89 300se check engine on when brakes applied
 

Have a 89 300se. the first time I step on the brake pedal after starting, the check engine light comes on and won't go out until I turn the car off.As long as I don't step on the brake the check engine light will not go on.the 89's did not have the lockout transmission, so you do not have to depress the brake to put in in gear like the 90-91's another post with the same problem posted 01-09-2007 under search type in 300sel check engine. cluster was never out of the car so I know nobody switch bulbs .this started a couple of months ago.I checked the brakes, sensors,pads, fluid level. I think their must be a weird vacuum leak someplace.. the booster , and hose is fine. no problem with the brakes at all. any thoughts.

For the sake of argument check to see if you have a brake light bulb out.

ALL BULBS ARE GOOD. FLUID IS UP IN RESEVOIR, GOOD PADS AND SENSORS. LOOK AT THE POST TITLED 300SEL CHECK ENGINE 2007

I would start by trying to isolate other sources of vacuum leaks. It could be that you have low vacuum, and using the brakes just puts it over the edge and triggers the light. You can disconnect the vac hoses one by one and cover the vac nipple with a rubber cap. Then dirve the car to see if the problem is still there - the one that doesn't give it is a vac leak source.
I would also verify with a MityVac that there is no leak at the booster.
If those don't do it, I would get the mixture checked and adjusted. It could be that it is fairly lean, and using the brake booster is enough to put it over the edge.

vacum hoses?
 

what vacuum hoses are you referring to. my car is a 300se gas engine. the booster hose to the intake manifold is good. no leaks. are there other hoses? the small rubber hoses near the air filter unit are new also.any specific hoses I can check?

a vacuum leak will not cause a check engine light on that car, it's too dumb. you have another problem. good luck, chuck.

I normally don't chime in twice but in this case I'm with dux. At the very least, have a close look at the brake light bulbs. Bulbs that are illuminating but are discolored can trigger the warning indicator.

A bulb filament can be broken but still illuminate if the filament is touching just a bit. There's enough electrical current to jump across the crack, but the crack could create enough resistance that the car thinks the bulb is burned out.

My best $0.02.

check the overload relay and fuses in the top.
disconnect the booster hose from engine and plug start car and push brake if light comes on it has nothing to do with vac. unplug the brake light switch at pedal and drive it does light stay off?
apply parking brake remove reverse bulb and put car into reverse which light comes on check engine or light out?