5 Teves, was the yahrtzeit of my wife's grandfather, Rabbi Dr Israel Frankel, a"h. This week's shtikle, a most appropriate one, is dedicated le'iluy nishmaso, Yisroel Aryeh ben Asher Yeshayahu.
When Yaakov reaches Be'er Sheva on his way down to Mitzrayim (46:1), he bring sacrifices to "the God of his father, Yitzchak." Of course, This was the God of his grandfather, Avraham, as well. However, Avraham's name is not mentioned. Rashi writes that the reason is because one is obligated more so to respect his father than to respect his grandfather. The very simple and obvious inference to be made from this Rashi is that there is, in fact, some halachic obligation of respect owed to one's grandfather, albeit less so than for one's father. Rama in Yoreh De'ah 240:24 writes that there is an obligation to respect one's grandfather. His source is this Rashi.
However, this issue is the subject of much discussion amongst the posekim. Mahari"k (Shoresh 30) declares that a grandfather is just like anyone on the street, so to speak. That is, there is no specific obligation of respect. There seems to be no official source given by the Mahari"k. One source that is suggested as the basis for Mahari"k's position is the gemara (Sotah 49a.) The gemara relates that R' Yaakov grew up in the house of R' Acha bar Yaakov, his maternal grandfather. When asked to bring him water, R' Yaakov declared, "I am not your son." As the saying goes, "Raise me but I am still not your son for I am but your daughter's son." (End of quote) This suggests that there is no obligation to honour one's grandfather.
However, the GR"A and others explain that there is a difference between paternal and maternal grandparents. The GR"A, Gilyon Maharsh"a and Mahari"l explain that the source is a gemara in Makkos (12a). The gemara discusses various laws pertaining to the go'eil hadam, the avenger. In certain specific instances, certain relatives of a murder victim are permitted to avenge their relative's death. A brother is usually included in this group. However, if a father kills his own son, under these conditions, the brother of the victim is not permitted to kill his father. Nevertheless, the gemara does conclude that the son of the victim (grandson of the killer) is not bound by this prohibition. This seems to imply that there isn't any obligation to honour one's grandfather. Rashi's language there is even more unequivocal, stating that one is not warned by the Torah to respect his grandfather.
In addition to buttressing the position of Mahari"k, the statement of Rashi in Makkos also seems to be in contradiction with the statement made in our parsha. R' Akiva Eiger (Teshuvos 68) and others answer that when one's father is not alive, there is no obligation to honour the grandfather. Therefore, in the case of the gemara where it is the father that has been killed, the obligation to honour the grandfather does not apply. However, in our case, Yitzchak was not alive either. Perhaps one can explain as follows: There are certain acts of respect and honour that can be performed when a parent is alive (e.g. bringing them a cup of water.) But when a parent has passed, obviously some of these acts are no longer possible but there remains a subset of honour that is still possible after death, such as the way one addresses the parent by name. R' Akiva Eiger is addressing the former. The obligation to refrain from avenging a relative's death is clearly one that pertains to the living. In the case where the father is no longer alive, it doesn't apply to the grandfather. In our parsha we are dealing with a show of honour after passing. There would still be an obligation to show such honour to a grandparent, regardless, albeit less so than to a parent.