The Volvo Parts, Accessories &
Performance Specialists Since 1963

Get Into Your Overdrive

2019-01-17 - Lee Holman, aka Volvogirl

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Overdrive Fundamentals

The overdrive unit on the M46 manual transmission is surely one of the most maligned and misunderstood parts on the Volvo 200 series. It's found in both the 240 and 260 from 1975 through 1986. The M46 is the younger sibling of both the venerable M41 found in the 1800 and in some 140s and Amazons, and the more robust M410 found in the 164 and 1800 EIES. The J-type overdrive is considered stronger and more able to withstand higher torque than the older D-type.

There's something about the Laycock de Normanville mystique that I just love. It's such a French sounding name from such a distinctly British com-pany. The invention of Edgar de Normanville, it was originally called the de Normanville epicyclic gearbox. It contains a sun wheel with a sliding cone clutch and a planetary gear set that run in an internally toothed annulus with a uni-directional clutch that drives the output, allowing for 0.8:1 gear reduction when the overdrive is engaged. In overdrive, the planetary gears are driven by the stopped sun gear on the transmission output shaft, which drives the annulus gear faster than the output shaft of the transmission. When out of overdrive, the power comes by way of the transmission output shaft through the one-way clutch internal to the annulus gear (output shaft) of the planetary gear set.

Often thought to be troublesome and unreliable by its detractors, I have found the M46 to be durable and really quite satisfying when its basic hydraulic and electrical needs are met. In fact, quite apart from improved fuel economy and allowing the car to sustain high speeds with lower engine RPM, hitting that switch and feel-ing my car slip into overdrive is one of my favorite driving delights.

Volvo was not the only company to use the Laycock overdrive units. Jaguar, Austin Healy, Alpine, Hillman, Triumph, MG and others used them, as well as the French Facet-Vega HI, which used the B 181M41 combo. Volvo does have the distinction of being the only non-British marque to have used them in such great numbers.

Of course, a number of things go wrong with the J-type overdrive units. In my experience, the most com-mon problems are fairly easy to diagnose and repair. Excluding major mechanical damage and worn internal parts the usual suspects are electrical faults and slipping, or failing to engage or remain in overdrive due to hydraulic problems.

If your overdrive unit isn't working at all, with no OD light in the instrument cluster, the chances are good that you have an electrical problem. Older 240s have a sliding switch in the shift knob to actuate the overdrive while the newer ones have a momentary contact switch that triggers a relay. This is the first place I look. I have seen cases where the wires that go to either kind of switch have become disconnected, and a little fussy work with needle nose pliers will allow the overdrive to operate. You can check the switch for continuity. Switch off -- open circuit = no continuity; Switch on -- continuity = —0 ohms.

Less common, but possible, is that the switch itself is gummed up. Disassemble carefully and use electrical contact cleaner with caution -- This stuff is generally super toxic. If your car is 1981 or newer, listen for the soft click of the relay, which is behind the vent directly to the left of the glove box (left hand drive).

Don't forget that the circuit is fused. It's an awful thing to spend time fault tracing only to end up finding a blown or corroded fuse. In my 1980 GT, the OD shares a fuse (#11) with the rear window de-mister, for what it's worth. If the fuse has blown from a circuit overload, look for damaged wiring in and around the shift lever shorting to ground. The harness was upgraded in 1978, and later cars may be less prone to this fault. Nonetheless, after 10 or 15 years, moving wires can break.

If the OD relay isn't doing its thing, the fault could be in the relay (P/N 1259750-6) but before you rush off to spend money on a new relay, be sure the wiring that goes up to and ~ft lever is intact. The wires that travel from the shifter and travel under the carpet are often suspect, as are many wires that have had to flex repeatedly. It's also worth noting that OD relays have sometimes been found to have aging cold solder joints that allow for intermittent operation. Replacement is one solution, but if you're clever with a soldering iron, you may be able to repair this kind of fault yourself.

