I'm in the process of doing a swap and have tried to figure out the connectors for C500 & C203. I've looked at the following pages:

C500 & C203 Discussion 1

C500 & C203 Discussion 2

Fiero_C203_C500_pinouts.pdf

Connectors and Pinouts

I noticed several of my wires don't match up in location of color to what is shown. So I was curious if anyone else has run into this problem and can offer any input. Specifically on pin A and P of the C203. Also I'm hesitant to test the wiring since I'm not quite sure what voltage is safe to put through the wires. I presume all the digital electronics in the gauge cluster are 5V and if I exceed 5V I could fry them?

Has anyone found a decent way to trace wires?

If anyone has suggestions or corrections please let me know. Also e-mail me at FieroNate@gmail.com to get an Excel version of the data below. It should make correcting it easier.


Here is what I recorded on my 87 GT 5 speed:

Fiero C500 Connector Chassis Side
Pin
Location Wire Color Wire Dia. Inch
A1 - | | -
A2 Black | | 0.079 Battery Ground to Engine
A3 - | | -
A4 - | | -
A5 - | | -
B1 Brown | | 0.089 ???
B2 - | | -
B3 Brown | | 0.089 Charging System Battery Indicator Lamp
C1 Light Blue | | 0.089 Backup Lights from +12 V ignition feed
C2 Light Green | | 0.078 Temp Gauge Feed from Sender (is this +12 volt ref, or the actual signal wire)?
C3 Tan | | 0.089 Tach Feed (signal wire??)
D1 Green/White | | 0.089 Fan Request from temp sender (+12 volt +??), does this go to the solenoid on the relay?
D2 Green/Black | | 0.089 (2) speed fan, high speed??
D3 Dark Green | | 0.079 Hot Temp Warning Light Feed (+12 Volt? or ground)
E1 Light Green | | 0.089 Backup Light Feed from Transmission (to +12 V or to ground??)
E2 Violet | | 0.186 Starter Solenoid Power +12V
E3 Pink | | 0.136 Ignition Power +12V (to ECM??)
E4 - | | -

Fiero C203 Connector Chasis Side (Black)
Pin
Location Wire Color Wire Dia. Inch
A - | | - Upshift (does this work on '87 GTs?)
B Orange | | 0.077 Fuel Pump +12V?
C Black/White | | 0.077 SES Lamp (service engine soon) +12V ??
D Light Blue | | 0.077 AC Request +12V to ECU?
E Tan | | 0.083 Oil Guage (is this the power, or signal) +12V or +5V?
F Pink/White | | 0.087 ECM Power +12V (is this in addition to the power found on the C500 connector at pin E3?)
G Yellow | | 0.083 VSS High (I presume this is an AC signal as there is no polarity associated with it)
H Dark Green/White | | 0.077 VSS to ECM. Is this pulsed DC or AC?
J Pink | | 0.083 Fuel Injection Fuse 1 (5 Amp) is the fuse on the ground or the +?
K Pink/Black | | 0.083 Fuel Injection Fuse 2 (5 Amp) is the fuse on the ground or the +?
L Tan/White | | 0.097 Power from Fuel Pump Relay to run fuel pump +12V
M Black/White | | 0.077 VSS/Speed Ground (I presume this is a ground to/from the ECU???)
N Black/White | | 0.083 AC Clutch Power from Fused Relay +12V??
P Tan/Black | | 0.083 Uknown??? Any ideas what his might be?
Q Violet/White | | 0.081 VSS Low (again I presume this is AC from the speed sensor)

Pin A of Fiero C203 is for the upshift lamp that was NOT used in all Fieros. Your Fiero may not have this wire.
Pin P of Fiero C203 is for the brake switch power supply to the torque converter clutch. If your Fiero came stock with a manual transmission, you will not have a wire in this pin location on the C203.

Concerning the gauge cluster, each gauge has its own power supply. I'm not sure where you got the 5v from but you shouldn't need to supply any 5v reference to the gauge cluster at all. The guage cluster connects to its own sending units which are independent of the ECM except for the VSS (speed sensor) depending on how you are going to wire that up.

