Anyone know how to test a 7747 ECM? 5v or 12v for CES light?

[GOY]

Here's what I have:

No CES light when the ignition is on the on position.
Can't run codes. The light just plain never lights.

I've replaced the bulb with a working bulb. I've resodered the joints on the board for the light, and tested it on the board - it works when 12v is applied to it. (Didn't burn out bulb)

Here's the question - how should I go about testing the wiring/ecm for failure? My first thought was to pull the ECM, then apply the required voltage to the 5th pin (I think it's A5)... but I'm not sure if...

A) I should apply 5 or 12 volts
B) If I do that, will it work even then, or does that light share a common ground that requires the ECM to be plugged in too illuminate.

Any help on diagnosing a fried ECM or bad wiring?

[93w/t1500]

If you can access a Tech I or other code scanner you can plug it into the ALDL and see if you can read the data stream. If you can see the computer respond to requests (A/C on for example) then it's prob ok. Depending on prices and the accessability of the local boneyard it may be cheaper/easier to pull another ECM to substitute in. If your instrument cluster is out you may be able to test the wiring. You'll need a set of wiring diagrams to figure out which pin controls the SES light and then pull the computer and check continuity between the SES output at the computer and the input at the cluster head.

[GOY]

I know it's pin A5 at the ECM conntector for the CES light - but is it 5v or 12v do you think?

I was going to venture a guess at 5, but I still have no idea if there is a common ground I would be needing that I wouldn't have be disconnecting the ECM.

[93w/t1500]

Usually the cluster lights are all 12V. There should be a common ground at the instrument cluster and a separate ground for the ECM. I think if you disconnected the ECM but left the cluster in and applied voltage to A5 your test would be valid. Here's a quick check; with the ECM disconnected turn your headlights on. If the cluster lights it has it's own ground.

[GOY]

Great idea! Thanks.

[ChevyTech]

Being you didn't say what year, model, engine, trans, and gvw/ emmissions system, I can only give you a general answer.

The service engine soon light gets power usually from the gauge fuse. The ECM connects the other side of the bulb to ground to make the light come on.
So the wire between the ecm and service engine soon light is grounded by the ECM to turn the light on, it is not powered by the ECM.

Hope this helps you figure out your problem.

[GOY]

1988 C1500, lo5/A4...

Sorry, I normally don't forget those details...

Would that fuse be a general fuse for the panel and not just that one light then? Secondly - so all I would do is have the key in the on position and then ground to a chasis ground to is if the light came on?

[ChevyTech]

Originally Posted by GOY

Would that fuse be a general fuse for the panel and not just that one light then?

I have not checked a 1988 wiring schematic but I believe it does not use a seperate fuse, and the gauge fuse feeds it.

Originally Posted by GOY

Secondly - so all I would do is have the key in the on position and then ground to a chasis ground to is if the light came on?

Yes - if you unpluged the ECM, turned the key on and grounded the wire that goes from the light to the ECM, the service engine soon light should light up.