Igniter Plug (3 Viewers)

[numby]

Farting around with my ignition system and I've decided to make up a wiring harness that will allow me to run different distributors on my '78. I've had vacuum retard (never again), HEIs (recently kaput), and just because I stumbled on a cheap-ish new 61080 Non-USA points model I'd like to run that for a while. I've never had the original electronic ignition system on this vehicle. Somewhere I have a box full of coils, igniters, and year specific distributors but it's tough to tell where one begins and the others end due to the expanding rust. Kind of like rust Ebola. Or rabies. Mad dog. I don't think I ever had the correct resister wiring harness. I doubt anything works anyway.

So... starting with what I've got there is a factory wiring harness on the passenger fender terminating in the green plug with two Black/Yellow wires. One appears to be 12awg and the other 16 or so. (There is a third smaller 18awg B/Y wire for the igniter capacitor with its own connector.) Both are 12v from the ignition and the '78 wiring diagrams show them to be split off the same wire. Question is: which of the two - larger or smaller - went to the ignitor and which one originally went to the coil?

Here's a section of the wiring schematic in question from Coolerman's site (Thanks Mark!)...

And here's a little drawing (also from Mark's site) showing the two-pronged B/Y plug at the very top labelled "MAle Plug"...

Thanks all - numby

 

[numby]

Plug number 4 in this pic...

 

[ToyotaMatt]

u need a the connector plug ?

 

[Curt]

Larger diameter wire goes to coil and should supply 9.6 volts with key in on position. Smaller wire goes to the igniter.

 

[numby]

u need a the connector plug ?

I don't need the connector on the main harness (No. 4 in the photo) but I could use the corresponding one from an igniter/coil harness that plugs into it. If you have one PM me. Or the harness...

Larger diameter wire goes to coil and should supply 9.6 volts with key in on position. Smaller wire goes to the igniter.

Thanks for that. I get 12.5v at both wires and at the smaller capacitor wire.

Is it possible that the larger wire from that No. 4 plug to the coil should be a resistance wire to knock the voltage back to 9.6?

 

[Curt]

78 was the first year of the resistor wire, shown in the schematic, my Feb 78 has one but others may not.

 

[numby]

Hmmm. 2/78 build date here as well (fine spline pinions on both ends). I see the resistor indicated on the wiring diagram shown as being upstream of the coil and condenser but the multimeter says 12 volts straight through. I haven't peeled back the wrap on my harness in years so it's easily possible that a previous owner removed the resistor portion. The gift that keeps on giving

 

[ToyotaMatt]

I don't need the connector on the main harness (No. 4 in the photo) but I could use the corresponding one from an igniter/coil harness that plugs into it. If you have one PM me. Or the harness...



Thanks for that. I get 12.5v at both wires and at the smaller capacitor wire.

Is it possible that the larger wire from that No. 4 plug to the coil should be a resistance wire to knock the voltage back to 9.6?

sent u a PM

 

[Curt]

Interesting. . .Well it gives you more options having 12.5 volts there. I remember there being a lack of info on when exactly it was added, the resistor is about 4' in length and follows the firewall.

 

[numby]

Images for Matt


It's the lower of the two plugs I need (male). Same as the Outer Vent Control Valve on a charcoal canister.

Thx

 

[ToyotaMatt]

 

[numby]

Interesting. . .Well it gives you more options having 12.5 volts there. I remember there being a lack of info on when exactly it was added, the resistor is about 4' in length and follows the firewall.

That would mean it's under the dash? There's nothing in my engine compartment that follows the firewall. I peeled back a bit of the insulation from the ignitor/ coil connector for a foot or so until I got down to where the PO had eliminated the fusible link (I repaired that long ago).

There were several loops of excess wire running to the headlights/horn/windshield washer which look to have been doubled up and buried to avoid re-crimping by whatever Toyota employee had assembled/wrapped the harness initially but no evidence of any splicing. Feeling along the harness all the way back to where it enters the firewall below the VR everything is quite smooth. Not conclusive evidence either way regarding a resistor wire having been fitted and removed or not.

I'm picking up a connector or two from RAGINGMATT to make up the new coil harness and will initially run a coil with a primary resistance of 2.9Ω which is within factory spec for an internally resisted coil (2.6-4.0Ω). If points life is short I'll add in a ballast resistor...if I can find one that keeps me below that 4.0Ω max. Likely it will be easier to buy a different coil with lower resistance.

