I was wondering if any one out there has the wiring diagram for the Over drive relay of a 1988 740T. I have the diagram for 1990 which shows wires to and from the 5 connectors of the relay (15,86,87,87a,31). When I look at the connections on my 1988 740T connection 87a is not used. Also 1990 diagram shows 12 volt at connection 86 and 31. I only got 12 V at 15. So I know that I cannot use the 1990 diagram for my 1988 Vovo.

 

What was your original problem?Those over drive relays are known to go bad.Whe they go bad the arrow on the dash stays on.

 

The car is 1988 740T with Automatic transmission. The problem is at random, the overdrive kicks in. Say I was driving at highway speed between 100-120 KM/hr. If I remember correctly, that should be around 2500 rpm ( 4th gear , may be ). When OD kicks in , the rpm goes up to 3500 , the car slows down by the engine and I need to pump more gas to maintain the speed to avoid rear end collision. The feel is no good especially when you have some one follwing too close. I normally will try to reset (turn off) the OD by pressing the OD switch a number of times. This normally does not work. However the OD will stay on for a while (at its own will) and drops by itself
When it is normal, I can use the overdrive switch to turn OD on and off with no problem. So I think both the switch and the OD solenoid is working fine.

I called the dealer, the new relay cost $80 + TAX and there is no exchange or refund. So I want to be sure that it is the relay and not some other external connection problem before I spend the money. I believe my 1988 740T had more than average electrical problems.

 

What you are explaining is the typical bad overdrive relay.If you notice that when this happens the arrow on the dash will come on when it kicks down into 3rd gear.You might be able to buy the relay aftermarket.Might be half the cost.Try to locate an Import Auto parts store and check with them.Alot of the relays are also used by Volkswagon.Hope this helps a little.