Hello,

I'm trying to troubleshoot a problem in a friends 1978 F150, 300ci, c6 transmission, about 120K miles. To be honest, I'm much more comfortable with manuals, and have exhausted the search function in the forums. I think I have it narrowed down to the torque converter or pump, but I'd feel better with some feedback.

The truck stumbles fairly baddly in reverse - maybe there's a hint of stumble in Drive, but reverse is much worse. It will move in reverse fine, but it feels a little rough doing it. The forward gears seem to function fine, shifts are crisp and precise, and there's no signs of slipping at all. The truck just made a 300 mile round trip on mostly interstates with no signs of trouble. I just drove it another 30 miles to confirm this, and the only thing I noticed was a slight shuddering letting off the gas at 60mph plus. Again, I'm not sure what is normal, it's so slight that if someone told me they all do that I would belive it.

I pulled the pan, changed the filter and fluid, and saw nothing worrysome. The pan had a slight even gray coating of "dust", but no particles or chunks at all.

What I'm thinking after reading the forums is that the T/C is not fully releasing at idle, either due to it being defective or not having enough pressure from the pump. I'm also wondering if the vaccum modulator could have anything to do with this? Is there a test I can do to confirm any of these things without just throwing parts (or a transmission!) at it?

I should add that the fluid level is fine, dexron/mercon fluid used.
Thanks for any insights!
Dave

 

The C6 converter is not a "lock up" converter so its not staying on when coming to a stop. Pump failures usually cause the trans to slip and sometimes they make noise when they are badly worn. Both the pump and converter wont have problems in just reverse, they are not effected differently from drive to reverse.

The modulator controls shift timing and wont cause a trans to slip unless it makes it shift too early.

Transmissions typically wont stumble, dont over look a engine miss. The engine should also have the problem in drive also.

Describe the stumble in more detail.

 

I'm picking up the truck tomorrow and I'll report back on the possibility of an engine miss, pull plugs, check timing, etc. I asked about tune up parts and was told everything - plugs, rotor, cap, wires - was changed about 4 months ago, but I'll verify that.

The strange thing is that there a marked difference between reverse and drive. In drive, you pull away from a stop normally. In reverse, it takes a little throttle to keep the engine running, and it just sounds different (I know, not the most accurate way to describe it I've read that reverse has a higher ATF pressure demand than the forward gears, does it follow from this that load on the engine would be higher at idle in reverse than in drive, and therefore more likely to expose a tuning problem?

 

I agree with jk080, I don`t think it is a trans problem. If it was me I would look at the rear brakes first. Good luck.

 

Here's the follow up . . . and I think it's good news.

Driving it again, it doesn't really feel like a miss, rather a shake . . . I bumped up the idle just a little and changed base timing from 8 to 11 degrees BTDC, which seemed to smooth things out a bit. Plugs look good, choke works, all cylinders are contributing. The big thing, which I'm surprised I didn't notice or think of before is a broken left motor mount. That would certainly be part of the shake . . . .

A new mount is on the way, I have high hopes this is it. Thanks for the advice!

 

were u abe to figure the problemm out about the shaking? mine does it to. i changed the cap, rotor, plugs, tranny mount, both motor mounts. i rebuilt the carb. and when i put in reverse it moves the truck backwards and badly but in drive moves it a little in drive. i turned the idle up. i was told it could be a cracked vaccum line , jw