My '86 300e had some idle problems and the antilock (ABS) warning light would come on, especially on a rough set of bumps. The problem was poor or loose contacts where the OVP plugs in. The OVP was free to loosen in the socket from road and/or engine vibration.

I understand that later models had some sort of a hold-down to prevent the OVP from shaking loose but my '86 did not have this. I don't know what your '88 has. I cleaned the OVP contacts with OxyKlean and then secured the OVP tightly to its socket with a zip tie. The problem has not returned for several months now so I think it is fixed.

__________________
1986 300E
1993 Volvo 940 Wagon
1984 Volvo 240 (daughter's)
1976 Dodge 1/2T pickup, gas hog