I bought this 245 and it was all good. Then had to replace the fuel pump relayer. Then replace thermostat. Then replace fan clutch... All good for a month till I have to fix my clock which it never been worked when I bought. After that, I start have overdrive light come on all the time. Pressed the button, nothing, still light on. I replaced the relayer, still have same problem! Don't think my 245 can drive on 4th gear. (i am deaf) i kept feel change from 1 to 2 to 3 and that its.

Possibly overdrive soldiered?

Oh before that I was mess around with relayer by shake, tapping, the light went off. It could be wiring?

 

If the light is on, then the overdrive is likely turned off. On one of my 240's I had to hold the button down a couple of seconds before it would engage/ disengage.

There is a block- off plate to defeat the overdrive so it's on all the time available online.

Here is a troubleshooting link:
cleanflametrap.com

 

if tapping the relay turned the light off, that suggests the problem is either the relay or the socket/wiring. its also possible its the OD pushbutton wiring in the shifter ****.

the way the OD stuff is wired is a bit wonky. if the light is OFF, then the OD solenoid is ON, and automatic overdrive is ENABLED. if the light is ON, then the OD solenoid is OFF, and automatic overdrive is DISABLED.

 

Just about everyone I know who owns a 240 has had to replace the OD relay at some point. It's unfortunate that when the switch, the relay, or wiring in the system fails, it defaults to OD being off (OD light on) and locking out 4th gear.

I had that issue, pulled the OD relay, removed the plastic cover and it looked fine. Couldn't find any other issues, so I just replaced the relay and the problem went away! This was about 3 years ago, haven't had any issues with the OD system since.

 

The common failure mode for most any relay is cracked solder joints on the little circuit board that holds it all together. you can't see these, but reflowing the solder on them can often fix them. the other failure mode for relays is excess corrosion/erosion on the actual contact points, at which point the relay is usually worn out beyond redemption.

 

as others have mentioned, one could choose to eliminate all the electrical stuff/gremlins associated with the o/d by installing a block off plate...

 

you could, if you don't ever tow anything, or drive in the mountains where you want 3rd gear engine braking on long descents.

 

right you are, on the other hand it's a moot point if the OP isn't towing anything,...and there aren't too many mountains around dallas that i know of!
all i know is driving these cars w/out 4th gear is downright annoying!

 

The very first car I had to sell on the lot was a 1991 240 that had an intermittent overdrive failure. A good source locally suggested that I simply change the relay and sure enough, solved!

 

Actually it was bad wiring! Relayer are fine, OD plate is working. I decide to replace the OD and found the wires gone bad! I left the old OD and cut the wire off, I am having hard time to install new wire inside the shift stick to outside of under car. I even used metal wire to hold wire and it failed. won't tuck into this rubber gasket or mount inside the shift stick wall. anyone got better idea?
thanks for those inputs, i know what to do next time!