First off, let me say thank you for all the good info you guys are putting out. I just bought an '88 F-350 Supercrew custom with a C-6 tranny. This is my first diesel and it's been interesting so far. We've taken it on a couple of 100 mile trips to the woods and so far she's brought us home. Even after shutting off twice on way back roads. I plan on doing some maintenance this week and have some questions you guys, and gals, might help with.

The vacuum pump, one of the lines goes to a tuna can looking device on the passenger side wheel well. This hose snapped off at the plastic fitting. What is this part ?

The tranny is concerning me, the fluid is not bad looking but she burns alot of it. Say, a quart every hundred miles. The pan is wet. I checked the modulator and it is not leaking and there is a little bit of vacuum at the hose. She shifts to the next gear around 1,800 to 3,000 rpms only if I let off the pedal. Is this normal ? Also, is it normal to have to rev to 1,800 rpms for the truck to even move on flat ground ? I'm afraid to try to haul anything at this point. It doesn't really slip, it's more like it's not grabbing or torquing up. At 65 mph, I'm revved up to 3,000 to 3,100 rpms. The speedometer and odometer read to high for the distance/speed. Could the tach be reading too high also ?

The emergency brakes are disconnected at the rear axle. I notice at the bracket where the two rear lines connect, one of the lines is sticking out further than the other. Is this right ? If so, the bracket will be contorted in order to hook up to both. The ABS light stays on too.

The fuel tank is leaking, pretty sure it's the front tank at a steady drip. Killing the fuel economy. Is there a spot I should check first ? Filler neck, low pressure pump, sending unit ? The fuel gauge is all over the place.

The temp sending unit is inop. Where is it located ? The tach, fuel and temp gauges are not reliable. Anyone know where the main harness plug is ?

I plan on doing all the fluids this week. What synthetic gear lube are you guys using in the rear end ? I bought this truck to take hunting, camping, and to pull a 7,000 lb boat. With 158,000 miles I expect to do some work on it. I ordered the manuals I could find at rockauto, waiting on them to show up now. I appreciate any help to direct me to the right places to start.

K.J. Killebrew

 

For your high rev shifts, check your kickdown linkage, it might be stuck, stiff, or getting caught up on something causing high shifts.


ABS light on might just be someone that did brakes on it didn't bleed the RABS valve in the frame rail (drivers side directly under drivers feet, inside frame rail), or the speed sensor in the rear axle is bad, or wires/connections for it or bad/dirty.

If the light bothers you, put a piece of tape on it, and take the bulb out. That Rear ABS is useless anyways.



Temp sensor is located on drivers side head, as shown here.






I'd suggest real gauges for the truck, the dash ones are borderline useless.


The tranny could use a good flush.

Is there an Aux. tranny cooler on the truck already? If not, add one, if their is one, add a bigger one according to whatever you plan on pulling.




Others will chime in for more info.

 

The tuna can is the vacuum loss indicator, signals loss or low vacuum in the braking system........ Did you get a red light on the dash?

With a c6 trans I believe you can suck ATF out of the trans via the modulator valve. On a gas engine it was visible from the exhaust..... clouds of white smoke??

As for the truck shutting of what was the fuel tank level at..... if at or near the 1/4 tank then that could indicate the fuel pickup in the tank is broken off.

As for the brake cable then it appears one is applied or stuck and one is not... best to check that out.....

Depending on your axle ratio then the odometer and tach speeds can be very high..... normally we see 65 mph at 3000+-rpm on the C6 trans with 355 ratio even worse with 410.

Have you reset your Vacuum regulator valve.....

As for tank leaking check the top for the sender could be he seal or a bad fuel line.

The coolant temp sender is on the block just ahead of the left head....... Take a trip to Dave Sponaugles pictures ( sorry if I spelled it wrong Dave)Seen on above post.....

Not sure if your year truck still has the IVR instrument voltage regulator a silver relay on the back of the dash cluster .. it's job is to ensure correct voltage goes to the guages.

 

this is what i used to like to run on the C6:
TCI Adjustable Vacuum Modulators - JEGS

 

First off welcome to the forums!
That tuna can looking thing on the inner fender is a vaccume sensor. There should be a couple wires running to it also. That broken port might have something to do with the tranny not shifting right, as it needs vaccune to shift, I think. I'm not an auto tranny guy, so some one will chime in on that.
I would guess the cables to the e-brake are frozen up, and need replaced.
Take the sheilds off around the fuel tanks, they tend to hold dirt and rust the tanks.

 

Wow ! Quick response. I've been racking my brain over that tuna can. You guys saved me some grey hairs.

