Hello,
My cruise control will not hold speed, it just drops speed as i go, it will accel when i press the accel button, but just will not hold the proper speed,
Any tips on how to correct this problem??
Thanks
CHris

 

Could be a vaccum leak at the cruise control module (under the Hood on driver's side fender)...

 

Does your speedometer work properly? any other driveability concerns? Depending oon the year rig you have. You should use an ocilliscope to check the wave pattern of the VSS.

 

This could get really long winded, but I will try to keep it short.

There are too many differences between years, engines, and controls to be specific to your question but most units work very much the same way.

Reserve plenty of time to test your factory cruise control unit. This could take a few hours to troubleshoot. Below is how to troubleshoot a vacuum assisted factory cruise control.

Factory 1980-1986 cruise control switches are located on the center pad of the steeringwheel with the cancel switch located at the brake pedal assembly for automatic transmission models. The servo unit is located on the left (driver) fender engine bay near the hood hinge. The harness connecting the switches plugs into the steering column harness and connects again to the servo unit. There should be a vacuum can on the left (driver) wheelwell which holds vacuum from the engine manifold to the servo unit. Some servo units use mechanical linkage to control throttle, while some others use a cable for throttle control. The vehicle speed is determined by a speedometer cable running from the transmission to the servo. The speedometer is driven by a cable from the servo unit to the speedometer.

For 1980-1986 the most frequent problem is; failure to maintain speed control. This is caused by a vauum leak either before the vacuum reservor cannister or at the servo diaphragm. The least expensive way to troublehoot a vacuum leak this is with a hand held vacuum pump beginning at the vacuum cannister hose connection to determine if the cannister will hold vacuum. If not, the cannister, vacuum hose to servo, or servo diaphragam is bad. These items can be tested one by one. If all are good, then the problem is before the vacuum cannister connection. Working your way back to the vacuum source one connection at a time is the only way to determine where the leak is.

If all vacuum connections check out good, then the problem lies with a switch, wire harness or dirty/corroded reciprocating common. Once the steeringwheel is removed, you will notice a copper ring attached to the steering column which makes contact with the base of the steeringwheel. This is the recprocating common for the horn, and cruise control switches. Brake cleaner applied to a Q-Tip is great for cleaning this item. If it is corroded, WD-40 or semichrome polish appied by a Q-Tip will do nicely. Check all wire connections to ensure they are tight and free of broken insulation.

Mechanical switches can be tested with a simple light probe with the switch depressed and held to determine if it looses contact or not. Normally, they either work or don't work and rarely fail otherwise. The most frequent problem is a bad contact either internally, at a wire connection, or common area such as the reciprocating common.

This was as brief as I could make it but hope it helps!