Help! P1870, No 4th gear

[JRS_C5]

Hello everyone, I'm new here. Just bought a red 98 C5 automatic last week. While driving I got a P1870 code, then it started to shift harshly. I looked it up and found out that by erasing the code the transmission shifts fine again. (the code sets max. pressure in the trans. to make sure it shifts). Later I noticed that if I was on the interstate and I pressed the gas 3/4 or more the car would downshift to 3rd, but it doesn't upshift to 4th, it hits the rev limiter at about 90mph. And one final thing, sometimes when driving steady at about 45mph the car seems to downshift by itself then upshift again to 4th. The transmission oil temp is somewhat in the range of 180-200.Anyone has any idea what this could be? Is it the TCC valve in the valve body? And why doesn't it shift to 4th in 3/4 or more throttle but it shifts fine at normal throttle?Thanks in advance for any help!Juan

 

[Cadillac Tech]

#01-07-30-023B: Harsh 1-2 Upshift, SES, MIL, or CEL Illuminated, DTC P1870 Set (Replace Valve Body) - (Mar 22, 2002)

Subject:Harsh 1-2 Upshift, SES, MIL, or CEL Illuminated, DTC P1870 Set (Replace Valve Body)​

Models:1996 Buick Roadmaster

1996 Cadillac Fleetwood
1999-2000 Cadillac Escalade
1996-2000 Chevrolet Camaro, Corvette
1996-2000 Pontiac Firebird
1996-2000 Chevrolet and GMC Light Duty Truck Models
1996-2000 Oldsmobile Bravada
with 4L60-E Automatic Transmission (RPO M30)
Built Prior to January 15, 2000 (Julian Date 0015)​

This bulletin is being revised to update VIN Breakpoint Date information and revise the Parts Information table. Please discard Corporate Bulletin Number 01-07-30-023A (Section 07 -- Transmission/Transaxle).
Condition​

Some customers may comment on a harsh 1-2 upshift and the Check Engine Light or Service Engine Soon indicator is illuminated.
Diagnosis​

Typically, these vehicles will have been driven more than 32,000 km (20,000 mi) before this condition occurs.
The scan tool may show a DTC P1870 set as a history code.
A harsh 1-2 shift or DTC P1870, caused by wear in the control valve body, may be difficult to duplicate when the transmission temperature is below 93°C (200°F).
Cause​

The condition may be due to wear in the control valve body. This wear occurs in the bore that contains the TCC isolator and regulator valves, and results in poor, or no, TCC apply.​

Important:

• DTC P1870 is a type B code. The conditions for setting the P1870 DTC must occur on TWO CONSECUTIVE TRIPS (ignition cycles, with a drive cycle) before setting a P1870 history code.


• When the conditions for setting DTC P1870 are met (first trip), the PCM commands maximum line pressure and harsh 1-2 shifts are the result.

• This may result in a harsh 1-2 shift with no history code if the conditions for setting the DTC required for the second trip are not met, on two consecutive trips (Ignition cycles, with a drive cycle).
• When the conditions for setting the DTC are met, on the second consecutive trip, a DTC P1870 is stored as a history code.
• When the P1870 code is stored, the PCM will turn on the Service Engine Soon (SES), Check Engine Light (CEL), or Malfunction Indicator Lamp (MIL). ​

Correction​

Install a control valve body with the revised TCC regulator and isolator valves. Refer to the Parts Information table below. These valves are used in all transmissions produced after January 15, 2000 (Julian Date 0015), and all of the service parts currently available through GMSPO contain revised TCC regulator and isolator valves.​

Important: If all of the following conditions are true, it is not necessary to rebuild the transmission or to replace additional transmission components beyond the control valve body.

• Transmission operation is normal before the transmission reaches operating temperature, or before DTC P1870 is set (no slips, flares, or missing gears).


• The torque converter is not blue or overheated.

• The transmission fluid is not burned or has no burned odor.
• The transmission fluid pan contains no abnormal debris (clutch material, bronze, brass, or metal fragments).​

​

 

[JRS_C5]

Yeah, I got that, I have the GM service manual. But that doesn't say anything about why it doesnt go into 4th under acceleration

 

[Mr. Lucky]

I had that exact same problem when I hooked up a Predator tuner and accidentally set the wrong gear ratio. In fact, I heard subsequently that some software revisions of the Predator set the ratio incorrectly when it appeared to be correct in the display.

 

[JRS_C5]

I had that exact same problem when I hooked up a Predator tuner and accidentally set the wrong gear ratio. In fact, I heard subsequently that some software revisions of the Predator set the ratio incorrectly when it appeared to be correct in the display.

Is there anyway to know if my car has been tuned? It did have an exhaust an intake when I bought it. Is it normal for it to chirp the tires when it shifts to second?

 

[Mr. Lucky]

I suspect a dealer could tell if they plugged in a Tech 2. I don't know if the tire chirping is normal. It's been so long since my car was stock (and even longer since I had runflats) that I don't remember.

Another thought...
Since your car has mods, I wonder if the previous owner swapped in a different rear end ratio w/o compensating in the PCM. It would essentially be the same scenario as with the Predator. If you have close to stock size (OD) tires, run it up to 75mph in 3rd gear and your RPM will indicate your gear ratio (i.e., 3150 = 3.15 gear ratio). 3.15 and 2.73 were the only ratios available from the factory for automatics.

 

[JRS_C5]

Tried that and I get about 3150rpm@75mph. So that's the way it should be. Maybe it's a worn 4th gear band :ugh the car does have over 100k miles :/