As for your C203 and C500 questions:

C500
A2 - connect to engine block ground

B1 - unknown / shouldn't need to wire it up to anything

B3 - connect to the alternator terminal "L"

C1 - connect to your backup lights switch

C2 - connect to your temp gauge sending unit (this should be for the temp gauge, not the hot warning light on the dash - you can ground it with the key on and if it pegs your temp gauge out then it is for the gauge. If it turns on the overheat light, then it is for the light)

C3 - tach feed (connect to ignition module or coil tach signal output)

D1 - Fan request wire - ground to turn fan on (connect to switch or PCM to control your radiator fan)

D2 - Fan2 request wire - ground to turn fan2 on (only used in Fieros that had 2 speed cooling fans as far as I know)

D3 - hot lamp control (you can ground it with the key on and if it pegs your temp gauge out then it is for the gauge. If it turns on the overheat light, then it is for the light)

E1 - connect to your backup lights switch

E2 - connect to your starter solenoid

E3 - connect to your igntion coil or igntion module's IGN 12v + input terminal


C203
A - for the upshift lamp on the dash which your Fiero may not have (grounding it with the key on would turn on the upshift lamp if your Fiero is wired for this)

B - this is the 12v + power supply from the F.Pump fuse in the IP (instrument panel) fuse box to the Fuel pump relay and oil pressure switch. I rewire the fuel pump circuit in all the swaps I do and don't use this wire to power the fuel pump.

C - the ECM grounds this wire to turn on the check engine lamp

D - A/C request - 12 volts comes in on this wire when A/C is requested (by the climate control panel AND A/C pressure cycling switch, if installed) to tell the ECM / A/C relay you want A/C turned on

E - oil pressure gauge feed. The oil pressure sending unit supplies this wire with a varying resistance signal to ground display the proper reading on the oil pressure gauge. DO NOT connect this wire to anything other than an OE Fiero oil pressure sending unit (or other GM 80 psi oil pressure sending unit).

F - 12v key on power comes from the ECM fuse in the IP fusebox on this wire - this is what you will need to hook up to the ECM (and share with the MAF sensor, if your engine uses one). This is a circuit seperate from C500 E3.

G - VSS high from sending unit to the speedometer. The speed sensor (or ECM) will provide a pulse on this wire to run the speedometer, 4000 pulses per mile. If you are using a newer GM ECM/PCM to supply the VSS signal to the speedo, then you will need to build and install this VSS interface circuit: http://www.gmtuners.com/fiero/speedo2.gif

H - 2000ppm VSS signal from speedo to factory Fiero ECM. You won't use this wire if you are using a newer than stock Fiero ECM that requires a 4000ppm or higher ppm VSS signal.

J - 12v key on power from INJ1 fuse. 12v + power (hot) with key in run and crank. I connect this to power all the injectors on the engine.

K - 12v key on power from INJ2 fuse. 12v + power (hot) with key in run and crank. I connect this to power other devices on the engine or electronic trans, such as the EVAP solenoid, EGR valve (if needed), Heated O2 sensor, etc.

L - this is the relay switched power supply from the fuel pump relay to the fuel pump. If you are doing the Fuel Pump Hot-Wire mod (http://www.gmtuners.com/fiero/hot_wire.htm) you are going to re-task this wire for use to power up the new fuel pump relay you will be installing using the HOT WIRE mod. If you are leaving the stock Fiero fuel pump relay wired up in your swap and not changing that wiring at all, then you won't change this tan/wht wire.

M - this is a ground supply to the speedometer unit in the cluster. Must be connected to engine block or a very good chassis ground.

N - this is a 12v + power supply from the climate control panel to the original Fiero A/C relay. This is the actual 12v + power that drives the A/C clutch. Hot with key on and ANY control (other than OFF) selected on the HVAC panel.

