1990 F150 trouble only when pulling trailer? I'm at a loss.
 

I have a 1990 F150 4x4 302, AOD. It is a relatively low mile truck(84,000). I haul with it quite often and I occasionally pull a car trailer with it. In the snow, in the mud, with the box full of engine/trans or heavy cast iron tractor parts it has never given me any trouble. It also didn't ever give me trouble pulling the trailer until recently.

Last fall the transmission went out and I had it rebuilt/reinstalled by a local reputable transmission shop. Since then I have had the same problem twice. Both times when pulling a trailer. The first time the trailer was empty and I traveled about 60 miles. When I pulled into a gas station and rpms came down, the truck started started sputtering and died... very similar to the way it behaved the time I had a catalytic converter go out on my 302 Tbird. At the time I was sure that was my problem. Towed the truck home, replaced the converter, it ran great again so I thought I was golden.

Yesterday I hauled a small tractor and mower and after about 180 miles it did the same thing when I slowed to turn a corner. Luckily, because I was on the road this time, I was able to downshift and accelerate and it came out of it with no more trouble the remaining 15 miles to my destination.

Any ideas?

Is the truck fuel injected, or carbureted?

K

Fuel injected

Lock up converter staying engaged to a stop?

I was wondering if the converter could be an issue. But, I thought I should ask around a bit before I called the transmission shop in case I was way off base.

The only thing that I can think of is the only link of the truck and trailer is the trailer lights being connected to the truck.

I had a Ford Aerostar brand new do that exact same thing and it was the throttle position sensor. It felt like you had not put the clutch in.

Are you using the overdrive switch when you pull the trailer? Being the trans has been replaced I would imagine something missed when it was swapped.

I make sure to turn the OD off when I am pulling anything.

Weird note on that, when I pulled with it PRIOR to the transmission rebuild, I would turn the OD back on when I was up to speed on the highway and never had a problem.

I'm wondering if I may be getting vapor lock? I didn't even consider this since I was under the impression that it was extremely unlikely in a fuel injected/electric pump vehicle. I suppose it would also make sense that it didn't happen the times I pulled prior to trans rebuild as those times were all in the winter or very short distances.

I suppose it could be the TPS as 68GTO82 suggested. But, if that were the problem, would it only show up after pulling a load for a while?

This would be my guess also, certain GM transmissions have a solenoid that will act up and keep the lockup clutch engaged as you come to a stop when it starts failing. You have to throw the vehicle in neutral to keep it from stalling.

GM also has a temperature sensor in their electronic control system in case the fluid overheats. It then tells the TCM to avoid anything that will generate more heat in the fluid. Locking the TC clutch would do that, but the brake pedal stoplight switch should override the TCC lockup when coming to a stop.

Having knowledge that with the TCC working the problem doesn't show itself might be evidence that the fluid temps are high enough under towing conditions that the problem shows itself. You could have more than one problem that is contributing to the condition when towing, but it would seem to me that it is fluid temperature related. This is just a guess from past experiences with GM products, I actually have zero experience with ford transmissions.

A scan tool may be of some help to determine what portion on the electronics controls, or sensors may be causing the problem.

It could also be some brake light switch back feed that only shows with trailer lights hooked up.

I'm supposed to take it to the transmission shop tomorrow so they can hook it up to the scan tool and see if it shows anything.

Have you considered a transmission cooler upgrade? Might not actually resolve anything, but your replacement transmission will thank you for it. Especially after the last one died prematurely. An OEM stacked plate cooler from an F250/F350 might even fall into place.

How big/heavy duty of a cooler do I need? Somewhere in my parts stash I've got a good used one leftover from the wrecked vehicles I have rebuilt in the past. The one I've got is roughly 14"x4" and probably 1 1/2" thick.

Took the truck in and scanned this morning. The only thing that showed was a EGR code. I wonder if that may be a past issue. Since I had no O2 sensor for almost a month and they work together could that have had any effect on the EGR code on the scan?

I was reading this thread before and thinking of a stuck EGR valve. If it's stuck open, it would cause a stall condition and likely be open a lot during a tow because of load.

I’d say yea, he wants the current one to last it would be a wise investment. He could likely find a used one for cheap in a yard easily since they made so many.

As I was reading along, EGR physically stuck open or the EGR position sensor stuck would have been my first guess after eliminating the Torque converter clutch by shifting to neutral. The position sensor sits on top of the EGR valve on those trucks and can be purchased separately. Easy to change and cheap to buy. If your code was for a position sensor specifically, I would just replace that first. If it was for a flow problem, then take the valve off and investigate further.

Been 3 weeks, any updates?

I really don't know how specific the code was. The guy that checked it just said it was showing an EGR code and to bring it back if it acted up again and he would check to see if it showed that code again(he cleared it that day).