Installing the aux gauge cluster in an 84 (Page 1/2)
RWDPLZ JUL 09, 11:59 PM
Before starting this project, I searched the archives and asked people here about it. A few people had claimed to have done it, but never posted how. Other people simply said 'it can be done, but 84 wiring is different' So using information compiled from multiple threads and my trusty 84 service manual, I was able to successfully install the aux cluster into my 84.

Here's the differences between the 84 and later years:

First, the oil pressure switch/sending unit

Shown at left is the 84 switch, right is the 85-87 sending unit. The difference is the 84 one is just a switch, which turns on a light on the dash when the oil pressure drops to 4 psi or below. The 85-87 unit is a variable resistance (0-90 ohms) sending unit, which varies in resistance depending on the oil pressure, causing the gauge needle to move around. The old sending unit must be removed using a special oil pressure switch socket. The new one has the same size 1/4" threaded end, and can be screwed in in place of it. Mine used a 9/16" wrench to screw it in tightly.

I had a bad experience with a Niehoff sending unit purchased from Autozone (probably now reboxed and sold as Duralast) which read 40psi max and read low overall. Replacing it with an AC Delco unit fixed the problem. I purchased it from an AC Delco distributor for just over $50. The dealership wanted $95. The sending unit is AC Delco part # 10045775

Here is the pinout for the earlier units and the 88 sending unit:

Wiring diagrams:

In this diagram you can see how the orange and tan/white wires are connected in the fuel pump circuit.

The tan signal wire:

The tan signal wire carries the signal from the sending unit to the gauge cluster. In the 84, it will turn the light on if the oil pressure drops to or below 4 psi. You can see the way it's wired here, the light will also come on if the car overheats since it's also connected to the coolant temperature switch. In this diagram you can also see that the tan wire for the oil pressure comes through the firewall on the C203 connector, while the coolant temp switch comes through on the C500 connector. Both join at the S214 connection point under the center console.

C203 Connector:

This is where I got the signal for the aux cluster. The tan wire, circled in yellow below, was cut 3" from the end of the connector. I then crimped on a male spade connector, and crimped a female spade connector to the other end, which gets shoved back under the console. This makes the wiring reversible later, and keeps the unused end from grounding against the frame. A tan wire is run from the cluster back to this connection under the center console.

C206 connector differences:

This connector plugs into the gauge cluster at the rear right of the cluster, behind the tachometer.

The main difference here between 84 and later is pin #6: In 84, this is where the oil pressure signal comes in, while on later years this is where the brown charge wire is located, and the tan oil wire is moved to pin # 17 (empty on the 84). Fortunately, it isn't necessary to move the wires around.

I soldered and taped up the harness going to the aux cluster at the points indicated on the connector: pin 9 (pink/black) for the 12V, pin 11 for the ground, and pin 15 for the illumination so the gauges will glow at night, and dim with the dimmer switch. These wires are then run up behid the dash to the aux cluster.

The only thing left is the brown alt lamp wire. In the 84's, instead of a bulb, Pontiac used a 10 ohm resistance wire

I didn't bother hooking up this wire since I'm not sure if the total resistanc evalue matters or not (10 ohm wire + bulb resistance) but it would probably be simple. The volt meter and oil pressure gauge work fine without it, you just don't have an idiot light for the alternator.

Testing the unit:

Once installed, turn the ignition key to START without actually starting the car. The voltmeter should jump to wherever the battery is currently charged (probably 12.4 or so).

The oil pressure gauge can be tested by hooking up a wire to the middle terminal (tan wire). I crimped a spade connector to a long length of wire. Hooking the spade connector to the middle terminal and touching the wire to a ground should cause the gauge to go into the red zone and the oil light to come on (zero resistance). Unplugging the sending unit should cause the gauge to peg far to the right (infinite resistance).

Hopefully this helps anyone who wants to install the cluster in their 84.


1984 Fiero SE

[This message has been edited by RWDPLZ (edited 07-10-2008).]

E.Furgal AUG 13, 06:20 AM
nice, is there enough room to tee the fittings, to have both the dum dum light and gauge?
I have no issue running a wire all the way back..
some in family (wife) need to see a red light to think there is a problem.
RWDPLZ AUG 18, 10:31 PM
The bright red light on the oil pressure gauge will light up, it's pretty noticeable. I thought about the tee idea, but it would mean more possible places to leak, and running extra wires. Felt like there was plenty of room, not sure, though.
fierosound FEB 12, 09:17 AM
Done it with my Indy using this website

My World of Wheels Winners (Click on links below)

3.4L Supercharged 87 GT and Super Duty 4 Indy #163

RCR FEB 24, 08:13 PM
Excellant write-up.


[This message has been edited by RCR (edited 02-24-2013).]

Ram426 APR 21, 10:01 AM
Is I correct to assume if adding the 120speedo to an 86 is just a direct replacement , and plug in?
RWDPLZ JUN 08, 04:57 PM
A little late, but if the 86 is a V6, then yes, plug and play. If it's a 4 cylinder, you'll need to swap the tach circuit board.
longjonsilver JUL 04, 08:37 AM
Thanks for the excellent writeup. Now i think that i can wire in the gages. Can i assume that instead of running a wire from the C203 connector, that i can just tap into the tan wire on the C206 at pin #6 ?? Since i am installing the 120mph speedo, i will also need to tee off to the #17 position? Or can i move the #6 wire to the #17 position instead?

reason for edit: clarify

I'm the original owner of a white ' 84 2M4 purchased Dec 10, 1983 from Pontiac. Always garaged, no rust, 4-wheel drifts are fun! 3800 SC swap to come!

[This message has been edited by longjonsilver (edited 07-16-2018).]

longjonsilver JUL 04, 01:48 PM
Well, that is exactly what i have done and it seems to work correctly. Pulled an oil sender off the same car that gave me the rally gages and the main gages. Hopefully it will work, as now the oil pressure is pegged max max max. The RPM of my Duke reads 1 and a 1/2 times the real RPM. i plan on installing a 3800SC into my 84 so i will live with the incorrect tach reading. In the meantime, it will make me do the math!


I'm the original owner of a white ' 84 2M4 purchased Dec 10, 1983 from Pontiac. Always garaged, no rust, 4-wheel drifts are fun! 3800 SC swap to come!

[This message has been edited by longjonsilver (edited 07-16-2018).]

longjonsilver JUL 04, 04:57 PM
Well, on a second thot, after a drive i find that my dash lights don't work, except for the right turn signal. The left turn signal works outside, but the dash light doesn't. i forgot to remove the handbrake on the test drive, because the light didn't come on. Any ideas?

I'm the original owner of a white ' 84 2M4 purchased Dec 10, 1983 from Pontiac. Always garaged, no rust, 4-wheel drifts are fun! 3800 SC swap to come!