Fuel Pump Diagnostics for the Turbo Coupe

Fuel Pump Diagnostics for the Turbo Coupe
By Jeff Korn

Introduction

For an engine to run, it needs air, compression, properly timed ignition and valve opening / closing events, and fuel.  In most fuel-injected engines, fuel is supplied to the engine from an electrically powered high-pressure pump inside the fuel tank.  Power is supplied to the pump through a fuel pump relay, inertia switch, and associated wiring.  The operation of the fuel pump relay is controlled by the EEC (electronic engine control) computer.  If a no start or dies while running concern is present, it is possible that a component of the fuel system is at fault.  The following article describes the theory of operation, and diagnostic procedures of the fuel system.

Theory of operation

Refer to the figure below that shows the basic wiring of the fuel pump electrical system for the 87-88 Turbo Coupe. 83-86 Turbo Coupe wiring is a little different, as explained below.

   

Component locations:

• EEC – behind passenger side kick panel
  
• EEC TEST connector – under hood
  
• Fuel pump – inside gas tank
  
• Inertia switch – behind trunk trim on tail light header panel (87-88), or, on 83-86, above passenger side rear wheel well.
  

Fuel pump relay – part of IRCM (integrated relay control module, the black box mounted on the front side of the passenger side strut tower under the hood) on 87-88, or, on 83-86, above passenger side rear wheel well.

The fuel pump relay has four terminals. Two are for the relay coil, and two are for the relay contacts. When the coil is energized, the contacts close, and 12 volts is supplied to the fuel pump through the inertia switch. (Note 83-86 inertia switch is in the coil circuit as explained below.) The inertia switch shuts off power to the fuel pump in the event of a collision. The operation of the fuel pump relay is controlled by the EEC. One side of the relay coil is supplied with 12 volts whenever the key is in START or RUN via the EEC power relay. If the EEC wants the relay to close to run the fuel pump, it grounds the other coil terminal. When the key is first turned to RUN, the EEC runs the fuel pump for roughly 2 seconds to pressurize the fuel system. This is called the prime cycle. If the key is only turned to RUN, the pump shuts off after about 2 seconds. If the key is turned to START, and the engine cranks, the PIP (profile ignition pickup) Hall effect switch inside the distributor sends an RPM signal to the EEC. Whenever the EEC gets an RPM signal from the PIP, it will also energize the fuel pump relay. If the engine dies, and the EEC no longer gets the RPM signal from the PIP, the EEC will open the relay to shut off the fuel pump.

Diagnosis

Please note that the instructions are a little different for 87-88 and 83-86, since wiring on 83-86 Turbo Coupe is a little different. The main differences are: 1) The fuel pump relay is a separate relay above the passenger side rear wheel well. 2) The inertia switch is located near the relay above the passenger side wheel well. 3) The inertia switch is wired in the relay coil circuit, not the fuel pump power circuit as it in on 87-88. The 83-86 wiring system with the inertia switch in the low current relay coil circuit instead of the high current pump power circuit is a MUCH better system than the 87-88 system, in my opinion.

Tools required:

• An EFI fuel pressure gauge (highly recommended!!). A tire pressure gauge can also be used, but I DO NOT recommend using one! Just pushing in on the Schrader valve to see if there is pressure present is NOT sufficient!!!
  
• A 12 volt test light
  
• A DMM (digital multimeter)
  

The first thing to do is to see if the fuel pump runs. First, listen for the pump to run its prime cycle when the key is turned to RUN, or turn the key to RUN, and use a jumper wire to ground the fuel pump test terminal on the EEC TEST connector (see diagram for details on which terminal to ground). The fuel pump should run constantly for as long as the fuel pump test terminal is jumpered to ground. If the pump runs, the problem is mechanical, not electrical. To test the mechanical part of the system, attach a fuel pressure gauge to the Schrader valve on the fuel rail, and measure fuel pressure with the pump running (key at RUN, fuel pump test terminal on EEC test connector jumpered to ground). Pressure should be roughly 40 psi. If pressure is much higher, like 60+ psi, either the fuel pressure regulator is bad, or the fuel return line to the gas tank is restricted. If the fuel pressure is much lower than 40 psi, like 20 psi, either the pump is weak, the fuel filter or supply lines are restricted, the pressure regulator is bad, or the short hose inside the gas tank that connects the pump to the bung on the mounting flange is leaking (rare, but not unheard of!). Note that the engine will not start if fuel pressure is much under 20 psi. This is why just depressing the Schrader valve to see if fuel squirts out is not a sufficient test. 20 psi fuel pressure will still cause gas to shoot out the valve, but pressure is still too low to run the engine.

