The Overdrive was not engaing sometimes. The button and wiring were in bad shape so I replaced those.

Still not working... The wiring and plug that connect to the solenoid was bad so I replaced those also.

Next step was to replace the Relay. Got a new one and still not working!

Now I have tested the wires: The Relay has 12V going in, Ground, and 12V everytime I hit the OD button,

but the other 2 wires (power to the solenoid and dashboard) don't have power when I hit the OD button....

If I tap 12V into that wire the solenoid will engage, so I know that the solenoid is working.

What am I missing here?

 

The strange thing, I got a hold of a used relay - 1347768, not the same as the yellow one, it has 1 less connector and it will engage the OD in all gears including reverse. It wont disengage when put in neutral and instead of lighting up the "5" light on the dashboard a different light ARROW lights up.

 

what year/model car is this?

 

It's hard to believe that the new relay is also defective.

With the new relay in, there is no output to the solenoid (yellow wire) when I press the button.

If I tap 12v + wire into the yellow wire I get the solenoid click and dashboard "5" light.

 

It's a '89 740 GLE

 

Here is a pic of the wiring:

Nether OLD or NEW relay sends 12v + when the button is pressed.

Also the NEW Relay is sending negative current constantly to the blue wire ... when it should only be recieving - signal from the 4th gear switch.

So this is confusing because I'm getting different results from both relays.

 

you can't just randomly swap relays, they all have different innards

I believe the yellow one is the correct relay, I have no idea what that red relay is for. note too, automatic vs manual OD relays are completely different, and circa 1984 or so, they all changed.

unluckily, i do NOT have the wiring diagrams for 740's, other than 1991-1992 (which are the same as 940's of those years). This is the diagram for a 1991

its not real easy to read the fine print, sorry.

• relay pin 1(15) is power from fuse 12, also connected to the 'hot' side of the OD button on the shifter.
• relay pin 2(31) is ground
• relay pin 3(87) is the power output to the OD solenoid.
• relay pin 4(86) is the OD button (hot when button is pushed)
• relay pin 8(85) is connected to the gearbox switch, grounded if you're in 4th, this prevents the OD from engaging in any gear other than 4th.
• relay pin 9(87b) is connected to the same signal as pin 3 and powers the OD light in the dashboard.

 

That's what I checked .
It has power (15) and ground (31).
The button also has power when pushed (86).
I'm just not getting any output to the solenoid - (nether the old relay or the new one).

 

The update to this situation.

As you can see in these pics, the new relay I bought has a more modern electronics. The Relay's have the same number but for some unknown reason it doesn't work.

 

I seen some videos of people taking about re-soldering relays and that was something that happens frequently with these older relays.

To the naked eye everything seemed OK but you need a magnify glass or in this case a zoom in with the cell phone to see cracks!

Several cracks were found and after checking continuity with a tester there were more bad connections. I ended up soldering almost all of the connections.

And voilà, the relay is working again!

 

But, the problem with the overdrive continued...

After checking all the wiring again I came to the conclusion that it could only be the 4th gear sensor that was faulty.

I had to remove the gearbox mount to lower the gearbox so I could fit a socket on the sensor, located on the top side.

Removed it and yes, it was also not working...

Ordered a new one from the dealer (1324496), installed and now everything is working properly.

 

just a note, 'proparts sweden' is a junk reboxer of generic often low grade chinese parts. not quite as bad as "Uro" but close.