Swapping cluster from idiot lights to gauges

[5150]

I know there are several threads on this, but still not getting it. I hated to revive a 3 year old thread so I'll start my own.

I have an 87 R10 that has idiot lights. I'm swapping it to a gauge cluster that I picked up from the junkyard. Here is a pic of the before harness?






I followed one of the older threads directions but I ran into a few problems.

1. My fuel gauge doesn't work(pegged at 3:00 and it has a new sending unit)
2. Oil Pressure Gauge doesn't work (pegged at 3:00 also)
3. Temp Gauge does not work(just won't move at all)

I grabbed a harness off of an 80's model with gauges at the junkyard, but decided to repin most of it. I have two/three wires left over. It appears one is the Brown 25 and the other is the Brown 50. Also there is a brown wire with a white stripe that I can't find on any wiring diagram. Here's a pic of the current harness:







After reading all of this I hope you're not as confused as I am.

[motornut]

I did that swap
I think the brown was batt light?
Needed power in/out before, now power/ ground
I left a brown unused taped off.
Sounds like a ground is off(3oclock)
Confirm the gauges are grounded.
All grounds went to one behind the speedo.

[5150]

Not a ground issue. Grounded the cluster and it change anything. I did ground the pink fuel gauge wire while it was plugged in to the cluster and it went to empty.

[motornut]

ok Can we see the back of your idiot cluster?
yours is different colors/spots
my 78 and 79

[ray_mcavoy]

Originally Posted by 5150

I followed one of the older threads directions but I ran into a few problems.

1. My fuel gauge doesn't work(pegged at 3:00 and it has a new sending unit)
2. Oil Pressure Gauge doesn't work (pegged at 3:00 also)
3. Temp Gauge does not work(just won't move at all)

(1) The fuel gauge pegging at 3 O'Clock is usually a sign of an open circuit somewhere in the sending unit wiring, sending unit, or ground.

The fact that you can temporarily ground the pink circuit #30 fuel sending unit wire and make the gauge read empty is a good sign that your connections up on the gauge end are okay. I'd recommend double checking the sending unit's ground ... make sure there isn't any dirt, rust, or paint where the ring terminal bolts to the frame. Also use an ohmmeter to test the sending unit itself ... sometimes you can get a dud even though it's a new part.

(2) and (3) could also be sending unit related. You're not still using the old warning light senders are you? The warning light ones are simple on-off switches while the gauges require variable resistance type sending units.

Originally Posted by 5150

I grabbed a harness off of an 80's model with gauges at the junkyard, but decided to repin most of it.

Out of curiosity, why did you decide to repin this harness? An early/mid 80's factory gauge harness should be a direct plug-in swap with any 1978 up to 1987 (or to 89 R/V series) gauge cluster.

Originally Posted by 5150

I have two/three wires left over. It appears one is the Brown 25 and the other is the Brown 50.

Those two brown wires (along with the "GEN" light bulb) formed the exciter circuit that supplies the initial field current to the alternator. These wires do not connect to a factory gauge cluster. But unless you're using a "1-wire" alternator, you need to connect circuits #25 & #50 together through a resistor for proper alternator charging at lower engine RPM. A 47Ω 2 Watt resistor will work with either the stock 10 or 12 SI alternator as well as the newer CS alternators. Or you could even wire a 168, 194, or similar small light bulb between those two wires and still have a functional "GEN" light in addition to the voltmeter.

Originally Posted by 5150

Also there is a brown wire with a white stripe that I can't find on any wiring diagram.

That is most likely circuit #419 which is for the "check engine" light. It would have originally been in cavity #14 of the cluster plug. This circuit/wire was a new addition for TBI engines in 1987 so you won't find it on the earlier year diagrams.

[5150]

I'll check the ground on the frame, but the same ground is also being used by the pump. I'm driving it so I would think that ground is good.

I repinned it because the original cluster was an idiot light cluster.

that's another question I had. I'm swapping it over to a serpentine belt system this weekend. I was wondering how to wire up the CS alternator.

So I don't need that wire at all(419)?

[ray_mcavoy]

Originally Posted by 5150

I'll check the ground on the frame, but the same ground is also being used by the pump. I'm driving it so I would think that ground is good.

You're right, the fuel pump working means the sending unit ground is okay. I was forgetting about 87 having a shared ground for the pump & sender when I wrote that.

Originally Posted by 5150

I repinned it because the original cluster was an idiot light cluster.

Yes, I understand that you repinned your truck's original harness for the conversion to gauges. But I thought you also said you went to a junkyard and picked up a harness from an 80's truck with factory gauges. I was wondering why you said you repinned that harness too.

Originally Posted by 5150

that's another question I had. I'm swapping it over to a serpentine belt system this weekend. I was wondering how to wire up the CS alternator.

The CS alternator should have a 4-pin plug labeled "P", "L", "I" or "F", and "S".

The "L" pin goes to the indicator light. This would be the brown (circuit #25) wire that used to connect to the #1 terminal on the SI alternator.

The "S" pin is for remote voltage sensing. This would be the red (circuit #2) wire that used to connect to the #2 terminal on the SI alternator.

The "P" and "F" pins can remain unused. The "I" pin (if present, not all CS alternators have this) can be used as an alternative "exciter" wire instead of the "L" pin.

Originally Posted by 5150

So I don't need that wire at all(419)?

Are you still using some form of electronic fuel injection? If so, you'll probably want to keep the "check engine" or "service engine soon" light functional. That can be accomplished by re-locating the brown/white ckt#419 wire to cavity #15 of the cluster plug. And you'll probably also want to transfer the corresponding lens at the bottom of the speedometer over into your gauge cluster (unless the gauge cluster came from an 87+, the existing lens probably says "choke" instead of "check engine").