On my 87 300sdl with about 175k, I have this problem every so often while driving on the highway, seemingly a strange combination: The abs light comes on, the tachometer goes to zero, and I think the a/c seems to cut out a little. Then after a second or two it goes right back to normal. I want to say I replaced the tach fuse and that helped (less frequent). Any thoughts on potential other causes?

I think I may have found the answer via Google:. Overload protection relay??

This is a classic case of the OVP relay going bad. When it opens up, you lose ABS, Tach, and Klima (A/C).

Before replacing the OVP, check charging voltage. The OVP could actually be doing its job. If charging voltage is <14.5V, the OVP needs replaced.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Thanks; will do!

You also lose transmission kickdown, something to keep in mind when you get on the highway.