[AllanC]

Vehicle is a 1985 Oldsmobile Delta 88 4 door sedan. Powertrain is 307 V8 with 4 bbl feedback carburetor and 3 speed automatic transmission with lockup torque converter clutch. The issue is that the CHECK ENGINE lamp is illuminated constantly with ignition ON / RUN and engine running or not running. I tried to connect an OBD I scan tool with proper, GM connector cabke but could not get the ECM (engine control module) to communicate with the scan tool. Scan tool is good and works though I have never used it with the GM specific cable to retrieve codes.

I found a picture of the GM diagnostic port under the instrument panel. There is a way to use a jumper wire to connect between terminals B & A of the port (ignition switch in the ON / RUN position and the CHECK ENGINE lamp will enter diagnostic mode and flash any 2 digit trouble code. This is similar to the procedure with Chrysler OBD I vehicles where one would do the key dance routine (ON-OFF, ON-OFF, ON) and get the lamp to flash. Below is an image of the connector and the terminals I connected.

However with this Oldsmobile it will do nothing. No blinking / flashing; the lamp remains illuminated. So now I am starting to wonder if something has failed in the ECM.

The engine will start and run and drive well but it does seem to be running slightly rich. You can accelerate the engine and you get an occasional black smoke puff. Default position for the mixture control solenoid is RICH if the ECM has failed.

Any thoughts or ideas on further tests to conduct before condemning the ECM as failed? I have been searching for a wiring diagram for the ECM but have not been successful. If I had a wiring diagram I would check the wiring between the diagnostic connector port and the ECM for proper continuity.

 

[ImperialCrown]

If you unplug the ECM, does the 'ck eng' lamp stay off? This may verify that the lamp is not a wiring or instrument cluster problem.
The cars that I remember jumping in order to read the fault codes had me connect the jumper in the connector before turning the ignition key to 'run'.

 

[AllanC]

If you unplug the ECM, does the 'ck eng' lamp stay off? This may verify that the lamp is not a wiring or instrument cluster problem. . . . .

Excellent idea but I did not try that. Power for the CHECK ENGINE lamp passes through the ignition switch. The ECM controls the ground path and when to illuminate the bulb. If there is a short to ground on the return side of the lamp wire to the ECM , it will always be illuminated and the ECM will not be able to control it. I will definitely try this test.

. . . . The cars that I remember jumping in order to read the fault codes had me connect the jumper in the connector before turning the ignition key to 'run'. . . .

All the instructions I read indicated to turn the ignition switch to ON / RUN and then jumper terminals B & A in the diagnostic connector. But it could be just a simple sequencing event of connect first and then apply power. I know that I turned the ignition switch first and then applied the jumper wire.

I found a somewhat crude wiring diagram from Chilton's online so I am thinking I should try and trace the B terminal wiring at the diagnostic connector back to the ECM and make sure it is continuous.

It will be next week before I visit the vehicle again and try these tests. As always, thanks for your insights!

 

[AllanC]

. . . .It will be next week before I visit the vehicle again and try these tests. . . . .

I tried using the jumper wire in terminals A & B and then turning the ignition switch to the RUN position. That made no difference. I removed the diagnostic connector from its mounting under the dash and back probed the terminal pins. Still no luck. CHECK ENGINE lamp still would not flash.

I found the ECM (engine control module) under the glove box on the driver side of the vehicle. It has 2 rather large connectors which appear to have about 20 wires in each. I am still searching for a wiring diagram p;inout at the ECM. I will try some GM / Oldsmobile forums and maybe I can find more information there.

 

[AllanC]

I have been struggling with this ECM issue for several months. Sometimes you have to step away from the problem for a period of time, think and then approach with a fresh mind for diagnosis.

I finally located a schematic for the fuse box in this 1985 Oldsmobile. It is attached. After studying it I noticed that there are 2 fuses present for the ECM: ECM - DEC and ECM / BAT. The latter is probably direct battery power and I am not sure the function for the former ECM - DEC. I was not expecting that but wondered if one of the fuses had failed that this might be the source of the problem with no communication from the ECM.

So I checked the fuse in the ECM - DEC slot and found that it was open. I replaced the fuse and now the ECM has come to life and is controlling the feedback carburetor. So the ECM has not failed though I had been thinking that was the problem.

I do not know why this fuse failed as typically a fuse does not fail unless it experiences high current draw. Time will tell if the condition repeats itself.

Take away from this is to NOT get misled with assumptions and to check all possible inputs and outputs on electronic modules. Since the CHECK ENGINE lamp was illuminated constantly I assumed that the ECM had power. I was only partially correct. Sometimes the solutions are obvious and right in front of us. One just has to NOT overlook the easy ones and get "slid"!

 

[85lebaront2]

If you need more, I have AllData Pro and should have most of the wiring diagrams.

A couple of things I remember from working on these, the feedback system can correct the mixture quite a way from being off to the point a bad float that would have manifested itself can actually be masked until gets to the point it will not run at all. The other was the canister purge system, some of these when they failed would actually pull raw fuel from the float bowl. Fun systems to work on.

 

[AllanC]

. . . . If you need more, I have AllData Pro and should have most of the wiring diagrams. . . . .

I DO appreciate the offer of wiring diagrams as having good documentation is critical. This car has been "hacked by multiple" mechanics over the years. Vacuum lines plugged, broken sensors so it will be a challenge to get it running well in any semblance of its original state. 1980s carbureted emission control systems with vacuum plumbing everywhere was just a nightmare. GM, Ford, Chrysler, etc all had "spaghetti systems" under the hood.

 

[85lebaront2]

Ok, I do remember well working on those systems. The feedback carburetor was a stopgap measure by Detroit to meet the emission requirements just like TBI systems were also a stopgap. Everything was driven by cost vs effectiveness. GM feedback carbs were actually pretty good, particularly the Quadrajet E4ME and E4MC models.

On spaghetti, I have a 1986 F350, 460 C6, I converted it from carbureted to MAF/SEFI and installed an E4OD transmission. The vacuum "snakes nest" was incredible. Two air pumps, each with a diverter valve with two different vacuum controls, EGR, Evaporative system, including two vacuum operated bowl vents, a distributor vacuum advance system that looked like it chased itself around the thermal vacuum and check valves along with a couple of restrictors. The bundle of plastic lines ran down the inside of the right valve cover under the two 3/4" diameter air pipes, perfect location Ford! Burned through one of the air pump control ones, damn thing would shoot a 2 foot flame out the tailpipe when you let off the gas and the air flow would ignite the gas in the exhaust.

If you want the diagrams, message me on here with your email and I will send them to you in Adobe pdf format.