Is there any dogmatic difference between Jehovah Witnesses Churches and Pentecostal Churches? Are they one and the same as far as beliefs are concerned? Untill recently I thought both the Churches were related in beliefs.
The short version is that they are not related at all. In fact in many major ways they are quite opposite each-other.
There are some similarities in the early roots of Pentecostalism. One of several main roots of the movement can be traced through the Foursquare Church. Although that particular church has since taken a bit more moderate approach, their beginnings are tied up in the affairs of one Aimee Semple McPherson and her teachings, "healings ministry" and other extensive influence. Because it had many roots and later many branches the Pentecostal movement is not as organized or definable as a sect, but it does have some characteristic teachings such as an emphasis on continued revelation, spiritual gifts, miraculous healings, etc.
On the other hand the JW's doctrine tends to be much more concrete. They believe in various specific end times predictions (some of which have morphed as dates come and go) and are very precise about things like terms of service to the church, doing good deeds, etc.
In summary, while they both began at roughly the same time and some parallels could be drawn in the way they first started, they are completely different entities. Early on they were driven by very different influences and have become very different beasts. Their teachings bear little resemblance to each-other and many of their teachings are mutually exclusive.
Basically, a lot.
Jehova's Witnesses are not really related to Pentecostalism
For one thing, Jehova's Witnesses teach that the Watchtower literature is the most accurate interpretation of scripture, while Pentecostals believe in scripture being relevant to modern life without having to rely on the leadership of a church to lead them; the Jehova's Witnesses believe in "divine guidance" which allows them to claim that certain parts of scripture are poorly translated/inaccurate.
There are major soteriological differences-Pentecostals basically believe that faith alone is sufficient for salvation, often including baptism in the Holy Spirit but not always, while Jehova's Witnesses have a doctrine that requires works for salvation (to prove that their faith is genuine).
In addition, Jehova's Witnesses believe that we are in the end times (with various apocalyptic predictions that haven't seemed to come true), while Pentecostals don't really have a defined statement on that as a larger movement.
Of course, the two movements are, to a certain degree, apples and oranges.
Jehova's Witnesses are a sect, and Pentecostalism is a movement-I attended a Pentecostal church, and they were relatively midline for Pentecostals, but there's no real organization that they were a part of, while the Jehova's Witnesses have a hierarchy and direct leadership. You may find individual Pentecostal movements that have different beliefs, or incorporated beliefs of other sects, but as a general rule they are not related to the Jehova's Witnesses, who predate them.
Massive differences. On the outside JW will appear to be a normal christian church. JW hold to a very different set of beliefs. Dig deeper and you'll find
Naturally, you will find that all denominations believe slightly different things. But Christian churches will all often profess a core set of beliefs that are the same - such as Jesus being God and have only the one bible, no other scripture added (such as the book of Mormon added by the Church of Jesus Christ of Latter-day Saints).
You will also find there can be churches such as Pentecostal Presbyterian and ones that are not Pentecostal.