P - brake switch 12v + power feed to the TCC (torque converter clutch) circuit in the trans. If you have a manual transmission application, then this circuit probably doesn't exist in your Fiero.

R - this is the ground supply from the Fiero speedo to the speed sensor mounted on the trans. If you are running your Fiero speedo off the PCM in your swap, then you don't need to hook this wire up to anything at all.

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OVERKILL IS UNDERRATED

Custom GM OBD1 & OBD2 Tuning | Engine Conversions & more | www.gmtuners.com

[This message has been edited by Darth Fiero (edited 10-18-2013).]

It appears I have no Pin A in the C203. I do have a pin P. Any chance I can wire the shift light to the wire run for Pin P?

I didn't realize that each gauge was powered on its own. I was however concerned about input voltages on sensor pins. In the '80s I believe most digital logic was 5V TTL, today its CMOS so I wasn't sure how tolerant of 12V to a sensor feed expecting 5V or less it is.

B3 I assume this is the + wire to control the voltage regulator or power it? (I'm using the newer Bosch alternator with a 2 pin Bosch style connector. I presume its a ground and a voltage from the battery to make the regulator work and thus regulate the field on the stator.

C1 & E1 I imagine it doesn't matter which goes where (+ vs -) as the trans as a simple switch that needs to be connected.

C2 I'm pretty sure the newer sensors on this engine run all the warning lights through the ECU. I'll have to look into the outputs available from the ECU or make a circuit that runs this gauge (from what I understand all the newer stuff runs a CAN bus signal from the ECU to the gauges... so I might have to pull data off the CAN bus).

C3 this is the pulsed signal for the tachometer? Is it expecting 12V + or 5V + to run the tach? Also any idea how many pulses/sec it takes to make it work? Again not sure the new ignition module outputs this. I'll have to study the ECU wiring a bit more to see how it handles tach output.

Pin C of C500: the SES light is on a pull down circuit?

Pin E I don't think I'll be reusing a fiero style oil sender. Can I provide this gauge with a voltage signal that simulates the variable resistance of the pressure transducer?

Pin E3 (C500) runs only the ignition? So I'll want this hooked to my coil pack control module only.

Pin F (C203) powers the ECU and other electronics requiring +12V?

Pin H why wouldn't I use this? I can generate a signal at 2000 ppm from whatever the ECU uses. Is 2000 ppm at the dash gauge or is it used somewhere else in the engine / ecu? I'll be running an F40 trans FWIW.
Thanks again for the help.

quote Originally posted by FieroNate: It appears I have no Pin A in the C203. I do have a pin P. Any chance I can wire the shift light to the wire run for Pin P?

You could run a new wire for the shift light if you want. Or use an existing wire that's already in the harness that you don't need. Doesn't matter.

quote I didn't realize that each gauge was powered on its own. I was however concerned about input voltages on sensor pins. In the '80s I believe most digital logic was 5V TTL, today its CMOS so I wasn't sure how tolerant of 12V to a sensor feed expecting 5V or less it is.

The OE Fiero coolant temp, fuel gauge, and oil pressure gauges output a 12v bias voltage which is pulled down to ground by a variable resistor circuit (sending unit). The OE Fiero speedo and tachometers both work off of time based pulse inputs. The speedo requires 4000 pulses per mile and the tach requires either 4 or 6 pulses per camshaft revolution depending on if it is for a 4cyl or V6.

quote B3 I assume this is the + wire to control the voltage regulator or power it? (I'm using the newer Bosch alternator with a 2 pin Bosch style connector. I presume its a ground and a voltage from the battery to make the regulator work and thus regulate the field on the stator.

B3 is IGN B+ that comes from the charge warning light on the dash (if you have one) or a resistor (or diode?) installed in harness somewhere if you don't have a charge warning light. This circuit is going to supply 12v + key on power to "excite" the alternator and induce charging (which can be grounded by the alternator to turn on the warning light on the dash without damaging the circuit). If your BOSCH alternator has an "L" terminal or a terminal that is supposed to be hooked up to a charge warning light, connect it to this circuit.

quote C1 & E1 I imagine it doesn't matter which goes where (+ vs -) as the trans as a simple switch that needs to be connected.

