Speed control quit working

[don340-4]

 My speed control started failing intermittently, now it is not working at all. I went through all the electrical tests that are in the shop manual everything checks fine, brake switch & throttle cable adjustment is perfect but it just won't engage. The information in the shop manual is very limited if everything tests ok & it still don't work replace the servo. I would like to get it working again but unable to find any new parts for this 1970 unit. Has anyone had any experience taking one of these apart & fixing the problem? or where I might get a reconditioned unit?

       

[GranCuda1970]

Just saw one on ebay not sure if it is the correct unit I remeber mine being round like that and I still have the mount bracket on the garage but it looks different. 

 http://www.ebay.com/itm/DETAILED-CRUISE-CONTROL-SERVO-70-71-72-73-74-CHARGER-ROADRUNNER-CUDA-CHALLENGER-/150889389579?pt=Vintage_Car_Truck_Parts_Accessories&hash=item2321b55e0b&vxp=mtr

[don340-4]

 GranCuda; thanks for the link, that is the correct unit, sounds like he was able to take it apart & check the diaphragm, I think mine may have a hole in it, I'll crack it open & check it.

[Chryco Psycho]

I assume both cables are good . you should be able to check for a vacuum leak without disassembling it just see if it will hold vacuum

[don340-4]

 Both cables are good the speedometer works fine, with the ignition switch turned on it will only hold vacuum if the rotary switch is in the off position, if the rotary switch is in the on position it will not hold vacuum.


   
       

[Chryco Psycho]

got me 

[don340-4]

 Me too! 

 

[GranCuda1970]

Never taken one apart but I bet there is a gasket in there that is dry rot. Try shooting a little squirt of penetrating oil into it pull side of the vacuum and check the vac again.

[GranCuda1970]

Out of the BOOk!!

Control ring in OFF position                                                                    Turn to On position   
Improper stop lamp or speed control switch adjustment                       see test and adjustment                                       
Faulty electrical circuit                                                                       See electrical tests
Fuse Blown                                                                                       replace fuse                       
Cable disconected                                                                             test and adjust control cable
Vacuum leak                                                                                    check vacuum lines
No resume when control ring is rotated                                            faulty circuit , Rotation fully toward resume

Speed control system 1970 Service manual  1-11.

[GranCuda1970]

you should not be able to engage it below 30 mph               speed control inhibitor

Service manual has somed detailed tests.

[GranCuda1970]

how to rebuild a servo 

[GranCuda1970]

http://www.imperialclub.com/Repair/Lit/Master/298/cover.htm

[GranCuda1970]

if it is a stuck servo vacuum control valve? Solenoid operated brake release valve system may be in brake mode dumping vacuum all the time..

[don340-4]

 GranCuda1970; Thank you for the Imperial club  link,  that is exactly what I needed, now I understand how this thing works or doesn't work. I think you nailed it (stuck servo control valve), I should be able to fix it now. Will update with results.

  http://www.imperialclub.com/Repair/Lit/Master/298/cover.htm

     

[GranCuda1970]