I'm gonna replace the vacuum modulator on the tranny of our '79 F-150 4x4 (351M/C6/CP-205) and when I hopped online to make sure the auto parts store had them in stock I see they are available as either a purple stripe or a brown/black stripe. . .

. . .I'm sure this probably has to do with shift points or firmness and I could always just pull the one on the tranny now and replace it with the same color strip modulator, but I'd like to know what the color stripes mean so that I can get the correct modulator for sure. . .just in case it doesn't have the one best for our use on it now.

Thanks!!!

 

Makes no difference.
What is the stamping info from the transmission.

 

They are adjustable, the "striping" just indicates roughly where the adjustment is set to.

Also you can adjust the length of the modulator pin.

There are 4 pin lengths.

1) 1.601-1.611
2) 1.634-1.644
3) 1.650-1.660
4) 1.666-1.677

A metal coat hanger works well.

Josh

 

So what does the adjusting the length do. . .i.e. adjusting it one shorter does what to the shifting and adjusting it longer does what?

Thanks for the info!

 

It adjusts the shift timing.

Earlier or later

ie:1st to 2nd @ 18 mph, 2nd to 3rd @32 mph vs 1st to 2nd @ 16 mph, 2nd to 3rd @ 30 mph

 

Turning the screw CLOCKWISE raises the shift points.

If, by chance, you had the shortest pin installed and had the screw bottomed out and wanted to go more you would then install the next longest pin and readjust until you are satisifed.

But, you cannot install a pin longer than 1.677 inches.

As for stripe colors, again it really doesn't matter. Black is the most common. Green is more for Diesels and Purple is sometimes used for low vacuum engines. There is also a White Stripe too (not the rock band)

So I would match what you already have, if that wasn't possible I would get a black stripe and fine tune it.

Josh

 

I'm picking up the black stripe unit tomorrow.

Thanks!

 

OK. . .I pulled the old modulator and of course I hit the 50-50-90 jackpot (i.e. faced with a 50/50 choice 90% of the time you will make the wrong choice) and the modulator I pull has the purple stripe.

I went ahead and replaced it with the black striped modulator and the truck idled in Park with a VERY noticeable lope that wasn't there 15 minutes before I replaced the modulator.

Will the modulator affect the truck's idle in parK???

I plan on getting a purple stripe modulator tomorrow. . .and unfortunately I forgot to measure the pin lenght.

Could anyway shed some light on this situation for me. . .the truck was running great and then it all the sudden will no longer idle in gear for more than a few seconds before it stalls out. I went ahead and replaced a lot of stuff that didn't necessarily have anything to do with the problem but probably needed changing while I was at it. . .here's what I replaced:
-cap, rotor, & coil
-plugs & wires
-fuel filter
-all vacuum lines
-voltage control module

and none of that helped so I figured it was the modulator. . .replaced the modulator with the black striped unit and the problem is now very much noticeably worse.

Do you think switching to a new purple striped unit and adjusting the screw to match the old length (~13/32", or 0.4" out from the face of the rear of the modulator)?

Thanks!!!

 

I do not think the modulator will effect the way an engine runs unless there is a hole in the modulator itself causing the engine to suck air or transmission fluid, try plugging the line at the modulator and see if the engine runs correctly.If there was a hole in the modulator or vacum line at the modulator, the carburator may have been adjusted to run with this problem at some time and now that the hole is gone the carb may need to be adjusted.
The above is based on the problem showing up only after changing out modulator.

 

I don't think it was the modulator either. . .I ended up replacing the intake manifold and carb with Edelbrock Performer intake and 600 cfrm Edelbrock Performer electric choke carb and the problem went away. . .now I have to reconnect the vacuum on the transmission modulator (I forgot to and there is a plug in that port right now) and adjust the shift points and I should be good to go!

Thanks for the help guys. . .I learned a lot from you!

 

Found a correctly sized "tee" (3/8" x /3/8" x 1/4") included in the intake manifold install kit that came with the Edelbrock Performer intake manifold (thank you Jeg's!) and I spliced the transmission modulator vacuum line into the power brake vacuum hose and voila. . .

. . .I guess I adjusted the modulator pretty darn good just by eyeballing it (of course I don't know how sensitive these things are, the window could be pretty broad) and my tranny is shifting just right and my engine is now purring like a kitten!!!