I'm assuming when the bottom half of the fuel tank selector switch is pushed in, then that's the rear tank ? Both times it has cut off, it's been on the rear tank. I switch to the front, pedal to the floor then crank til it starts. I definitely know I need to get to the top f both tanks. Either drop them or pull the bed. Any recommendations ? It will probably be just me and my tools, my boys are too young for the heavy stuff.

Farmert, are the rear brake lines just for the parking brakes ? At least the two that meet under the rear driver side frame ?

Ford, I went ahead and ordered a new green stripe from rockauto, I adjusted the one in there now with some success.

PLC, I'll look for that voltage regulator. Would it be in the engine bay or the cab ?

John, I lubed the snot out of the kickdown and it seems like it wants to hang up at the IP throttle connection. If it has ever downshifted, it must be too smooth for me to notice. Most of the time it does seem to end up in 1st from a stop though. I'm used to a tach surge or the feel of an automatic downshift. Should I expect this ? Is this part of the shift modulators function too ?

I read in an earlier post to hammer down on the throttle to tow any thing. I guess my 3,000 rpm/65mph numbers aren't so bad ? Will a heavy tow make it 3,000/55 ?

Thanks Again, I'm feeling a little better about this rig already.

 

Oh Yeah ! How do I adjust the VRV ? I inspected it and can't see a way to adjust. It does have a vacuum coming out of it. I plan on getting a brake bleed kit and using the vac pump/gauge to check the amount.

John, it came with a pillar gauge set, oil, temp, voltage and they are inop right now. Plan to fix and probably swap the voltage for tranny temp. I'll be looking for the cooler.

Thanks some more !

 

-Swim welcome to FTE!!!! take some pics when you get a chance!!!

 

The ABS warning light could be the result of a bad sensor at the hub. Replace it if you want or take out the bulb. You could also bleed off the valve on the driver's frame rail to see if it goes off.

My '91 F-350 front tank was leaking at the top - the last gallon or so after fill-up. The steel band which straps the tank in place had eaten the rubber padding away and rubbed a small hole in it. I removed the band, sanded the hole and put a dab of pig putty on it. No more leaks.

The last 4 inches or so of each pickup tube is rubber. It hardens over time and breaks off. Replacing them is easier if you remove the bed. I have a dump kit, so replacing both of mine a couple of years back was easy.

The transmission problems would concern me. Almost 8 years ago, mine was rebuilt for similar issues. I would take the advice of others here 'cause most of the guys on this forum are pretty sharp. Have the trans flushed before dropping $1500 ~ $1800 on a rebuild. Again, add a larger trans cooler.

 

Yes the cables are for the parking brake. One from each side that meet just back of the cab on the drivers side frame rail. One cable then runs to thepeddle in the cab. The cables from each wheel can ge dirt and moisture in them, and they will bind up. Usually a new set is th fix.

 

Well guys, my parts finally showed up so I dropped the trans. pan last night. What a mess, the pan had about a quarter inch of sludge in the bottom. Cleaned her up, replaced the filter and took it for a spin. She shifted much better and smoother but still not perfect. I had WAY too much fluid in it and had to pump that off. Ran her again and it did not shift as good as being overfilled ?

This truck has some serious leaks; power steering pump, transmission line that runs to the bottom of the radiator ? and oil leaks from the top back side. I plan to clean the undercarriage then pinpoint them one by one.

It turns out the fuel leak I had ran the back town out in about 100 miles. On a quarter now for the fron with only 200 miles on it. This thing is getting expensive quick.

The PO had the oil filter on so tight I had to drive a tire tool through it to get it off. I guess the guy was sleeping during the hand tight lesson.

Thanks again for all the help.

 

Nope dont put these filters on hand tight they will loosen on you, about 11/2 turns after the gasket touches. If your VRV is adjusted right you can ditch the kickdown rod you dont need it, it will downshift with the VRV.

 

Well think of it this way: If this was a powerstroke, you would already be out more money than you already have spent.

 

I got a Chiltons manual and there is nothing in there about adjusting the VRV. Anyone have any pointers ?

I knew when I paid $1,500 for the truck I would have to do some work. So, I'm not complaining...............................yet.

 

inside the nipple where the vacuum line goes on, there should be a small screw. In / out for higher / lower shifts, i dont remember which is which, but give it 1/2 a turn and see what happens. Sounds like you got a rig thats been rode hard and put away wet more than a few times. Dont judge all the IDI's /pickups by that though, theyre really great rigs 99% of the time.
My old C6 would shift better overfull too... i remember being told that it means time for a rebuild, but i dont know what exactly the problem was. A flush could make it better, but could also make it worse. But imo $80 is better than $1200. Yours sounds like its slipping fairly good though, might be junk in the valve body or an internal leak.