I want to thank you for the help. I have had a truck in the shop for a few weeks with this problem. I tried the linde software with no success. The tip to reset function 11 worked. I wish I found this site sooner.

Yes different battery, connected it to a different control system (SEM on a Hyster). At this time I checked the RPM of the motor and it is running at 3800, I'm checking this on the side of the comutator thru the brush holder hole. We are running 5 AMP on the field wires with no load. I plan on testing a different truck at a customers facility.

Did you try another battery?

It is a E25C/600 48V. THe driver said it did go 11-12 MPH. In one day it started to go slow then sped up about 4 times. Then at the end of the day it was going slow about 6 MPH.

I just noticed that your original post said the truck went 7.8 mph.
I assume that the full model of the truck is E20C/600. If so the theoretical maximum travel speeds are:

36 volt- empty: 8.7 loaded 7.6
48 volt- empty: 11.1 loaded 10.2
Hope this helps.

The dash light switch is there, but only one switch, is the Interlock switch an option? I did by-pass this switch no change in speed. The controller was bought new by the customer from Linde at Summerville NC.

The park brake switch is behind the mast and above the differential.
Where did you buy the parts, specifically the controller?

Still don't have this truck fixed, anything new out there to try?

The encoder was changed. The motor was rebuilt. The original controller is in it. The new one is a brand new one. I changed it after your suggestion. I'm checking on the interlock park brake switch. The manual said it is by the reservoir, but all I have there is the switch for the light. Any ideas on this?

Did you change it before or after my suggestion. Did you check the drive motor encoder? Where did you get the controller?

Thanks for the information. I did change it, but it didn't make any difference. We apparently have other issues.

I'm assuming that this is a 347 series truck.
If so, we had the same issue with both an original controller bought from Linde as well as a controller that we purchased from Flight Systems. We spent days on trying to resolve this issue trying different controllers, replacing the accelerator pot and on and on and on..
Although the default setting is quite different, try setting function 11 to 51. That is the only way that we could get our truck to attain full speed.