1987 740 Turbo M46 trans type p overdrive repair advice

Salomons Driveway · Post by **Salomons Driveway** » 06 Jun 2015, 04:16

I am working on a 1987 740 with manual transmission. Recently the electric overdrive stopped working, so I replaced the wiring harness to between the relay box and transmission. The overdrive worked for two months and then stopped working again. Now when I push the overdrive switch in fourth gear, the light comes on, solenoid receives power and according to my mechanic, moves as it should. However the overdrive still does not engage.
Anyone had success fixing the problem with Ipd's seal and gasket kit?

Post by **jimmy57** » 06 Jun 2015, 09:47

The overdrive uses a lined two-sided cone clutch that gets pulled against an angled brake surface to stop the clutch to accomplish the shift to overdrive. The inside lining is pushed against the beveled surface on output shaft to give you reverse power flow. Forward power flow is through a one way clutch. The cone clutch has a bearing in a 4 bolt plate. The bolts are long and pass through the center of the OD housing. The bolts each have a spring that pushes the clutch against the output shaft. The housing has two bores with pistons on the opposite side from the cone clutch. two steel bars bridge between to of the bolts and are over the pistons. When oil pressure is raised under the pistons they push out of the bores and lift the bridges which in turn lifts the cone clutch off the output shaft and pulls the other friction lining into brake ring and stops a gear that makes the overdrive planetary gear set function. Your piston seals are likely worn out so the oil is leaking and not lifting the pistons.
You could have worn piston bores also. There is an oil pump but I have not seen that wear out unless the whole trans is full of metal filings. There is an oil pressure relief valve under the plug on bottom that can break its spring. The pistons get a low oil pressure when OD is off and then when the solenoid stops an oil flow the relief valve goes to a higher pressure for the pistons to lift the clutch to applied position. The oil pressure is controlled or the pistons woud bend the bridge bars.
The unit is easy to disassemble. Removal is not hard but one "trick" should be observed that can make it easier to slide the unit off trans shaft: kill engine in reverse by letting out on clutch before you lift it to remove OD unit. Your year model P type may not have the two piece spline set that can bind on shaft but it can't hurt to do the reverse kill trick in case it does have the two piece unit.

Salomons Driveway · Post by **Salomons Driveway** » 06 Jun 2015, 11:31

I removed the overdrive and replaced the piston seals with P-type seals from IPD as well as the relief valve spring and seals for the three round covers inside but the overdrive still does not work. I am going to try replacing the pump seals, but wonder if that will really make a difference.
How can I tell if the piston bores are worn out?

Post by **jimmy57** » 06 Jun 2015, 13:30

It is obvious. Groove in bore where o rings have been riding.

Check the solenoid. Blow through hole in end, open not powered sealed when powered.

Salomons Driveway · Post by **Salomons Driveway** » 06 Jun 2015, 21:52

I removed the solenoid and replaced the two o-rings on it. tested using a 9-volt battery and plunger moves foreward as it should. It took a while to grind down a 25mm open end wrench until it was thin enough to remove the solenoid. I reinstalled the solenoid, but the overdrive still does not work. I am going to change the pump gasket and try a pressure check next. If that does not work, I guess it is time to check the clutch itself.

Salomons Driveway · Post by **Salomons Driveway** » 22 Jul 2015, 07:50

I took the overdrive apart a second time and tried to run the pump by hand with trans fluid in the pan on the overdrive. The fluid does not build up enough pressure to go out the release valve. instead it leaks out the top of the pump cylinder. Are there any seals in the pump, or do i need to have a sleeve put in the cylinder to allow it to pressurize the fluid?

Salomons Driveway · Post by **Salomons Driveway** » 29 Jul 2015, 06:15

I finally bought and installed a used overdrive, and overdrive now works using the same old solenoid. Unfortunately, the overdrive stops working after the first 10 miles of use. I guess it is worn out too.

93Regina · Post by **93Regina** » 29 Jul 2015, 11:15

Salomons Driveway wrote:overdrive stops working

Viscosity Index....one needs to find an oil with the highest viscosity index.

Compare viscosity index to what you have to this product: Super Shift® Racing Transmission Fluid SAE 10W

Viscosity Index (ASTM D2270)......................140

Get the highest you can find