M20B25 DME Diagnostics
For an M20B25 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 when it sees timing data from a rotating engine. 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 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. It isn’t uncommon to find that the inline fuse is corroded and not proving full power to the DME. You can cut off the heat shrink tubing and have a look at the fuse. If okay, it can be sealed and protected with several layers of good electrical tape. If the fuse is iffy or bad, I cut the fuse out, splice the wire and place a 60a lug type fuse between the small cable and the power point in the engine bay. The lug type fuse will bolt right in.
To check these signals you will need to remove the back shell of the DME connector connector so that when appropriate the connector and the DME can be mated for tests. Some of the tests require that the DME be connected and others must be done with the DME disconnected (like continuity tests).
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). All of the DME grounds are via the small bundle of brown wires that attach to the right side shock tower.
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.
For an M20B25 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 when it sees timing data from a rotating engine. 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 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. It isn’t uncommon to find that the inline fuse is corroded and not proving full power to the DME. You can cut off the heat shrink tubing and have a look at the fuse. If okay, it can be sealed and protected with several layers of good electrical tape. If the fuse is iffy or bad, I cut the fuse out, splice the wire and place a 60a lug type fuse between the small cable and the power point in the engine bay. The lug type fuse will bolt right in.
To check these signals you will need to remove the back shell of the DME connector connector so that when appropriate the connector and the DME can be mated for tests. Some of the tests require that the DME be connected and others must be done with the DME disconnected (like continuity tests).
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). All of the DME grounds are via the small bundle of brown wires that attach to the right side shock tower.
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.
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