Can anyone refresh my memory regarding speedo heads.
Are the refurb speedo heads different than the earlier ones?
And how can I tell the difference?
Final question, does anyone have a wiring diagram for a Cummins refurb RML?
Thanks

Rob Duker

My bus number (if any): RML2355

rob i have a wiring diagram for a refurb cummins RML .i can put in post let me know if u still need one

My bus number (if any): RCL,RM

I've got a copy of the wiring on CD and have a rebuilt speedo head in stock, bought in for a customer abroad who then changed his mind! This one is to suit the later type gear panel.

Early speedo heads are of the Smiths 3 phase type requiring a 3 phase pulse generator driven from the back of the gearbox.Refurb speedos are the single pulse type getting its pulse from a hall effect sensor on the propshaft.This signal is linear an needs to be amplified by the 551 control panel before its sent to the speedo head.Early speedo heads have 3 terminals numbered 1,2and3 whereas later ones have a positive and negative and 1 only pulse terminal.

My bus number (if any): RML2551

I have been looking through my Speedo heads and thanks to the info from Steve, I have a RM later speedo head but it needs a new glass and bezel fitting.
Can anyone suggest a company that could do this for me?

Roy do you have the contact details of where you get your speedo heads serviced, maybe they could replace the glass and bezel.

Regards

Rob

My bus number (if any): RML2355

Perhaps a clock maker/repairer might have the glass or be able help somehow? Worth a try..

My bus number (if any): RM158

Airlec at Rye 01797224466 the complete recon head cost me about £180!

Rob

I have used Airlec for parts before. Very good service from an ex LT man.

Steve, the old speedo generator produces an AC output which increases with speed, although I cannot remember the voltage range. I used to test them with with an AC voltmeter or a star connected set of lamps (so thinking about it the maximum voltage must be around 41.5V between phases to give 24V between the phase and the star point).

The newer type are of course pulse generators. We had a test set for the pulse generators on MD, M, LS and I think T vehicles. Not sure if they are compatible with the RML refurb generators though.

My bus number (if any): RML2276 etc

David you are correct that the signals generated on the old speedo generators are 3 phase ac.The speedo head is like a miniature 3 phase motor requiring the negative going phase to form the return path of the positive going phase of the previous winding in rotation.I did in fact produce a solid state phase splitter to allow a single pulse signal to drive an old type smiths speedo head and it did work, although i did experience some callibration difficulties.The idea was to use the new type pulse signal to drive an original speedo head without the need for an original type generator.I haven't had time to perfect this but i'm confident i can get it to work.

My bus number (if any): RML2551