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Anti-Lock and Brake warning lights on

The vehicle has done 225,000 miles. The transmission has been rebuilt 2 years ago and a lower mileage engine fitted recently. It drives well. But since the engine was fitted, the Anti-Lock and Brake lights are on when the engine is running. The parking brake is off and the brake fluid reservoir is full.

When I switch the ignition on, the warning lights come on briefly and then most go off including these two. As soon as the engine starts these two warning lights come on and stay on. The brakes work fine.

I have plugged in a basic code reader but it does not show anything.

Any ideas?

ABS systems work by information about wheel speed from sensors located on each wheel. The first thing to suspect is that when the new engine went in the speed sensors either got disturbed or were not reinstalled/hooked back up properly. Did you inquire at the shop that dropped the new motor in?

This agrees with my suspicion. I took it back to the shop but the guy could not see anything obvious. He cleared the codes but after about 30 miles they came back on. They did clear themselves yesterday but soon came back on. Looks like I need to book it in and let the guy investigate thoroughly.

Just having the low brake fluid warning light on is enough to disable some ABS. I don’t know if this applies to your car, but it probably does. I would trouble shoot the low brake fluid warning light first. It sounds like it is somehow shorted to some circuit that is active when the engine is running.
You would need a GM specific code reader to get the OBDII codes related to the ABS. If you do, you may find that it is only disabled because it thinks the brake fluid level is low.

My mechanic traced this to a faulty releay in the ABS circuit. $30 part fixed it.