My truck has been starting to run the RPM's up higher over the last few days. So I checked the tranny fluid level (with it warmed up and running in park - as per the directions on the dipstick). It wasn't even registering at that point. Was at the "full" section when parked cold. My trans code on my door sticker is K which I understand is a C6. My dash indicator shows PRND12. No OD. The fluid on the dipstick is red. I went to AutoZone thinking I'd just grab a few quarts and dump it in. I didn't realize there are so many different kinds. Info I've found in here says Mercon, another says Type F, then someone else mentions that you can't mix them. All the Type F is sold out at my AutoZone. I'm afraid to put the wrong fluid in. My plan was to have the fluid drained and filter changed over the winter when I'm not working the truck. But for now I have to get it topped off so I don't burn out the transmission. Any idea what I need in there? The dipstick says "Use Mercon". Doesn't specify IV, V, VI, Type F, +4, multi, etc. I don't think they sell just a plain old "Mercon" anymore.

 

At the time your truck was made MERCON was the right fluid. There was only one type of MERCON then.

Since then MERCON has been discontinued. MERCON V is now the right fluid for your transmission.

 

Ugh😩 Ok. I literally searched the web for over 4 hours last night trying to get at least a few sources that stated the same thing and I couldn't lol. Some say Type F and some say it will destroy the transmission. Some say Mercon V and some say that will destroy it. Both were specifically stating for the C6 lol. Does it matter what's in there now? I've owned the truck for about 3 years but never had to add anything or change the fluid.

 

Use the stuff at Walmart labeled "Automatic Transmission Fluid". It's way cheaper than Mercon V and is compatible with your transmission.

 

You obviously didn't find Ford TSB 06-14-4. That states that you can use either MERCON or MERCON V in a 1987 or newer C6. Since MERCON is no longer manufactured that pretty much leaves MERCON V. Try putting Ford TSB 06-14-4 in the Google search bar.

Ford stopped using Type F in the C6 before 1987, but I don't remember the exact year.

 

I believe the dipstick in my '87 U150 called for type H ATF. It is currently filled with MERCON since MERCON V was not available back when it was serviced and then later rebuilt. Type F was only for the power steering.

 

Type H was later renamed MERCON.

 

i have about 3 quarts of castrol ATF (mercon compatible according to the label). can i mix it in with mercon V for my next fluid change ?

 

1973/79 Ford truck parts catalog sez:

XT 2-QDX .. Automatic Transmission Fluid - "MERCON" ~ 1977/79 C4/C6.

 

Yes, you can.

 

Ok so I topped the transmission fluid off with Mercon V. It wasn't registering on the dipstick before. I put in exactly 1 quart and it brought it up to the crosshatched area on the dipstick (right in the middle). I checked it hot and running in park. I checked twice. When I drive it it still runs the RPM's up high before shifting. It does it from 1st to 2nd and from 2nd to third. What's odd is if I'm in 1st trying to go into 2nd if I let off the throttle slightly it will shift sooner but if I keep my foot on the throttle it will runnthe RPM high. What else could it be now that the fluid level is correct??

 

I decided to look under the cover of the throttle body and noticed one of the cables with this white plastic sleeve is cracked. I'm not sure if that would have anything to do with my symptoms but there is a good amount of tension on that cable with a good size spring behind it. What is that cable? Versus the cable to the right.

 

It is probably a leak in the vacuum line to the transmission modulator or a bad modulator.

 

I pulled the vaccum line off at the modulator. I don't have a vacuum tester but you could clearly hear the air sucking in. I put a latex glove on my hand and it sucked the glove to the tube really strong and held it. I'm pretty confident it isn't the line at least. So if it is the modulator itself why would it shift up once I back off on the throttle? It looks like the modulator is adjustable because of the little screw in the middle where the vacuum line connects. I don't know if it's worth even trying to adjust it or just replace it. I can't imagine the part is all that expensive.

 

Okay, dumb question for the OP; did some trans fluid drip out of the line/modulator when you pulled the line off?

If so, replace the vacuum modulator.

Also, you might want to check the kickdown linkage for proper adjustment.

The reason it upshifts when you slack the pressure on the throttle is the vacuum signal increases, telling the transmission through modulator pressure signals it should be in a higher gear due to lower engine load/demand.

<useless Transmission-1 syllabus information follows>

(Please excuse my nomenclature. Haven't taught in almost 25 years)

A C6 is a simple beast. It uses Governor pressure (road speed) and Vacuum modulator pressure (engine load) to determine upshift/downshift speeds. There is also Throttle pressure (a direct link from the throttle linkage to a lever on the left side of the case) to force downshifts.

Governor pressure is on one side of the 1-2 and 2-3 shift spools and a bias spring plus modulator pressure is on the other. If you just put the transmission in gear and release the foot brake on a level road, the governor will begin to put pressure on the 1-2 spool. Once the pressure gets high enough to overcome the spring and any modulator pressure, the spool moves to the "2" position and the trans upshifts. The same goes for the 2-3 shift. If you apply throttle to the engine, now we have some opposing pressure to the governor pressure, delaying the upshift.

That is why an automatic trans upshifts when you lift the throttle.