brettb

So I recently bought an actual barn find a 1974 Oldsmobile 98 I'm fantastic condition everything works except the rear defrost switch it won't switch over to the "on" side it just stays at the "off" position. Its not stuck it'll move but just go back to the "off" side how can I fix this Irreplaceable part? Since no one remans or sells this exact piece. Also I have cruise control and there's the normal cruise control button on the turn signal switch but there's also a cruise control switch on the dash by the heater controls that looks identical to the Rear defrost switch...so how do I work the cruise control do I use one or the other or both? Thanks for the help

jaunty75

About the rear defroster, I believe that's the way this switch is supposed to work. It prevents you from leaving the defroster on indefinitely and overheating the glass. Pushing the switch closes it momentarily and activates the defroster. After a few minutes, it shuts itself off. Also, the defroster only works, I believe, when the key is on and the engine is running.

Have you actually tested the defroster when the window is fogged to see if it works?

As far as the cruise control, there may not be a dash switch. Just push the button on the turn signal to set it, touch the brake to turn it off. Again, have you tried to operate the cruise control? These usually had a safety mechanism built in so they would not operate below something like 30 mph.

Do you have an owner's manual for the car? It would probably shed more light on both of these issues.

brettb

Yes I've tried to use the cruise control on the highway but jaunty there are 2 cruise control switches one on the turn signal and one on the dash they both are factory and say cruise control on them. I haven't used the rear defrost because I haven't had to lol. And I do have the owners manual

jaunty75

I'm sorry. I misread your post. Yes, you should turn on the dash switch and then depress the button on the end of the stalk to activate the cruise control once you're at a speed above 30 mph. Once the cruise control is activated, pushing in and holding the button on the stalk causes the car to decelerate until you release the button, and it will then cruise at that lower speed unless it's below 30 mph. There is no way to accelerate using cruise control switches or buttons like there is on cars now.

Once activated, you can cancel the cruise control by either tapping the brake pedal or turning off the dash switch.

If the cruise control does not work, there could be any number of causes, but the most likely is a leaky or missing vacuum line as the servo is controlled by engine vacuum. You would want to check the line between the intake manifold and the servo. If this checks out, it's probably time to get a factory service manual and go through its troubleshooting steps.

As far as the rear defrost, I know you have to wait for a humid, rainy day to really see if the defroster works, but does the little light on the tip of the switch go on when you activate it? There is, I believe, supposed to be a small red light on the switch to tell you it's on. If it does, at least you'd know that much of the circuit works.

brettb

Okay thanks for the help next time I take it out on a long drive I'll try that out. I don think vacuum lines is a problem I replaced them all with brand new ones. So if it doesn't work than its the cruise control system itself. Also I'll see if there is a light on my defroster switch. And try it out next time its moist inside the car. But that might be a while seeing how the car has such low mileage I only drive it once every week or every 2 weeks to maintain that.

jaunty75

It could end up being the servo itself. That's just a diaphragm that the vacuum pulls on. If it's got a hole in it or is otherwise worn out or damaged, no amount of vacuum pulling on it will move it.

I have this issue as well. I've had my '73 wagon for about 3.5 years now, and to this day I have yet to drive it in weather where the rear window fogs up, so I have no idea if the rear defogger works. I'm not sure I would bother with fixing it if it didn't as I have so little need for it.

Just as a point of reference, on my car, the switch does stay in the ON position when I move it that way, and the little red light goes on. Yours springing back after pushed momentarily to ON might have been a change between '73 and '74. I find it hard to believe that your switch, if it has failed, would fail in such a way that it would suddenly start springing back to OFF when it didn't do that when working properly.

jaunty75

You probably already know this, but the servo I'm talking about is circled in yellow in the photo below. This is on a '73 98. My wagon does not have cruise control.

brettb

Well I know my 98 was made in January or February it was bought new in Feb.1974. I live in Portland, OR so I'll need to use my defroster as everyone's windows fog here in the winter whether the car is new or old. I'll see if my defroster switch comes on today after I get off work. And my cruise control looks like the one you put up of the cutlass. Also your lucky to have that 73 Custom Cruiser most of them have been crushed or demolition derbied. I wish classic cars wouldn't get crushed. Anyways thanks for all your help. I'm going to make a video of my 98 soon and post it on YouTube and get it on here as well.

jaunty75

That's a 98. It should look very similar to your car.

brettb

Oh a LOL. my bad. No wonder why it looked similar. Thanks for all the help.