Correct. It doesn't matter how this is wired up to the backup lights switch.

quote C2 I'm pretty sure the newer sensors on this engine run all the warning lights through the ECU. I'll have to look into the outputs available from the ECU or make a circuit that runs this gauge (from what I understand all the newer stuff runs a CAN bus signal from the ECU to the gauges... so I might have to pull data off the CAN bus).

No ECU that I have ever seen is going to supply a varying resistance signal to run the Fiero temp gauge on the dash. You will need to connect this directly to a coolant temp sending unit.

quote C3 this is the pulsed signal for the tachometer? Is it expecting 12V + or 5V + to run the tach? Also any idea how many pulses/sec it takes to make it work? Again not sure the new ignition module outputs this. I'll have to study the ECU wiring a bit more to see how it handles tach output.

Yes, pulsed signal for the tach. On a stock Fiero engine, this would be a 12v to ground pulse thru a filter. For a 4cyl tach, you need 2 pulses per crank rev (or 4 per cam rev). For a V6 tach, you need 3 pulses per crank rev or 6 pulses per cam rev. You can recalibrate the tach to work with a V8 if you change a certain capacitor on the circuit board of the tachometer in the dash. I have instructions about this on my website.

quote Pin C of C203: the SES light is on a pull down circuit?

Ground to turn on check engine light.

quote Pin E I don't think I'll be reusing a fiero style oil sender. Can I provide this gauge with a voltage signal that simulates the variable resistance of the pressure transducer?

I don't know if you will be able to get the stock Fiero oil pressure gauge to work off of an ECU output. As far as I know, every ECU I've ever seen can only output digital signals, not variable analog signals. Why not just use a simple 88 Fiero oil pressure sending unit rather than trying to reinvent the wheel here?

quote Pin E3 (C500) runs only the ignition? So I'll want this hooked to my coil pack control module only.

Yes this is an IGN1 (hot in run and crank) key-on 12v + power supply from the ignition switch for use to power your ignition coil or coil pack/module. Keep in mind this circuit is only protected by a fusible link and not a standard fuse.

quote Pin F (C203) powers the ECU and other electronics requiring +12V?

I recommend you ONLY use this to power your ECU(s) and MAF sensor (if you have one). IGN1 circuit, hot in run and crank only.

quote Pin H why wouldn't I use this? I can generate a signal at 2000 ppm from whatever the ECU uses. Is 2000 ppm at the dash gauge or is it used somewhere else in the engine / ecu? I'll be running an F40 trans FWIW.

The stock Fiero speedometer has a 2000 ppm output that travels to pin H of C203 then on to the stock Fiero ECM's VSS input. The Fiero speedometer supplies this signal to the ECM. THIS IS NOT a 2000 ppm signal the ECM generates and sends to the Fiero speedo. If your ECU can use a 2000ppm VSS input, then you can use this circuit. Otherwise, it is useless.

[This message has been edited by Darth Fiero (edited 10-24-2013).]

Is the C500 connector something that Fiero owners should think about replacing at some point?
Mine looks like a time bomb waiting to go off.
Or if it ain't broke don't fix it?

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1984 Indy Fiero

The nasty jelly that GM used to protect the connections? That is just what GM used.

Are the power wires getting hot and melting C500? That is a problem on some 84s. GM didn't run the fuse box power through C500 85-88

If you ask me the worst part of wiring for a new harness is the C500 connector in the engine compartment. Lot's of gook and replacement connector pins there are hard to find. I've been using recycled ones from an old Fiero connector. The good news is that not many new connections must be made there. As Ryan said the Lt Green (or yellow wire to the C2 pin) comes from the center pin of the GM three wire ETS/CTS . That sensor supplies the PCM and gauge with temperature related resistance values (not voltages).

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite.
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