1996 Ram 1500 4x4 5.2/auto (46RE I'm assuming). Tranny rebuilt about 10 years/100,000 miles ago (current odometer 226,000). Was working fine till just recently.

Had the heater core and AC evaporator (or whatever sits beside it) replaced a few days ago at a shop I totally trust. Picked it up and drove it to work (45 miles one-way) no issues with the overdrive (above freezing outside - upper 30's/low 40's?). On the way home (0230 in the morning and at/below freezing outside).. if I'm saying this right... when I got on the highway the overdrive (OD) would unlock (RPMs would go up a couple hundred) and then within a second or two it would lock up again (go back a couple hundred to its normal OD RPMs). It would do this a couple times over about a minute, and then it would stay locked in OD the rest of the trip home (no more RPM fluctuations). Oh; cruise control doesn't seem to want to work either now (a minor concern).

I was wondering how this could be related to the heater core replacement since it seems to have started right after the repair. I know they drop the dash and it is a PIA to replace the heater core. I did a search here (of course!), and two possible suggestions were to replace the TPS and the brake switch. I guess the brake switch could have been knocked out of adjustment. So a TPS (what-the-heck - they're relatively cheap) and brake switch are on order. I'll try the cheap/easy possible fixes first.

But I'm wondering why the outside temperature would make a difference(?). I know if it is way below freezing the OD won't engage, but why this OD unlocking/locking issue is temperature related?

Thoughts/suggestions? I want to try this before delving into the tranny replacing solenoids and sensors.

 

Cruise is a separate issue. If your RPM is only jumping 200 RPM or so, that the throttle position sensor being weird. Replace the sensor, problem will go away.

 

Replaced the brake switch and the TPS. Got my cruise control back, and will have to wait for a freezing weather day to see if it stays locked in OD (vs locking and unlocking a couple times initially). It doesn't do it when it is above freezing.

Also swapped out my tune with my Hemifever tuner from the stock tune to the 89 tune. Seems to keep it out of OD longer when getting on the highway (a good thing).

 

Are you using the Radiator cooler?

 

I'm sorry I don't understand the question.

I replaced the heater core with another brass heater core.

I have an additional factory tranny cooler in front of the radiator (had the dealer install it when I bought the truck)..

 

I am guessing here and saying that the transmission isn't getting warm enough to stay in OD when it's cold out

 

I am not entirely sure the PCM really cares all that much about the trans being too cold..... My trucks (96, and 98) will both shift into overdrive after having driven less than a mile, regardless of outdoor temp. Even when it was -12 degrees outside....... If the fluid gets too warm, on the other hand, that will indeed disable O/D.

A fair bit of the time though, it isn't O/D that is the issue. Its TCC lockup. If the RPM change when it drops out/in is 200 or less, that's TCC. O/D is a significantly larger change.

 

See my truck won't use OD until the trans is warm. I thought since it only happens when it's cold that that might be it

 

I remember years ago when driving in subfreezing weather the truck wouldn't shift into OD on the highway (Montana winter - cold and snow). I was stressing over it since I was far from home, but came to find out that if it is too cold out, it won't shift into OD.

Haven't really had it happen since, but like I started this thread with, it recently did the lock-unlock (couple 2-300-ish RPM increase) lock-unlock-lock-unlock and then stayed locked in OD for the remainder of the trip home. This was in below freezing weather. Above freezing it does just fine. Someone suggested changing the TPS and brake light switch, so since these are cheap and easy, I did that. Cruise control is working again, and I'll have to drive the truck in below freezing weather again to see if it does the lock-unlock thing again. I drove it about 150 miles yesterday (after changing out those two parts) and it drives fine, but temp was in the 50's. If it acts up in freezing weather again, then I'll go to a transmission shop and see about the Torque Converter controller and solenoids and sensors.

 

I honestly believe the trans just isn't getting up to Temp. Last year in PA we had way below zero temps and it took my truck 45 minutes to warm up the trans to use OD now that is with a 180° thermostat