My 80 started downshifting going downhill? (1 Viewer)

[kilmarkid]

Went on a 2500+ mile road trip and encountered a strange issue. 1st day we went from bay area to Bishop over the Tioga pass. Coming down Tioga I turned off OD (overdrive) to slow vehicle without braking. This has been a standard procedure of mine on long downhills.

Near the bottom of Tioga I hit the OD back on and that is when it got strange. I am just coasting downhill and suddenly my revs jumped. It seemed like it dropped out of OD causing my revs to jump. I was not on the throttle, just coasting. It happened a few more times as i approached hwy 395. Pulled onto 395 and the problem went away. Does fine climbing and on flat roads.

I never touched the OD button for the rest of the trip and it down shifted fine on climbs. But at various times it would downshift or come out of OD when coasting downhill.

It is a 96 with 315's, 5.29 gears, and 330k miles. Checked and it did not throw any codes.

Does anyone have any idea what could be causing this?

 

[Malleus]

Perfectly normal.

Torque converter lockup; I assume you were idling? Regardless of speed, if the ECM senses the throttle vale fully closed, the lock–up solenoid will disengage. Lock–up is also speed dependent, if the throttle is open. Lock–up is also engine temperature dependent; if the engine coolant temperature is 131°or less, the ECM will prevent lock–up.

 

[Malleus]

This is from the 1996 service manual (RM451U)

The 1995 service manual (RM432U) has a little more information in it

FWIW, the test procedure for the lock–up control system is to operate the truck at 59–mph and add a small amount of throttle. If the engine speed increases, lock–up is faulty; if it stays the same, lock–up is functioning normally.

Same transmission, and I would assume the same ECM table.

 

[LongDuck]

Were you running Cruise Control at the time? Just curious, as I've had the 80 do some strange things when it can't maintain speed for whatever reason. Probably unrelated,

 

[kilmarkid]

Thanks for the insight guys.

Strange thing is that this behavior has never happened before going downhill. Engine had been running for hours so was up to temp.

Will try the 59 mph test with speedo at 59 and again with gps at 59.

I was sure something was up with the OD control button. And no was not using the CC.

 

[efjayatee]

On my experience coming down say Sonora pass, once I tap my brakes, the gear will downshift (acted like OD/Off), that is normal. But to kick-down while coasting.....never happened to me. Only when I tapped on my brakes as I'm coasting down a hill at 60-70mph

 

[kilmarkid]

Yea efayatee having owned my 80 for over 19 years and 210,000 miles, this behavior had never happened before.
It seemed like the OD actuation was malfunctioning. Does anyone know how the OD button works? Is it an electric mechanism?

On a 30 year old vehicle with 332K miles, I think mine is wearing out.

 

[Malleus]

From the service manual (transmission section):

(from the Cruise Control section):

The transmission control circuit makes decisions on which gear to use based on several inputs, one of which is the O/D switch. Yes, it is an electrical wwitch, but it's not directly connected to anything, except the ECU (and the combo meter light).