I was just troubleshooting the glow plugs in the '79 240D I bought a few days ago. The seller said the "glow plug light never goes off" (before starting) and I had given it around 30 seconds, and indeed it never went off in that time before starting the car. It does go off when I hit the starter.

Anyway, in the course of testing the voltage, the light finally did go off after about 60 seconds. This seems like a long time -- I am in Southern California and the temp is usually anywhere from 55-105. Or is 60 seconds (maybe 45 seconds) normal?

The voltage I measured at the glow plug fuse and at both #1 and #4 plugs was 10.7 volts. Is this low? I'm wondering if I need a new battery. The battery looks pretty old (and it is a correct white Mercedes-Benz brand battery!). Voltage at battery with car off is 12.75. Once the light went off I could also touch the end of the plugs with my fingers. They were warm to the touch, but not hot. Not sure what's normal... they certainly never "glowed".

Oh and there's been a veg oil conversion on this so there is an electric fuel pump energized along with the glow plugs.

Thanks!