley , things thast wer good wer 1a,2a, 2b,3b,

[BrandonG]

just tell me wer the main relay is and wat relays go wer in that group of 3 and did u backprobe cuz i dont see how lol i just pulled off connector and checked that way :) i wanna redo the first line odf stuff just gotta know wer main relay is and connector 101

[BrandonG]

just tell me wer the main relay is and wat relays go wer in that group of 3 and did u backprobe cuz i dont see how lol i just pulled off connector and checked that way :) i wanna redo the first line odf stuff just gotta know wer main relay is and connector 101

http://www.r3vlimited.com/board/showthread.php?p=2335283#post2335283

pics of my relay set up here and i found main relay and its good

[jlevie]

When you are working through the tests, if you encounter a failure stop and find out why you aren't seeing what you should.