Electronic Control Module / ALDL (lack of) Codes

My "new to me", 'high miler' (300,000 km), '89 Suburban (350 gas, 4X4, automatic), has been flashing her "SERVICE ENGINE SOON LIGHT" at me.
I first noticed it last weekend on her "maiden voyage", an 1100 mi trip to NJ, pulling a trailer (empty south, loaded with a car north). It only did it in the big hills, when I applied other than gentle throttle pressure. It would do it when I demanded extra throttle (up hills or accelerating to pass), and when I let off the throttle (on down grades). And it only did it with the fuel tank LESS THAN HALF FULL!!! On the return trip, with the light coming on at anything other than very even throttle pressure, and with just under half a tank of fuel, I pulled in for fuel at Binghamton NY, and it idled rough coming up to the pumps. I fueled up (16 gallons to top it up), started her up (idled great), and headed out immediately (didn't stop for a rest, it didn't cool down). I was pulling a trailer with a light car on it (Model T). The light didn't come on again until the fuel tank was down to 1/4, and I was accelerating up a steep hill. Then the light then would come on at anything other than very even, easy, throttle pressure, until I again refilled with gas (it again idled roughly when I pulled into the gas station). The light went out and stayed out until I got home, and until the tank got below 1/2 a few days later. Seemed like a fuel delivery problem to me: I replaced the fuel filter, put an additive in the tank (STP Concentrated Fuel Injector Cleaner), and tried her out. Acceleration seemed to improve, idling definitely improved. I drove her freeway, town, hard, easy; used up the tank of fuel; light only came on once, briefly. Yesterday I took her for a 300 mile run. Full tank of fuel, ran great, light didn't come on until tank was less than 1/2 full. I fueled up, light did come on twice, briefly, after fill-up. Today I decided to check the codes at the ALDL. I used a trusty paper clip and jumped from contact A to contact B, and turned the key to the on position (did not start the truck). The "SERVICE...." light flashed a feeble 1 - 2 code three times, then attempted to dully flash something else (light was very weak), then went dead. I tried again.....nothing. I took out the paper clip; started the truck; turned off the truck; tried to get codes; nothing. I now notice that the "SERVICE ENGINE SOON" light no longer comes on when I turn the key to the on position (guess maybe I blew the ECM fuse? I'll check that!). What gives? The truck seems to run very well. Mileage is acceptable.....13 mpg(Canadian gallons) in D with trailer in tow on the trip to NJ and back: 15 mpg(Canadian gallons) yesterday, in OD, combined freeway, city, towns; no trailer; new fuel filter seems to have fixed the idling problem, fuel pump seems to be delivering adequate fuel. Thanks for any help forthcoming; I'll go out and check the ECM fuse. Keith.
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Re: ECM fuse. I just checked the fuse, it tests good.
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Try and get it on a scanner, or disconnect the battery and start from scratch, see what happens, sounds like it might be seeing a lean condition when the tank is low, possible impending fuel pump problems, also how long has the fuel filter been in there. You will loose any history codes, if stored, when the battery is disconnected.

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Hi Shep; I installed a new fuel filter last week; after the NJ trip, before the 300 mi trip.
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Ok, check it for codes, you may see 44 or 45, autozone does it for free. Post back. Don't disconnect the battery you may loose history.

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Well, I bought a "Code Reader" (I wasted 46 bucks!), and I still can't get any codes. I get 2 strong 1 2 codes, then the light fades out on the third 1 2 code, then just an occasional very faint glow from the light. It acts like there is no, or not enough ground. Will I blow the ECM if I jump directly from port B (Diagnostic Terminal) at the ALDL to a good ground on the dash? I'll look for the ECM and see if appears to be properly grounded. I've also noticed that the "Service Engine Soon" light does not come on when I turn on the igniton. Any suggestions? Thanks. Keith.
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