Assuming the wiring is in order and the relay is clicking happily when you press the switch, you should have a light in the instrument cluster telling you that the OD is on. With earlier models which have the sliding

switch you won't hear a relay because there isn't one. In these models, when the wiring is intact, the light should be lit if the key is in the run position regardless of what gear you are in. This doesn't mean that the OD is engaged in all gears. There is a 4th gear switch to insure operation only in 4th gear. This is an essential safety feature for the OD.

Operation when reverse gear is engaged is said to cause fatal damage to the OD unit internals. Cars 1981-1986 have the relay, and will switch out of overdrive whenever you shift out of 4th gear and will not engage OD again until the switch is depressed. Some folks, who appreciate this convenience, have upgraded their ear-lier cars to include an OD relay.1 The older type will engage the OD immediately when you shift into 4th gear if the switch is left on.

If the wiring is okay, shift the car into 4th gear, ignition on, and operate the OD switch while listening care-fully to the underside of the car for the distinct click of the OD solenoid engaging. If it doesn't seem to be operating, check the wiring at the solenoid. These terminals should be clean and snug.

The yellow wire to the solenoid should have 12 Volts and the ground connection must be solid. If there's still no click, you can check the 4th gear switch on the right hand side of the transmission top cover. Note that there is another, very similar switch on the left side. That's the back-up light switch. You may be able to reach up to check this switch, but you may not have much access. If you need more room, this can be accomplished by supporting the transmission with a jack and removing the crossmember to lower the transmission -- only just enough. Don't forget to detach the exhaust bracket before lowering the tranny.

If your overdrive seems to want to work, but is slow to engage, shuddering and slipping, or dropping out of overdrive when the transmission is warm, then it's likely you have a hydraulic problem. The first thing to check in this department is for the proper amount of the correct lubricant. In the case of the M46, type F (or G, no longer available) ATF is specified. The M41 approved fluid is 80/90 weight gear oil. In either case, synthet-ic lubricants are recommended particularly in hot climates and for heavy use. The use of anti friction additives is specifically prohibited according to the Laycock manual. The oil used must provide both lubrication and shear qualities that allow the internal friction clutches to operate effectively. I know many people are pleased with the use of high quality synthetics, such as Redline MTh, claiming it has improved their overdrive opera-tion. I'm not sure I buy this version of better living through chemistry, but certainly clean lubricant of the cor-rect viscosity is critical to proper function.

Get in a Little Deeper...

Because the overdrive shares the transmission fluid, in order to drain the overdrive, you will need to drain the transmission, as well as the overdrive sump. Be certain the area is uncontaminated before removing the transmisson drain plug and overdrive sump. Any dirt, lint, or dust that gets into the unit can cause a malfunc-tion. When you drop the sump, watch for the suction filter, which can drop right out when the pan is removed. Keep everything scrupulously clean as you work, but on no account use water to clean any parts. According to Laycock, "...this will also affect operation of the overdrive." They specify the use of petrol or paraffin (kerosene).

Once the pan is off, and the oil has ceased dripping, look up and see the access plugs for the pressure relief valve, the pump, and the pressure filter. Each threaded plug has two holes in it, and can be removed by turning counter clockwise with the proper tool. That's the catch, of course, anything like this requires a lot of special tools - or maybe not. In fact, a few special tools can be mighty helpful, but I have managed without the 17 listed for overdrive service in the manual. You can easily make a pin spanner for this using a flat steel bar and a couple of roll pins of the correct diameter. Drill holes that are the same distance apart as the holes in the plug -same for each plug - and push the roll pins in to the desired depth. Be sure to have the handle long enough to give you enough torque

At any rate, the Pressure (dashpot) Relief Valve (PRV) is next to the solenoid, the pump is in the center, and the pressure filter is the big one on the other side. You will want to take the filter out for cleaning, even if you are just changing fluid, and to be certain there are no telltale metallic particles in its folds.

