My wife is Japanese and she said that by themselves each pair of characters do not mean anything. If you have any more characters, she may be able to work out what they mean.
The links I posted above are from Japanese Wikipedia, they say that 日産ディーゼル工業 (Nissan Diesel Motor Co., Ltd.) is often mentioned as 日デ in its business field, and 日本車輌製造 (Nippon Sharyo Ltd.) is abbreviated 日本車両 or 日車.
I don't know how you would pronounce them, if they have a pronunciation at all. The first one might be "nissha." I don't think the first one is associated with Nissan.