P1870 REVISITED By: Jim Dial (ATSG) Senior Technical Consultant Just when you thought you had all of the P1870 Torque Converter Clutch slip codes under your belt, another vehicle comes along.... and here we go again! I am sure that most of you are familiar with the 4L60E P1870 TCC slip code and Torque Converter shudder problems related to the code. This was usually related to Torque Converter Regulator Valve bore wear out, causing a loss of Torque Converter Apply pressure, allowing the Torque Converter Clutch to slip, and we all know what fixes are available for that. Now we have a 2000 model year Isuzu Trooper and BMW models, equipped with the 4L30-E transmission, showing up into shops with the same P1870 code. You guessed it, the same kind of TCC Regulator Valve bore wear is the cause here also. The biggest problem, is that there is little printed material to tell us where the TCC regulator Valve train is located and how it works. We all know that if you don't know how it works, you cant even begin to to fix it! History: In the 2000 model year, Isuzu Trooper and some BMW models went to a Pulse Width Modulated Torque Converter Clutch apply strategy. Isuzu Rodeo, Honda Passport and Cadillac Catera stayed with the typical ON-OFF TCC apply, that they have had since day one. This strategy change, on these limited models, has created some serious confusion in parts interchange and the identification of parts. Parts Change: In 2000 production, Isuzu Trooper and BMW models received the Pulse Width Modulated Converter Clutch Apply Strategy. This created the need for a PWM Solenoid, which happens to be the same solenoid as the 4L80-E PWM solenoid, as shown in Figure 1.

This, of course, changed the internal wiring harness because of the connector change. The pump changed from having two valves to four valves. The additional valve trains are the Torque Converter Regulator Valve and the Torque Converter Enable valve , as shown in Figure 2.

Special Note: ATSG has named these two valves based on valve function. If you happen to find a factory manual that names them something else, don't be confused. The Torque Converter Control Valve also changed and now has a bore plug with an o-ring. The pump plate and Bell housing also changed because of the valve additions, and of course the Torque converter went to a High Carbon or woven Clutch type.The Point: The main focus of this article is to show how it works. The re are no known hydraulic circuit diagrams that are available for the PWM TCC apply, so I decided to fix that. Refer to Figure 3

for a partial hydraulic circuit diagram of the TCC OFF. Figure 4

shows a partial schematic of the TCC when it is "ON." Notice, the PWM solenoid, when it is "ON" is in charge of stroking 3 valves simultaneously. (1) The TCC Control valve, which basically is the switch valve. (2) The TCC Enable valve, which controls the spring side of the TCC Control Valve and the Exhaust path for TCC release oil. (3)The TCC Regulator Valve, thru a .049" orifice in the pump plate, which controls Torque Converter Apply pressure, based on the spring pressure and the balance feed , thru a .042" orifice, from the middle of the two lands of the TCC Regulator valve, as shown in Figure 5.

This is the root cause of the P1870, the TCC Regulator valve sleeve is what wears , also shown in Figure 5, and creates a loss of TCC apply pressure. The great part about this sleeve is that it is sold separately from the pump and is easily replaced during overhaul, for the cost of less than a fast food lunch!
The Isuzu part number for the Sleeve is 8-96018-472-0 and if the valve is scored up the part number for it is 8-96018-518-0. Now you know how it works, and ATSG recommends replacement of this sleeve on every overhaul. We hope to see you in the upcoming 2005 Seminar where we will cover this problem a little more in depth, showing and comparing all of the ON-OFF and PWM changes. I would like to thank some people who really helped make this information possible, Ted at Ted's Transmission, Alex Biliski at Santilli's Transmission, Dynamic Dino and John Forrester.