Bad DME?

[bcrdukes]

Hey folks,

Are there any possible ways to test whether an ECU/DME is good or bad?

[jlevie]

About all you can do is to swap in a known good DME.

I don't know what problem you are trying to diagnose, but if it is a no start condition the following may be helpful:

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

Power on DME pins:
27 Start Input
18 Unswitched 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 fronm 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 & 47, 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.

[bcrdukes]

Hi Jim,

What I'm trying to diagnose is a hard start issue. This only happens when fuel pressure is at zero. I've checked fuel pressure with a gauge and it holds fine. This hard/long start issue occurs only when the fuel pressure is at zero. A check valve at the fuel pump was installed (new from Pelican Parts.) I've also replaced the crank position sensor and coolant temperature sensor. The car runs a lot smoother but otherwise, no luck. I've also replaced the FPR for good measure and ensured there were no vacuum leaks including the check valve between the throttle body and charcoal canister.

Otherwise, the DME/ECU throws code 1444, which is nothing. The car runs rich so my gas mileage is poor. Upon a cold startup (assuming fuel pressure being zero) the car burns very rich where my spark plugs are soaked in fuel. I'm at a loss here. The only thing I can nail it down to now are bad injectors, AFM, DME/ECU, or perhaps an O2 sensor?

Any leads?

[smooth]

My wife's car had a bad DME and the symptoms were no turn signals and no CEL at smog time :(

[nando]

^ on an E30? cause E30 ECUs have nothing to do with the turn signals. lol

[smooth]

Obviously an E30.

[jlevie]

Hi Jim,

What I'm trying to diagnose is a hard start issue. This only happens when fuel pressure is at zero. I've checked fuel pressure with a gauge and it holds fine. This hard/long start issue occurs only when the fuel pressure is at zero. A check valve at the fuel pump was installed (new from Pelican Parts.) I've also replaced the crank position sensor and coolant temperature sensor. The car runs a lot smoother but otherwise, no luck. I've also replaced the FPR for good measure and ensured there were no vacuum leaks including the check valve between the throttle body and charcoal canister.

Otherwise, the DME/ECU throws code 1444, which is nothing. The car runs rich so my gas mileage is poor. Upon a cold startup (assuming fuel pressure being zero) the car burns very rich where my spark plugs are soaked in fuel. I'm at a loss here. The only thing I can nail it down to now are bad injectors, AFM, DME/ECU, or perhaps an O2 sensor?

Any leads?

If the FPR and pump are good and the pump turns on as soon as the engine begins to rotate, the fuel system pressure should be normal within one or two revolutions of the engine. A weak pump could take longer to pressurize the system and thus make for hard cold starts.

But I suspect something else is going on here. That would be that the injectors (or cold start valve on an ETA) are leaking and flooding the engine when the car is parked. That would explain the hard cold starts, zero fuel pressure, and running rich on a cold start.

[nando]

Obviously an E30.

how was it obvious? you didn't say "my wifes E30".

My wife drives a Nissan Sentra, so if I said my wifes car and you assumed E30 you would be wrong. ;)

[smooth]

how was it obvious? you didn't say "my wifes E30".

My wife drives a Nissan Sentra, so if I said my wifes car and you assumed E30 you would be wrong. ;)

If you start posting about your wife's Nissan Sentra on an E30 board in the M20 section in response to someone trying to diagnose their DME then I might be wrong to assume you were referring to an E30 DME but I think at that point everyone would wonder wtf you were doing posting your comment in the first place.


@bcrdukes
The point is that bad DME's can cause strange issues that go unresolved for a long time. Hers didn't throw a code, either. If you have access to a known good DME, swap it but I don't know if I would recommend buying one simply to test.

[Thunderzol]

Hello,
This might be obvious to many but not to me:
When diagnosing DME pins you need to have it plugged into the main harness (as the harness provides power to it from battery). If the DME is fully plugged, how do you check the pins? Remove the DME cover and test the back of the pins?

In a sense it would be easier to understand this troubleshooting process if clarified which section involves testing the harness itself, the pins on the front of the DME, and the pins on the back of DME while plugged and powered up.

I’m totally challenged with low voltage electrical, so please correct me.

Thanks

[McGyver]

This might be obvious to many but not to me:
When diagnosing DME pins you need to have it plugged into the main harness (as the harness provides power to it from battery). If the DME is fully plugged, how do you check the pins? Remove the DME cover and test the back of the pins?

In a sense it would be easier to understand this troubleshooting process if clarified which section involves testing the harness itself, the pins on the front of the DME, and the pins on the back of DME while plugged and powered up.

I’m totally challenged with low voltage electrical, so please correct me.

You're going to want to look at and understand the Electrical Troubleshooting Manual (ETM) for your car. Some manuals are missing info, so find the one that is most similar for your car. (I use the 1989 325i manual for my 1991 325i).

http://www.wedophones.com/BMWManualsLead.htm

jlevie posted great advice on the order of trouble shooting. Some wires should always be hot and others should always be grounded. You can unplug your DME and test these. You may be able to check switched power at the other end of the wire from the 55-pin DME plug. You can also get really thin probes to poke through the wire insulation to get to the conductor. Don't take apart your DME.

[TobyB]

"Backprobing" is the ever- so- Cartman word used to mean

'Pull the back of the housing off of the ECU connector, and carefully jam something back down in there to make contact while not shorting it out on anything'-

that connector needs longish little whisker- wires to get a decent contact.

I freely admit that I have a 12" extension jumper made from an old ECU and a junkyard harness that I use for this. It
took an afternoon to put together, but it makes testing all sorts of problems so much easier.

t
I miss Jim.