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You can try to pull the codes. You have to find the connector attached to the big wire harness under the driver side dash, and ground it momentarily with the ignition on. I've tried this with my truck but I can't get it to flash anything. The RABS I diagnostic connector is where I mentioned earlier (should be there on the '88) or if you have RABS II, it will be somewhere behind the glove box. A mechanic I know says that the module or the valve goes out and it's an expensive replacement. He says that if the customer doesn't want to pay to fix it, he just unplugs the module so that the light will go out. The only thing is, if you do that, you have to remember you don't have abs. The brakes will still work fine, but the can lock up.
Dang right the replacement valves are expensive. When I bought my '94 van, the ABS light was on. The charge for the repair was almost $900 from the Ford dealer to the dealer I bought it from. And they used rebuilt parts.
1988 Ford, I don't know if you figured out your problem yet, I was scanning through the posts and figured I'd give you my experience. My light was on in my 87 150 from the day I bought it. Searched all over the truck, every part of the ABS system for two years looking for the problem. Finally unplugged the connector on the side of the brake fluid reservoir and one of the little clips inside the terminals was broke. I pulled the terminals out of the connector to be able to see this. Spliced in a new terminal and light has not been on since.