I prefer the use of a syringe type pump to add fluid to the transmission, as it can be an awkward and messy process without the right tools. A funnel with a very long hose could work, too, but it might take two to make it work well. Figure on 2.4 US quarts (2.3 liters) of Type F Automatic Transmission Fluid for an M46 and 1.7 US quarts (1.6 liters) 30W for an M41.

They say a change will do you good, but if fresh fluid doesn't restore your overdrive, you may want to test the operating pressure before removing the unit for overhaul. There is a test port by the solenoid. Instructions and proper operating pressures are found in better shop manuals. It involves installing a pressure gauge with a long hose and driving the car while reading the operating pressures. Trying it with the car on jackstands is dangerous! It's best to have a helper to read the gauge, if possible. Otherwise a dynamometer or rolling road is recommended. You can make your own gauge.1 Be sure that you use fittings that work on your unit, and a long enough hose to reach inside the car comfortably while driving.

Speaking of manuals, here's a quick review of the popular manuals for the 240 Volvo regarding their treat-ment of the Laycock overdrive. There is no real substitute for the green Volvo Service Manual, section 4, group 43, "Overdrive, Repairs and Maintenance" available by calling 1-800-25-VOLVO (that is, unless you can find a copy of the none-too-common Laycock manual). The Bentley Volvo 240 manual has good informa-tion for in car service procedures. Haynes 240 manual devotes more than 6 full pages to the Laycock over-drive unit - perhaps not surprising from a British publication. It has good graphics and explanations but lacks such detail as operating pressures. They also show how to make some tools, rather than just list the Volvo part numbers. Forget Chiltons, they barely mention the Laycock overdrive in their drivetrain chapter.

If operating pressures are out of spec, you may find the fault lies with a sticking solenoid, or it may be that the unit simply requires new seals. A solenoid stuck closed is a problem, but one stuck open is a cause for immediate concern, as this will keep the overdrive engaged in all gears - including deadly reverse! (If you're interested in checking the operating pressures, you'll need a gauge that reads from 0-600 psi and the appropriate fittings.)

Another tidbit for those who want to go about resealing their OD is that Volvo has superseded the old pis-tons with a larger piston That does a better job. At around $25 a piece, I have been told they are well worth it. This is certainly true if the old ones show any sign of wear.

If you are going to remove the overdrive, it is highly recommended to drive the car and shift it into over-drive and then, with the clutch pedal depressed, disengage the overdrive. This will relieve any torque that remains, and should allow the unit to slide off the transmission output shaft easily. You can also remove the driveshaft, and accomplish the same thing. I have seen a few cases where the overdrive unit was all but impos-sible to remove from the transmission because of internal pressures that couldn't be released. This is much worse if the whole transmission is out of the car. It can still be a struggle to remove, in the car, if this detail is omitted. More than one promising junkyard unit has been left behind for this reason. I've heard that the unit can be disassembled in place in this case, but have never had the pleasure, myself.

Once removed, clean the overdrive it well externally and plan to perform the operation in a clean place, expecting a certain amount of oil leakage. For this reason, the kitchen table may not be the ideal location.

Take the case apart slowly, loosening the fasteners evenly, to keep equal spring pressure on the clutch return springs. Be careful about all the little parts, lay them out and cover them so that they can neither become dirty, nor get jumbled about. Don't just pop them into a jar, and hope to sort through them later. Proceed methodically, and simply remove one 0-ring at a time, replacing it with the new.

Lube each part and 0-ring with a bit of ATF, as you go.

Examine the other parts. Assuming that you find no internal damage, such as worn clutch facings or dam-aged or worn gears, you should be able to put your overdrive back into service for many more miles. Once more, you can enjoy the way the car surges forward, as though freed of a burden, when you touch the switch. You can have improved gas mileage, and a more comfortable ride at lower RPM. Best of all, you can proudly say that you fixed it yourself!