hi all, i've got a 78 300d and on top of the thermostat housing is a temp sender with its wire/tube broken off. i cant find anything in my haynes manual regarding this sender. what is its function, what is it sending signal to? mike

Hi,
We Mikes need to stick together.

If your 78 300D is similiar to my 77 450SEL, the unit you are describing on top of your water pump is the 212F switch that kicks on the auxilliary fan located directly behind the hood grill if the coolant temp goes above 212F.

The fan is an essential part of your cooling system - esp if running AC - and ought to be put in operating condition ASAP.
Mike

__________________
Love driving my '77 450 SEL!
124,000 miles

that might explain why the auxillary fan went on when i closed the hood the other day. i just pulled the relay to keep it from running on. but i did just find a small bundle of wires on the right side of the engine compartment that are not connected to anything. a couple of them look like they go to the ac. ill have to sort that out.