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Project: Diagnosing and repairing overdrive - Volvo 240, AW70 / AW71 transmission

This project contains all of the overdrive components for reference, but you're quite unlikely to need them all. (Note: Make sure the OD solenoid includes the two O-rings. If not, the part numbers are 1239834 and 1239835.)

I've split this into two sections, diagnosis and replacement. Before going crazy and throwing a ton of parts at this, you should figure out which part of the overdrive system is actually faulty - keeping in mind that multiple problems can be present, especially if it hasn't been working for a while.

DIAGNOSIS:

The transmission can shift into overdrive (4th gear) when a solenoid on the side of the transmission is being powered, which permits transmission fluid to flow through it. Power is provided through a relay, which is switched off and on by a button on the shifter.
(Side note: when the up arrow light on the instrument panel is on, OD is off. The relay toggles power between the two when the shifter knob button is pushed.)

(1) First, look at fuse #11. Remove it and make sure there's no corrosion on the contacts that might cause a poor connection. It's probably a good idea to clean the contacts regardless. Disconnect both terminals from the battery. (Negative terminal: disconnect first, reconnect last.)


(2) Under the car, locate the solenoid on the driver's side of the transmission near where the shifter linkage connects (see photo). Examine the entire length of the wire, following it until it disappears into the car. There will be a connector before it goes into the car. Examine the connector, as poor connections aren't uncommon.

If the wiring is damaged, you can splice in a new section of wire. I suggest using adhesive lined heatshrink tubing if you do this, although it's not strictly necessary. You may also need to replace the wiring that leads into the car, which goes into the front of the shifter box. It can be tricky to get a new wire through the hole - I worked from inside the car, and pushed the new wire through using long tweezers (the only thing I had that could reach down far enough).


(3) Test the shifter OD button on the side of the shifter knob. To do this, first remove the glovebox. (Open it, remove the screws around the edge, close it, and pull it out.) The relay is inside and to the left, near the center A/C vents (see photo). Disconnect the relay, and on the plug, test wires 1 and 4 (see photo). There should be continuity when holding down the shifter button, but not otherwise. If necessary, replace the button.


(4) With the car off, the relay connected, and the ignition in position II, push the OD button while holding the OD relay. You should feel it click. You should also hear the solenoid click from under the car. (Note: the relay and solenoid will only click if the ignition is in position II.)

(a) If the relay doesn't click, disconnect it and try powering the solenoid by connecting a spade jumper wire to terminals 3 and 1 on the plug. (Careful not to short from 1 to anything metal.) You should hear the solenoid click. If so, your relay is likely bad.

(b) If the solenoid didn't click when jumpered, disconnect the solenoid connector under the car and use a long wire to touch it directly to the battery positive terminal. If it clicks now, you have a wiring problem - recheck the fusebox as well. If it still doesn't click, and you're sure the solenoid wire is good, then the solenoid is probably faulty.

(c) Test the solenoid resistance with a multimeter. First, test it from the relay plug, terminals 3 and 2. It should read about 13 ohms. If not, then under the car, unplug the solenoid, and test the resistance from the solenoid end of that plug to the body of the solenoid - make sure the probe gets through any crustiness on the solenoid, or resistance will be artificially high. If it reads about 13 ohms now but not before, you have a wiring problem. If the readings are still wrong, you have a solenoid problem.

(d) If the previous step indicated a wiring problem, test resistance from terminal 2 to a good ground. Since 2 is supposed to be a ground, you should have minimal resistance. If not, locate the grounding point and fix it - I *think* it's on the metal frame behind the center console.


(5) To absolutely diagnose the solenoid as good or bad, it must be removed and tested. Removal instructions are in below, in the replacement section. To power the solenoid, connect the wire to 12v positive (I used the car battery) and the negative around one of the bolt holes.

Test the solenoid with and without power. While unpowered, no air should flow through. While powered, air should flow. If the solenoid clicks, but air does not flow through, the solenoid is either faulty or clogged. If it's clogged, you might be able to clear it with compressed air while powered, but otherwise it must be replaced.


REPLACEMENT:

Relay:
Open the glovebox, remove the screws around the edge, then close it and pull it out. The OD relay is on the left, near the center vents (see photo). Simply disconnect it and put in the new one, tuck it back where you found it, and replace the glovebox.


Button:
Simply work a small screwdriver or similar object around the edge and gently pull it out. Once out, disconnect the wires and put them on the new switch. You'll probably need needle nose pliers to do this. Push it back into place. Make sure you didn't rotate it, or it won't fit right.


Solenoid:
First, a warning. The solenoid covers two holes that lead into the transmission. Transmissions do not like dirt. The area around the solenoid is filthy, and bits of sand will be wedged around it. Therefore, it is absolutely essential that you clean the area thoroughly. Compressed air, brake cleaner, a toothbrush, bent paperclips, a wire brush, and a small mirror are some of the tools I used to clean this area. Make sure you wear eye protection while doing this.

Once you are SURE that you have the area clean (make sure no debris can fall from above, either), you can remove the solenoid. You may need to disconnect the shifter linkage bar to make room. The solenoid is held in place by two 12mm bolts. Due to the cramped space, you may have trouble fitting your wrench in there, depending on its shape and size. Before you actually remove either bolt, make sure your tool will give you the space to remove BOTH bolts - it'd be annoying to get one bolt out, but not the other.

Transmission fluid may leak out once the solenoid is off - I'm not certain, because my transmission was drained when I did this. Be ready with rags and eye protection, just in case.

With the solenoid removed, use a mirror to make sure the mounting area is clean. Make sure the two old O-rings came off also. Use a lint-free cloth as necessary, taking caution not to get dirt into there. Now install the new solenoid.

Make sure that both O-rings stay in place on the solenoid, and make sure you orient the solenoid correctly. It can 'sort of' fit on backwards, which is a bad thing. If you use a mirror to look at it from above, it will be obvious.

Tighten the bolts to 9 ft/lbs. A torque wrench isn't really necessary (not that you're likely to fit one in here). Just get them snug, without gorilla arming it.

Route the wire appropriately and plug it in. You'll want to make sure it has something on it to guard it from chafing. I slit a vinyl hose and put some cable ties on it. Make sure the wire cannot be pulled too close to the drive shaft - mine was destroyed after getting caught on it, which led me to this repair.

Item   Price (ea) Core (ea) Qty
Project Total
Item Subtotal: $0.00
Core Charges: $0.00
Grand Total: $0.00
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