My 87 came with a resistor bridged right across the bulb holder itself.
The '88 cluster I replaced it with had it soldered on the flex circuit.

 

Correct that is indeed the resistor.

When you swap the cluster you remove the resistor, and you also remove the connection between the Red/Green and Green/Red wires.

It's very simple to swap the wires at the regulator connetor. No cutting or splicing needed. You just need a wire removal tool or an icepick to unlock the terminal from the connector to swap. This would also negate the need of buying a resistor.

At the regulator Move the Green/Red wire to the stator terminal, and remove the white/black wire and tape it up, then connect the Red/Green and Green/Red wires together at the cluster connector seems to be the simplist way around that problem.

 

Learned that from you!

 

Ok, with the exception of not having the oil pressure sending unit yet the job is complete. I tested the charging system with the idiot light cluster removed and it was not charging. In conclusion, pins 12 and 13 did to be jumped for a proper charge. Other than that, I followed the spreadsheet and everything seemed to work accordingly. The pin swaps were correct and the shunt is definitely necessary for proper charging/protection. The ammeter appeared to slowly reduce charge as the battery charged (indicating to me proper function). The tach works properly. The temp gauge works properly. The oil pressure gauge bounces showing it does have power...when I get the proper sending unit and install I will update. It took me about 3-4 hours, mostly waiting for the soldiering iron to heat up and telling my kids to stop driving me nuts. There were some other wiring issues (non-factory) that created some confusion but all in all, a successful outcome. Thanks to everyone to helped me out with this!! You guys are an unbelievable asset to the forums. Thank you!

 

The way you have it hooked up now will not give you any problems UNTILL you have a problem with the charging system.

I still highly recommend moving the Green/Red wire from the "I" indicator post on the regulator and placing it where the "S" stator post is. Then removing the white/black wire from the regulator all together as it states in the factory wiring diagrams. This will keep a dead short from happening, if your altenator decides to quit on you going down the road, as the regulator will try to turn on the Indicator in your dash that's no longer there. There will be a dead short between your ignition switch and ground if the altenator decides to quit while driving down the road as example. This is because the light and resistor have both been removed.

 

I am thinking if it was going to blow up, it would have done it already, since the light always comes on when you turn the key but the engine is not turning yet. If the engine is stalled out for some reason the light will come on also.

I still agree I would switch it over to the factory wiring scheme at the regulator, but this is how things are figured out sometimes. By that I mean going ahead doing it the supposed wrong way, and finding out it works anyway.

 

True, but how long do most people leave their key switches in run without starting the engine? Probably not long enough to create enough heat to cause damage. The only protection on this circut is the fuse link on the solenoid.

I might just be paranoid on this, I hope I am, but I do see it being a potential problem in some situations.

 

Hey guys....back in town and just catching up on this. From what I'm reading, it appears we did solve the question with two working solutions. 1974F100, I'm glad everything worked out well on the swap and that the estimates of effort required were pretty close. But I do agree with 81-F-150-Explorer that the red/green and green/red at 12 and 13 either have to be bridged with a 500ohm resistor or moved from the I to the S terminal on the regulator. I think either will be effective. I've got schematics that show those wires bridged with a resistor on gauge models also, just in the harness instead of the cluster. I'll try to do a FTE friendly version of the instructions that show either the those moves as options that hopefully can be turned into a tech tip. But in the bigger picture, I think it's pretty cool we all figured out how to do something we've heard for years couldn't be done.

 

I can't speak for others here but, pertaining to myself, I never promoted anywhere that it
couldn't be done but, more to the point, that it couldn't be done EASILY without first
going through all the work described in this thread (with much of it by you).

I've got the 81 factory wiring diagrams and my truck had a cluster with idiot lights in
which I wanted to make the ammeter work safely & correctly, but I was unwilling to
spend the time involved in unwrapping the harness to see for myself where the wires
actually go + I didn't have a gauge cluster to even look at much less disassemble.