 

[ToyotaMatt]

hey numby ,

your connector order will be @ the post office first thing , Thanks ...........

 

[Curt]

I was curious so i hacked into mine.

 

[numby]

Dang! So the grey wire spliced to the heavy B/Y wire is the start of the resistor wire...

I just went back and re-examined the loom at the point in your photo.

Sure enough. There's a lump right between the two connectors that I missed the first time. I'll try and get time to cut it open in the next day or so and post back.

 

[ToyotaMatt]

I was curious so i hacked into mine.

Dang! So the grey wire spliced to the heavy B/Y wire is the start of the resistor wire...

I just went back and re-examined the loom at the point in your photo.

Sure enough. There's a lump right between the two connectors that I missed the first time. I'll try and get time to cut it open in the next day or so and post back.

i want to throw something out there , while u guys are Splaying Harnesses open .

i have found these to be 100% positive in every way shape , & form . if u dont know about them already , its the Most efficient

way to remove the outer Harness Jacket , wont cut or nick wires , and Most important Very Safe , as u pull the Hook Blade Towards u

motion . using a straight Blade is Outdated & Dangerous , I switched up after 16 stitches a few years ago Splaying a Harness

and the knife slipped and went right across the top of my Left hand .



just my :

 

[ToyotaMatt]

​

 

[numby]

I peeled back the insulation* and sure enough - there it is - a big, fat pink resistor wire with a line of little blue diamonds. But I is stumped...

From Coolerman’s site:

“Note: If you are replacing your 5/1978 to 7/1980 OEM distributor with a D.U.I. distributor, you cannot use the LARGE OEM Black/Yellow ignition wire to power the D.U.I! This wire is actually spliced to a 1.2 Ohm pink Yazaki RESISTOR wire inside the harness and was used in place of a ballast resistor. Using this wire to power the D.U.I. will result in the voltage at the D.U.I being about 10 volts not the full 12 volts needed.”

Uh, I did exactly that for about 18 years with no ill effects - that is - I ran an HEI distributor straight from the larger B/Y ignition wire and never even considered the resistor as I always had 12+ volts at the plug. No difficulty starting and better top end running than with a brand new OEM points distributor. (More on this in a follow-up post)

Whether Curt and I have anomalous 2/’78 FJ40s with resistor wires is of no real concern at this point. What matters to me is that I do have one and yet the wire in question still puts out a full 12.5 volts and not the correct 10 volts (9.6V). So I went through everything with a test light and my multimeter.

I checked the function of the resistor wire and it does seem to have approximately 1.2Ω of resistance regardless of which direction I checked it. That leads me toward thinking that it must be getting the full 12V from another B/Y wire downstream of the resistor wire. But I sure can’t find it.

I checked all the other B/Y wires on that side of the harness and all that show continuity (to the voltage regulator, to the outer vent control valve) also show that the resistor wire is in place and functioning to spec and not being bypassed by any of those. I thought it might be the smaller gauge companion B/Y ignitor/coil plug wire but they split before the resistor section and that checked out with the multimeter. (Note: the very small B/Y condenser wire splits off the large wire after the resistor.) I even disassembled the coil/ignitor plug on the main harness to see if there was some tomfoolery inside but the findings were no different. I can only think of two other options.

The first is that some other wire (switched or no) has been spliced into the large B/Y wire downstream of the resistor.

The second is that the resistor has been dramatically shortened inboard of the firewall and though it is still showing 1.2Ω of resistance but perhaps under full operating conditions there’s not enough of it remaining to prevent a full 12+ volts from getting through. I don’t even know if that’s possible.

For the first option I checked every powered wire on that side of the harness including the passenger head light, turn signal, horn, alternator, windshield washer pump, heater blower motor, fusible link, etc. Nothing showed continuity with the big wire. Counting the wires I have 22 factory wires coming through the firewall and 26 terminations in the engine compartment. The extra wires seem to be due to the splits in the W/B ground wires at various locations. I saw no non-OEM colored wires or anything that appeared like monkey business. I didn’t probe every single wire at the firewall but nearly. I could have missed something here as it was getting late and I’d been at it since about 7:00am.

