I've owned my 85 4+3 for MANY years now and have never had ANY problems at all with my (routinely serviced) DN overdrive. I had it out a week ago and it worked perfectly. No noises, OVERDRIVE ENGAGED indicator light worked as directed also. Today I tried t use the overdrive and it wouldn't engage and the indicator light wouldn't turn on. I checked the relay and wiring to it and it was fine. I even checked the DIC with a paper clip to see if the OVERDRIVE ENGAGED light operated. It did. The engine was at operating temperature, so I know that part was OK too. Could a low amount of overdrive transmission fluid be a consideration at this point. Or should I consider the console mounted on/off switch to maybe be at fault. I really don't think there are any internal transmission parts going bad as it always worked smoothly and perfectly. I have no problem putting the car in reverse gear. Anything I'm missing here?

 

Good question. I've had several C4's, and still have my '87 Convert, owned since 1998, all with the Doug Nash 4+3 and little problems

Here's the problems and path that I usually follow,

If you haven't checked or changed the trans gear oil and OD ATF, then check the OD fluid level, see last test re TRANS Fluids first.

First check the miniature (momentary) OD mechanical Switch and the two Soldered wire connections that is installed at the lower end of the Shifter. You can un-hook and pull off the Leather Shift Cover up and over the Shift ****, but check the procedure first as their easy to tear.

Look down inside the console and follow the wire harness, 2 wires, that lead to the shifter. Look for a broken wire at the switch. I make a thin wire with a hook on the end and Tug on the wires to verify their connected. If they look solid, then you'll need to get a DVM and measure the resistance in the switch when open and closed. The switch must only show a few Ohms resistance when the switch depressed and several Meg Ohms when open. You might need to adjust the long Rod that travels from the OD Button to the Switch, But I've hand or personally heard this adjustment to be an issue. I have replaced my OD switch but it was about 15 years ago. I don't remember, but you might be able to push down on the top of the OD switch wit ha long bent wire, like a TIG rod. If you think that the switch is good, and search for this procedures, I think it's already been posted.

If the Switch function is good then the next step I take is to check the OD Relay. Review the attached diagrams. This relay is on the firewall next to the fuel pump relay, if I remember correctly.

You'll need to find a post on testing this circuit on this forum. But I recommend you Don't test the B terminal, Tan / Black wire with a Volt or Ohm meter ! You can test the OD Lamp and Relay with the ACDL connector, but best to find the post that's already been completed.

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TRANS Fluids,,,,, If the above tests, then It would be time to test the both Fluids in your 4+3 trans, the automatic fluid in the OD is important. There is a Fluid Pressure switch in the OD, find on the attacked diagram. So if you haven't check the fluid recently, it might be best to check prior to the above tests. But change the fluid after you've gotten the OD operation.

Every 4+3 trans I've owned have some seeping out of Speedo senor or pressure switch, so checking the fluids is very important.

It none of the above gets the OD operation, do post and I or someone else will be able to assist you.

good luck,

Michael

 

I personally would check to see if the od unit was getting voltage at the transmission. If not work your way back to switches and relays.

 

Thank you guys for your responses. I found the problem! I removed the old console mounted overdrive toggle switch (84-85 E only) and had it checked out. Contacts were dirty in the switch. It just so happens that I bought an NOS switch years ago "just in case" I needed one some day. They are VERY hard to find. I installed the new switch and everything works perfectly again! I'm a real goofball with this car as I never leave anything non-working. I always immediately fix the things that need fixed. When I ever do decide to sell this car, (not cheaply) someone will get a perfect running and looking 85 4+3 C4.

 

Great New !!