Many of the guys out here are interested in a quick fix - plug & play, as it were. Wouldn't
you agree that getting to this point took some specialized (maybe that's not the right
word I want but I hope you get the idea) knowledge & time & discussion with others and,
finally, somebody able & willing to spend the time to actually try it?

 

Chris, I'm with you on this. The words "can't", "can't easily" or "I wouldn't do" (all phrases from other threads on the subject) are semantics. But my point is that it took a collective effort from some very good and experienced minds on here to hash this thing out, and the testicular fortitude by 1974F100 to actually do it. In the end we have a relatively simple road map for others to follow. Not quite plug and play, but a workable plan that can be done in a few hours by someone with average skills. I think it's a win for the forum.

 

FYI, plugged in the new oil sending unit and it works fine (as expected). Cluster lights do not work though. I haven't had time to investigate that part yet. Or am I missing something obvious?

 

Is it just the cluster, or do the lights around the heater box and headlight switch not work also? If all are out, check the fuse and make sure the headlight switch dimmer isn't turned all the way off. If it's just the cluster, make sure you have light blue/red at pin 5 and black at pin 2. Make sure the pins are inserted all the way into the connector and aren't backed out. Also check that the copper strips on the printed circuit are positioned correctly and haven't shifted. That problem caused me to eliminate the printed circuit on my cluster and hard wire the whole thing.

 

While I have a few minutes tonight, here are the step by step instructions I promissed to close out this thread. Please note this is based on an 81-86 gauge/tach cluster. Pins 17 and 18 should be reversed if adding an 80 gauge/tach cluster. It's been tried and the reports have been positive to date. Disconnect the battery before starting any of this.

Move the Black/Light Green wire from Idiot Light Connector (ILC) pin 7 to Gauge Cluster Connector (GCC) pin 1.

Move the Black ground wire from ILC pin 9 to GCC pin 2.

Move the Yellow/White fuel sensor wire from ILC pin 11 to GCC pin 4.

Move the Light Blue/Red dash lights wire from ILC pin 10 to GCC pin 5.

Move the Red/White temp sensor wire from ILC pin 8 to GCC pin 6.

Splice a (factory color) Red/Orange wire from the Red and Black/Orange fusible link on the starter solenoid to GCC pin 8. Add one end of a 30 inch length of 10 ga wire to the the splice at the solenoid. This wire is the shunt to protect the ammeter circuit.

Splice a (factory color) Yellow/Light Green wire from the black/orange wire at the B terminal on the alternator to GCC pin 9. Add the opposite end of the 30 inch 10 ga shunt wire to the splice at the B terminal.

Move the White/Red oil pressure sensor wire from ILC pin 14 to GCC pin 10.

Splice a (factory color) Dark Green/Yellow from ignition coil negative to GCC pin 11. This is the tach input.

Splice a (factory color) Black/Light Green wire to the Black/Light Green wire at GCC pin 1 to GCC pin 12. This is the tach power wire.

Move the double Red/Yellow brake warning positive wire from ILC pin 1 to GCC pin 13.

Move the Purple/White brake warning negative wire from ILC pin 2 to GCC pin 14.

Move the White/Light Blue right directional wire from ILC pin 3 to GCC pin 15.

Move the Light Green/Black high beam indicator wire from ILC pin 4 to GCC pin 16.

Move the Dark Green/Light Green seat belt indicator wire from ILC 5 to GCC pin 17.

Move the Light Green/White left directional wire from ILC pin 6 to GCC pin 18.

Disconnect the Light Green/Red and Red/Light Green wires from ILC pins 12 and 13. Connect these wires with a 500 ohm resistor between them. You can use the resistor from the back of the ILC, but make sure its well insulated. More compact resistors are available at Radio Shack.

Change the oil pressure sending unit from an idiot light type to a gauge type.

Optional based on discussion in the thread: Move the Light Green/Red from the I terminal on the voltage regulator to the S terminal. Remove the Black/White wire from the the regulator and seal the end. There has been some concern voiced that not doing the above could lead to a dead short in certain situations.

 

Thank you for your effort in this great write up!
Reps sent.

 

My thoughts exactly!

Except the forum won't let me rep......