I have been trying to de-bubbify the wiring in my 85 TA. Came across something in the choke heater circuit that I am struggling to understand:

History: Car was originally a VIN G L69 engine. Been previously swapped to a 350 with a Demon Carb

Symptoms: The choke light on the dash never turns off. That is what started this process.

What I have diagnosed:

1. The electric choke on the Demon carb is connected to the choke heater wire from the firewall (light blue). This wire is powered by the gauge fuse.
2. The brown wire from the bulkhead to the alternator had been cut and was no longer connected to the alternator.
3. There was a generic horn relay installed in place of the correct choke heater relay.
4. The exciter wire from the ignition switch to the bulkhead connector had insulation broken off of it in chunks and had been cut.
5. There is a diode connected in series from the bulkhead connector to the choke heater relay terminal. (this is where I'm confused - see pic)

Diode wired in series between bulkhead connector and choke heater relay. Current cannot flow from alternator back to the relay.


What I have done:

1. I lengthened and reconnected the brown wire from the bulkhead to the alternator.
2. Removed the horn relay - do not have a spare choke relay, so trying to figure what to do here...
3. Spliced and soldered the exciter wire back together, cutting out the sections with the missing insulation

Question:
Most of the wiring on the car has been easy to identify as being screwed with in the past. The socket that appears to incorporate the diode are not typical of the twist-and-electrical-tape connections found elsewhere on the car. My understanding of the choke heater circuit is that current flows back from the alternator when it is running and activates the relay, turning off the choke indicator lamp. The way that it is wired with the diode would prevent the circuit from functioning properly?

My thoughts are to bypass the diode, then insert a jumper between the bulkhead brown wire and the light blue wire that connects to the electric choke on the carb and the indicator lamp. When the alternator is generating power, it will send current to the choke, powering it, and also to the lamp, turning it off due to lack of a voltage drop.

Does that make sense?

From my factory service manual:

From my 1985 FSM

 

Had a thought early this morning, and flipped the diode that is in that connector (actually, diode stayed in the same place, I swapped the terminals). That allowed current to flow from the alternator back to the relay circuit.

Next, I used a paper clip as a temporary jumper wire between the two terminals.

Temporary jumper between the lamp/choke terminal on the right and the alternator feed on the left.

When I tested, the circuit performs as intended. With key on - engine off, the choke lamp is lit. Once vehicle is running, lamp goes out. Electric choke is receiving battery voltage.

 

Congrats good man! This circuit perplexed me at first until I understood its complete operation, definitely wired different than most normal relays.

 

Brn/wht is the resistance wire.

The diode is there to allow current to flow from the ign sw through the resistor wire to the alternator, which makes the resistor wire, in effect, the alt exciter wire. It should be turned with the banded end (cathode) toward the brown wire. Not sure why it's not shown on the schematic; however, with the hard splice connection shown on the diagram at F8, it would be possible for the alt to feed as much as 1.5 amps or so back into a short on the orange wire circuit (wire 306? on the drawing) should that ever occur. Maybe something REAL BAD happened mid-model-year and they made that change, would be my best guess. The diode only allows current to flow one way - from the ign sw, toward the alt - and not the other. That will allow the alt to work properly at all times, which it might be doing now, but also might stop doing at any time if the rotor loses its permanent magnetism.

The alt grounds the brown wire when it is NOT working, and puts 12V on it when it is. The choke relay operates whenever the ign is on and the C/H fuse is good, and the alt is NOT working (the condition under which the brown wire is grounded inside the alt). This de-powers the choke heater itself. The Choke light is in series with the heater, such that when the heater is NOT powered, its very low resistance looks pretty much the same as a short to ground as far as the bulb is concerned, so it lights. When the alt is working, there is 12V on both sides of the relay, so it releases; power is fed from the C/H fuse, through the normally closed contact, to the light blue wire and onward to the choke heater. Since at that point there is then 12V on both sides of the Choke light bulb, it goes out.

You could use just about ANY SPDT (5-terminal) automotive relay for the choke relay. All you would need to do is wire it up just like the drawing shows, with the C contact of the relay and one side of the coil connected to the terminal connected to wire 250, the other side of the coil connected to the terminal with wire 25, and the NC contact connected to the 3rd terminal. Regular male slide terminals would plug right into the "convenience center" without having to cut splice or otherwise sodomize anything. 85 & 86 are the coil; 87 is the C terminal; 87a is the NC, and 30 is the NO. No idea why they picked those #s, butt hay, that's how they're all numbered. No doubt you can tie-wrap up the relay to something nearby.

So, put the diode back as it was, and make up a relay as described, to make it back like factory. This would restore the original proper operation of the circuit without the chance of unexpected weirdness when a fuse blows but power is still available through the resistor wire.

 

Thank you for the detailed explanation! That certainly answers a lot of questions, and I will definitely try out the relay idea, have a lot of 5-prongs in the shop.

Here is a puzzle... The wire leading from the diode to the bulkhead F8 is brown/white, same as the resistance wire. However, the other side if the connector is brown and connects to the relay terminal B, just like in the schematic. I have a separate brown/white wire that is the 10 ohm exciter wire. It was broken in one spot and some insulation missing, so I cut and spliced it back together. When probed, it tests 10 ohms between ignition switch and F8.

Sitting beside the car now. Going to stick my head back under the dash and see if I can uncover other secrets!

 

Completed this little project this morning. Got sidelined with a door bushing / rust repair.

I ended up using a standard 5-prong relay as described above. Based in my past use of relays, my wiring is a bit different than sofakingdom detailed, but achieves the desired result. Here are pics with the wiring description:


Used factory colored wires from some spare harnesses that i have. The A terminal is split to connect to one side of the coil (86) and common (30). The B terminal (single brown wire) connects to the other side of the coil (85). Lastly, the C terminal (blue wire) connects to the Normally Open terminal (87a).

When the coil is energized (key on, engine off), the choke heater power (A) is NOT connected to the choke heater (C). With the engine running, current flows up from the alternator (B) and the coil is no longer energized (no voltage drop between A and B). This opens the relay contacts and makes the connection between A and C thru terminals 30 and 87a.


Male spade connectors fit nicely into the original relay socket. Looking at this pic though, I think I will go back and put some heat shrink around the female terminals that connect to the relay. Preventative measure to keep anything from grounding out.


Tucks nicely in above the bracket and zip tied in place.

So this is my current solution. Was a fun mental exercise to figure out the relay,l. My intention is to fuel-inject the car in the future, so I am looking for a temporary solution. Hard to swallow paying $20 for a relay, and I have a bunch of these common ones in my goodie bins.