temperature gauge wont work

[slash5guy]

hello all, need a little help.
i recently swapped a 300d motor with a 300 sd motor in my 1985 300d. simple enough, though there are settle differences. the trouble I'm having is i have no engine temperature gauge. i do have all my other gauges, though the RPM's are low...another topic. i have changed the sending unit, no effect. sometimes the needle goes to the top (directly), sometimes it doesn't move. any help, guidance, or another link would be great. thanks.