I should mention that there was no fusible link on this vehicle when I bought it. The ammeter was just hard-wired in. I’ve done this on Toyota vehicles in an emergency but only long enough to get the vehicle to safety. It means there was likely an electrical issue in the past on this 40 but I don’t know if that would’ve burned out the resistor wire or not or if that’s even possible.

I don’t have time right now to get under the dash and confirm the integrity of the resistor wire or tear the whole passenger side harness apart as I need to a get a distributor back in this beast and drive it. I sure would love to hear some explanations for this issue if any one has one. Third option = I missed something.


*Thanks for the suggestion Matt. I’ve cut myself more times than I’d like to admit. Had to give myself a few stitches a time or two as well. I will look for your gut hook blade.

 

[Curt]

The wire can get full 12.5+ volts when the vehicle is cranking to give the coil extra juice at startup. There is a smaller diameter BY refered to as the "resistor Bypass" that runs from the starter. I just run the straight 9.6 volts wire to coil with no issues even in cold. I installed a trollhole dizzy using the 9.6 volts and it fired up but i wired in full 12.5 volts to keep the electronics in it happy. Back to stock now anyways.

 

[Coolerman]

This drawing might be closer to what you have in your truck.

The 12ga coil B+ wire starts at the key switch as BY (connected to IG), is spliced about 18" down to the Pink 1.2 ohm Yazaki resistor wire then near the firewall is spliced again to a 12ga BY going to the two pin connector. The small BY in the two pin connector is also spliced into the 12ga BY wire near the key switch, but BEFORE the Yazaki resistor wire. The small BY wire will always have full battery voltage.

So how does this circuit REALLY work, and how do you determine what voltages should be where with a resistor wire or a ballast resistor?

The Coil B+ Circuit
Note: We are going to use +12V for the battery voltage for simplicity when in reality it should be closer to 13.x volts. If you do this on your own truck, use the battery voltage you measure in all the calculations!

The actual coil circuit is a series circuit consisting of the battery, the resistor wire and the coil primary winding. Ignoring the battery resistance and the interconnecting wire resistance, you have the resistor wire at 1.2 ohms and the coil primary winding at say 4 ohms.
Ohms Law states that : I = E/R (Current (I) is equal to voltage(E) divided by the total circuit resistance (R). Since this is a series circuit, simply add up the known resistances. 1.2 + 4 = 5.4 ohms. Divide the battery voltage by the total resistance to get current measured in amps so I= 12/5.4 = 2.307 amps.

Now that we know the series circuit current, we can calculate the voltage dropped across each resistance in the circuit.
Lets start with the voltage drop across the Yazaki resistor wire.
To do this you rearrange Ohms law to be E=I X R. (voltage drop is equal to total current divided by resistance of the resistor wire) So E=2.307 X 1.2 meaning the voltage drop across the resistor wire is 2.768. Since voltage drops have to add up to the source voltage of 12V in a series circuit we simply subtract the 2.768 from the source of 12V to get 9.228 volts dropped across the coil.

Now to address why you seem to get a full +12V on the large BY wire. The first question is how did you measure this?
If you had the plug disconnected and measured the large BY wire to ground you will get the full battery voltage! Why? You have no other resistance in the circuit besides the meter (which is a very high resistance) and the 1.2 ohm resistor wire. The meter being high resistance will only allow milli-amps to flow. No current flow means, no voltage drop, so you will read almost the FULL battery voltage. Now if you plug the connector in so that the large BY wire supplies +12V to the coil B+, turn on the key, and measure from coil B+ to ground, you will read battery voltage minus the drop across the resistor wire because the coil and resistor wire will allow 2.307 amps to flow resulting in voltage drop. (Don't leave the key on for a long time!)

Now if you had it hooked up as above and measured the voltage AT THE COIL B+ terminal while CRANKING the truck AND you have a starter with the ballast bypass function AND it was hooked up correctly (see the black/white wire in the attached diagram labeled Z26A1) then you would once again read full battery voltage. WTF? Why? Well the purpose of the ballast bypass relay built into the starter is to provide the full +12V to the coil but only while cranking. Once the truck starts, the relay drops out and the coil is once again supplied power through the resistor wire or ballast resistor.

So in short you need to measure the voltage at the coil B+ WHILE THE TRUCK IS RUNNING in order to see the reduced voltage.

I have no answer as to why the DUI seemed to run fine with the reduced voltage other than they don't seem to know their own products capabilities... or due to some creative re-wiring by the PO, it was always getting full battery voltage.