For some reason, my wipers don't work at random times when I turn the knob on my 71 Duster. I took the knob off to make sure it wasn't slipping and catching at random, so thats not the case. Is there a reason why they don't work, say 1 out of every 5 times I try and use them?

Failing wiper switch is probably the most likely reason, but could also be a failing park switch if it's a 3-speed wiper. The park switch is located inside the motor gear housing.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I have had this happen on my 1980 D-150 truck and my 1972 Duster. The brushes aren't making good contact with the commutator on the motor. There are two long bolts that hold the motor housing in the wiper assembly. Undo them and gently pull the commutator/magnet assembly out. The brushes should slide off as you pull the assembly out. Gently clean the brush faces and the contact surfaces of the cummutator. The brushes are spring loaded and can be made to hold the brushes off of the motor comm while you put the motor back in. It is best to put the comm in the motor body (permanent magnets pull on the comm too much otherwise) then insert into wiper body and release the brush springs.

Worked for me.

bwhitejr

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'72 Duster (Performance 360)
'83 Ramcharger (Performance 318)
'80 TrailDuster (360)
'80 D-150 Truck (See Below)
CompCams 252S, Holley 390cfm, Offy manifold
Ported, Polished and Gasket Matched
P4286813 Springs,0.040 Overbore,
0.090 Shaved Head

On my Dart, it was the switch. I "fix" it by simply wiggling the switch back and forth between speeds. It seems to run more reliably when slightly off center from the detent in the switch's rotation. It scared me the first time it happened, but it has never failed to work with a couple of rotations of the switch.

Sam

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If there's something partially binding up, the breaker may be opening. It is internal to the switch. If breakers get cycled too much they can get weak and open at a much lower current than spec.

To test the motor's current draw, just put a ~20A ammeter in series with the battery and run the wipers with the ignition switch in AUX. You can remove the arms easily to avoid dragging the blades across dry glass.

Too low a current, and you probably have a high resistance or open at the switch, connectors, or brushes.
Too high a current, and you probably have binding or a partially shorted motor.

Be aware that the 2speed motor & wiring is quite different from the 3/variable-speed. The later requires a good ground at the switch, so something as simple as
re-tightening the holdown nut may fix it. If you whole dash is ungrounded, you'd have more problems than just the wipers.

Have you checked the ground wire on the wiper motor? Sounds stupid but my Valiant had a problem with the wipers not working some time back and I cleaned the ground wire and voila! wipers! Just something simple to try........

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'68signet

"Living analog in the digital age."