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What is this resistor in the engine bay?


jimmyfloyd

Member
Joined
Nov 7, 2017
Messages
15
Vehicle Year
1986
Transmission
Manual
1986 Ranger 2.9L 5spd 4x4

Trying to narrow down some issues. One of them seems to be this piece. i see it has a resistor label on it, but can't seem to find a replacement part. It has a Dark Green/Yellow wire on one side, and either a solid dark green, or Dark Green/Yellow out the other. Seems to go from the Ignition coil to the other side of the motor. Maybe to the TFI module?

Any help on where to get a replacement for this would be helpful. As shown in the pictures, the wire is in bad shape right at the edge of it, and I believe it is grounding out on bumps and causing the truck to stall.
 

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That's the ballast resistor for the coil.

I don't know where to get a replacement either.

Tape up the exposed area for now. That at least should tell you if that wire grounding is part of your issue.
 
Ok, that gives me something to go on. Thanks for confirming that.

Now I need to see if this needs a specific resistance or if I can get one that is close.

And it will be taped up for now. There are other issues to be solved (hard cold start, seemingly a misfire, and tracking down why the muffler exploded yesterday...)
 
Ballast resistor was used on older model coils, made them last longer
No, it wasn't because older coils were 6volt models, lol, old mechanics tale
(Tractors were different :))

1.20-1.40 ohms range is fine, this reduces running voltage(13.5-14.5volts) to about 10volts at the coil.

But you do need a direct voltage to coil on startup or coil would only get 5-7volts thru ballast resistor.
When starter motor is active system voltage drops to 9.5-10.5volts, starter is a big amp draw, so ballast resistor drops voltage too much

Ballast resistor needs to be by passed for better starting

There were a few ways Ford did this:
4 Post starter relay(solenoid), it will have an "I" post, the "I" post connected directly to the Coils "+" terminal
When starter relay activated the "I" post got Battery voltage just like starter motor did, so coil got full battery voltage only when starter motor was active.
NO....you can not use starter motor post in place of "I" post, 10volts from Coil "+" via ballast resistor would try to activate starter, melt wire or engage gear

Second method was in the ignition switch, probably have this in 1986
In RUN the system voltage ran thru the Ballast Resistor to coil "+"
In START the system voltage ran direct to coil "+"

Third, TFI system has a separate START and RUN terminal, by passing ballast resistor


Hard cold start can be a few things, but one of them could be low voltage at the coil while cranking, should be 10volts, if not then direct wire is bad/disconnected
 
Last edited:
Last time my exhaust exploded it was a bad O2 sensor.
 
That’s not a ballast resistor. It’s s 22k ohm resistor that steps down current sent to the ecu for RPM info.

It was not sold as a replacement part and is unlikely to cause a stall. It will cause a fault code to be set.
 
AndyB. said:
That’s not a ballast resistor. It’s s 22k ohm resistor that steps down current sent to the ecu for RPM info.

It was not sold as a replacement part and is unlikely to cause a stall. It will cause a fault code to be set.
Click to expand...

I am new to ford, so I have been reading a bit on them. This seems to come off the coil wire and is tied into a 3 wire junction just inside the wire loom.

For a test, while the truck was running I grounded the bare wire portion of this, and the truck stalled out almost immediately. I tested this because the way it was, it could easily ground out on several items depending on how it swung, so this lead me to believe it was the ballast resistor. Is there a way to check it conclusively to find out which it is so it can get replaced?

I have included some pictures of the other parts of the engine bay to show what else is there. It does have the Grey TFI piece next to the starter coil and battery with the 3 connectors out the side.
 

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Andy B is correct

That resistor connects to the Gound(-) side of the coil, it runs to Pin 4 on EEC(computer), it is the "tach" wire for the computer

Drawing here of lay out: https://cdn.shopify.com/s/files/1/0255/2163/files/88mustangIGN.gif?1317

Bottom center of drawing

And yes if you were to Ground that bare wire engine would stall, coil couldn't "spark" with full time Ground
 
Ok, more to this. Had a shop look at it, and they just replaced this with a wire. The truck ran OK, but still not 100%. Recently, we found a couple of these resistors in the junkyard and the owner has installed one. Still waiting to hear back on how they are doing, as he just did it yesterday.
 
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