Rostra Cruise Intermittent fualt

I have a Rostra 250-1580 fitted to a 1961 Valiant manual (Borg Warner T5) It works well apart from a bit of surging at light loads, I believe this is not uncommon. My problem is that it drops out for no apparent reason on a straight and level road, sometimes it will work for an hour then drop out for no apparent reason. If you reset it it may drop out in a minute or work for another half hour. I am running magnets on the drive shaft and have the blue tach wire conectected to the coil neg. I have been wondering if it is a heat related problem as it seems to be worse as the day heats up. This model has all the electrics in the servo unit and so is in the engine bay with all the engine heat. Has anybody else had similar problem? I am open to suggestions. I am getting a bit frustrated as I have just replaced the older Rostra unit 16R which was only months old as it stopped working. The dealer told me the unit had failed, when I fitted the new unit it would not work either, it turned out to be the coil on the magnet VSS unit. Now the new unit is also giving me grief. Please help! Mal

Intermittent problems can be very hard to find. Start by making sure any power and ground connections are good and not loose. If this all checks, check your stop lights. If all don't work, then this could be your problem. If it still fails at times, the problem could be an ignition spike on your tach line. If your vehicle is a manual transmission you have probably hooked up the tach wire and set the cruise for a manual trans. Try disconnecting the tach wire and set the cruise for an automatic transmission. Warning, be doing this the cruise won't disengage when you depress the clutch so you will have to tap the brake pedal first. Do this only as a temporary test. If the cruise seems to work correctly, you should install a clutch switch onto the clutch pedal which will disengage the cruise. I never use the tach wire for this reason. I always prefer to set the cruise up for auto trans. and use a clutch switch. You will find the cruise works smoother, as it only refers to a VSS signal and not a tach and VSS. To fix your surging problem make sure that the throttle cable for the cruise is tight at rest but doesn't increase the throttle. It should only have 1/8 inch of play in it. I am not familiar with your Rostra part numbers so I am not sure which cruise you actually have. It is an electronic one or a vacuum one. If the unit is new, I doubt whether it is bad. Rostra units are very reliable.sparky

check your vacuum lines to make sure they aren't leaky.  Sparkie is correct that the electronic ones are very reliable, but the vacuum ones that are a few years old are sometimes questionable.  If you contact rostra, they may be able to get a replacement for that model.

All Rostra units are reliable. I used the vacuum units for over 20 years. Failures were very rare unless they were very old and well used. I am not sure whether you can get a speedometer cable sensor that will fit your application anymore. If the magent kit is installed correctly, it will work just as well. The sensor needs to be about 1/2 inch away from the magent as it passes over it. You should be able to just fit your pinky finger between the head of the bolt through the sensor coil and the magnet. Make sure that the sensor/bracket are mounted to the engine. You want them to move in unison over bumps. If you mount the sensor to the body, it will constantly change the distance between the sensor and the magnet. This will also cause the cruise to vary in speed while set. Make sure that the magnet is mounted close to one end of the drive shaft to reduce the up/down movement of it too. It is possible that the only problem with the cruise dropping out is a spike in the tach wire. I might expect the cruise to be at fault if it were very old. Cold solder joints on a circuit board can cause it to cut out as they tend to act up after the unit gets warm.sparky