About a year and a half ago, to cure my washer not working I replaced the wiper switch. Recently the washer stopped working again, and I came up with a new issue that when I turned on the wipers only the high speed would work for the first few minutes, after which the low speed and intervals would work. It turned out to be a three-pronged problem.
a) I found that the ground wire from the switch was loosely crimped in its ring connector. That is more important than you think - the switch requires that extra ground to work properly. It only works on high sped until you ground that wire.. when I grounded the ring connector, I could also feel a little zing through my knee up against the side of the truck. Check that ground.
b) I had read elsewhere in the forum that the 'regulator' box (the name varies but it is the additional box cabled between the switch and the main wiring harness) is prone to broken connections. There is one screw through the bottom lip of the dash, on mine about halfway from the steering column to the parking brake, which holds the regulator bracket in. Unscrew that, work the box out, unplug the cables. There are two screws holding the bracket to the box which also hold the box together; unscrew them. You can then remove the side of the box, work the strain connector up the now-exposed slot it rides in, and expose the board. Work the board loose so you can inspect it to see if any component legs have physically broken - if not, great! Flip the board over to expose the solder joints - remelt the solder on all the connections (this is '70s through-hole stuff, nothing fancy like SMT or BGA), making sure you have a nice tight solder joint. This takes less than 5 minutes.
c) I don't know whether the bad ground was screwing the washer up or whether I had a bad contact in the switch - I dribbled contact cleaner down the shaft into the switch and kept working the washer action on the shaft. Must have worked; I started getting contact (I was checking with the continuity test part of my multimeter.)

Reassembly was the usual pain but with the three actions (reworking the circuit board solder joints, fixing the ground, and getting a little contact cleaner into the actual wiper switch) my wipers work on all speeds all the time and the washers work. If you suddenly only have high speed on your wipers, check that ground! and if you have to run your wipers on high a few minutes to get the pother functions working right, don't be afraid to go into the regulator box to fix it yourself. Hope this helps someone.

 

Do you have delay wipers? That's the only regulator box I can think of. And I have had trouble with those in Fox Mustangs, when simply unfastening them in order to work on something else like the pedals or whatever. On my old '82, I never again could get the delay function to slow down the wipers. Fast, slow, and the washers, still worked, but the delay mode only rendered the fastest delay setting. I don't think I was astute enough then to consider the box grounding via its attachment point...
But I did have my '77 F150 totally apart a couple of months ago, and I don't remember any control box. It seemed like my wiper motor is permanently hard-wired to the harness, and that there was just a straight uninterrupted lead to connector for the dash switch.

Nice work ferreting out, and rectifying, that problem!!

On a somewhat related note, I noticed my washer not working several months ago in the old cab, but the fault was much more simple: The **** was bottoming out on the dash bezel! Much easier fix too.

 

The ground wire which seemed significant is the one on the switch - I used the same screw as I used to screw the regulator box in place for the ground point. Yes, I have interval wipers; otherwise the harness plugs into the back of the wiper switch. Had a heck of a time finding the correct wiring diagram; most only show the wiper switch without the interval wiper box.

 

The intermittent wiper switch is the same: 1971 Torino; 1973/79 F100/350; 1978/79 Bronco.

The intermittent wiper governor (aka interval aka delay module) is either 1971/72 Mustang/Cougar & 1973/77 F100/350; or 1978/79 F100/350 & Bronco.

 

Hey Bill, do you happen to have the part number for my '74 F250? I figure if I can get a part number, I can start looking for a replacement while I try to remove my faulty one from the dash. Thanks.

 

Try renewing the solder joints on your old one before buying a replacement - you might get lucky and save yourself some $. You could also check continuity through the wires while you are in there. (Sorry, I would much rather rework than replace!)

 

D3TZ-18C476-A (replaced D1ZZ-18C476-A) .. Intermittent Wiper Governor / Obsolete

1971/72 Mustang/Cougar; 1973/77 F100/350.

No Ford dealer or obsolete parts vendor has either of these part numbers, none on ebay either.