TENYAS now plan to modify all their vehicles in a rolling programme and they should be on the road by the end of September.
TENYAS patient services director Tim Lynch said: "Following delivery of the first batch of these vehicles we decided on further improvements.
NEAS director of accident and emergency Paul Liversidge said: "We came up against the same problem when we first had Mercedes vehicles and have already spoken to TENYAS about our solution and we have arranged for our fleet manager to provide them with any help we can.
But they failed the targets for urgent ambulance journeys to hospital with TENYAS ranking the worst in England.
During that time Teesside was under the control of the then TENYAS which has now become the Yorkshire Ambulance Service.
The number of emergency calls received by TENYAS during the same time was up 6.
John Darley, TENYAS director of patient services, said: "Most of the issues identified by UNISON representatives require local negotiation to resolve and these are all resolvable through negotiation.
The management at TENYAS is confident that with the actions we have put in place, and through further negotiation with Unison, we will be able to resolve the issues identified without the need for any form of industrial action.
But Ian Walton, director of governance at TENYAS
, said: "There are no plans for redundancies.
Posters are now being displayed in all TENYAS vehicles to show that violence against staff will not be tolerated.
Tony Wearmouth, TENYAS controls assurance manager, said: "This is not acceptable and we hope these posters will be a stern reminder.