740 Manual Transmission Not Kicking Into Overdrvie

cslayman · Post by **cslayman** » 03 Jan 2012, 02:30

Hi,

My 1988 740 Turbo manual transmission is not shifting into overdrive. I checked the fuse and it's OK. Is there a simple way to determine if the problem is the 5th gear microswitch on the gear knob or the solenoid on the transmission? If I short the switch, should that kick in the overdrive unit? Is there a troubleshooting procedure in the Forum?

Thanks,
Charlie

Post by **billofdurham** » 04 Jan 2012, 15:30

Have you checked the relay?

Manual transmission relays don't often fail, unlike those on the automatics. To diagnose a suspected relay problem, watch the dash light. If it illuminates when you push the button on the gear shift, the relay is OK and the overdrive not engaging is caused by something else. If the dash light does not come on you have either a blown fuse or a 4th gear switch that is not working. Plug your relay in and with the engine OFF, ignition ON, gear shift in 4th, try to engage the overdrive. If the dash light comes on, does the solenoid by the overdrive unit click at the same time? If it clicks and still no overdrive, you need new actuating piston seals to get it working again. If the dash light does not come on, pull the gear shift further to the rear and right and, holding it there, try the button again. If the dash light comes on then goes out when you move the gear shift, the 4th gear switch is bad.

N.B. Manual transmission relays (blue) and automatic relays (white) are two different devices and are not interchangeable.

Bill.

cslayman · Post by **cslayman** » 10 Jan 2012, 19:39

Hi Bill,

Thanks for the reply. Fuses are OK and the dash light is not coming on. I even short circuited the shift button in 4th and nothing happens. So it sounds like it might be the relay. Where is it located? Do you have an online troubleshooting manual you can point me to?

Thanks,
Charlie

Post by **billofdurham** » 12 Jan 2012, 12:33

The relay is in the relay/fuse board behind the ashtray in the front of the car. To access the relays remove the ashtray and fuse panel cover (push the plastic snap at the bottom). Then prise off the snap-on cover over the cigarette lighter with your fingernail or a small screwdriver. Remove the two Philips screws securing the storage box. One of these is at an angle. Remove the storage box by gently tugging on the left side, pulling it and the cigar lighter wire out from the console. You can release the fuse/relay board by pressing two catches and pull it into the car to make work easier. The overdrive relay is at position 'I' on the attached :

740 '88 Relay Board.pdf: (48.56 KiB) Downloaded 230 times

The usual test for a relay is to replace it with a known good one. However, the overdrive relay is the only one of that type fitted so you can't do this. You can use a small jumper wire to replace the relay for testing purposes only.

With the ignition key in the "off" position take a small piece of wire (14 to 20 gauge) and strip it on either end. Insert each end of the wire into the 87 and 30 relay block terminals and turn the ignition key to the "on" position. The relay is now bypassed so the overdrive should now work. If it does you need a new relay. If it doesn't you need to investigate further. Do not drive the car with the jumper wire in place.

Bill.

cslayman · Post by **cslayman** » 12 Jan 2012, 22:22

Thanks! I'll try it over the weekend.

cslayman · Post by **cslayman** » 16 Jan 2012, 20:15

Hi Bill,

Well, the 5th gear OD fixed itself before I could do any troubleshooting. Darn! But you've given me the tools to dive into it if the problem returns.

Thanks,
Charlie

Post by **billofdurham** » 18 Jan 2012, 12:18

You've got to love cars that repair themselves!

Thank you for the update.

Bill.