If the pump doesn’t run its prime cycle, or doesn’t run when the key is in RUN, and the fuel pump test terminal on the EEC TEST connector is jumpered to ground, you have an electrical problem.

If you have determined you have an electrical problem, do the following:

1. Verify that the inertia switch hasn’t been tripped by pressing down fairly hard on the reset button. Try to start the car. If it starts, problem solved! If it doesn’t start, proceed to step two.
   
2. Verify that the EEC is powered up (via the EEC power relay). If you have a CEL (check engine light) this is easy, as the CEL will come on when the key is turned to RUN if the EEC is receiving power. If you don’t have a CEL, turn the key to RUN, and use a DMM and measure the voltage at the TPS (throttle position sensor) electrical connector between the orange and black wires. If the EEC is powered up, you will get close to 5 volts. If you get no voltage, you have an EEC or EEC power relay problem.
   
3. Assuming the EEC is getting power, listen for the fuel pump relay clicking when the fuel pump test terminal on the EEC TEST connector is jumpered to ground and disconnected, and that the relay clicks on and off when the key is turned to run, indicating that the EEC is trying to run the prime cycle. If the relay does not click, use a 12 V test light connected between test point A (see diagram) and ground. The test light should light up with the key in RUN and START. Test point A is at pin 24, the red wire, at the IRCM on 87-88. On 83-86, Test point A would be at the red wire at the pump relay socket. Note an open / inoperative inertia switch could cause the light to not light up on 83-86. If the light doesn’t light on 83-86, disconnect the inertia switch, and jumper the switch connector. If the light now lights up, replace the inertia switch. If the light does light up, use a DMM to verify continuity from terminal 18, tan / light green wire of the IRCM to pin 22 of the EEC for 87-88. If the light does light up on 83-86, use a DMM to verify continuity from the tan / light green wire of the relay socket to pin 22 of the EEC. If these tests check out OK, and the relay still doesn’t click, replace the IRCM (87-88) or the fuel pump relay (83-86). If the relay clicks, proceed to step four.
   
4. If you can hear the relay click, do the following: On 87-88, back probe pin 5 of the IRCM connector (pink / black wire), and connect the test light between pin 5 and ground. This is test point C on the diagram. Energize the pump relay as mentioned earlier. The light should light up. If it doesn’t light up, replace the IRCM. On 83-86, back probe the relay socket terminal with the pink / black wire, and connect the test light from the pink / black wire to ground (Relay must be in its socket for this test). Energize the pump relay as mentioned earlier. The light should light up. If it doesn’t light up, use the test light to verify that the yellow wire at the relay socket is hot at all times. If the yellow wire is OK, but the light doesn’t light up when on the pink / black wire when the relay is energized, replace the relay. If the light lights up when the relay is energized (all years), proceed to the next step.
   
5. On 87-88, disconnect the connector on the inertia switch. It will probably be necessary to remove the switch to get the connector off. With the key OFF, use a DMM set on ohms to measure the resistance of the inertia switch. It should be zero ohms. If it isn’t zero to a few tenths of an ohm, replace the inertia switch. If the inertia switch checks out, measure resistance from each terminal in the inertia switch connector to ground. One should read infinite. This is the power feed from the fuel pump relay. The other terminal should read roughly an ohm (the resistance of the fuel pump). On 87-88, this point is indicated as test point D on the diagram. This terminal feeds power to the pump. If both terminals read infinite (or very large) resistance to ground, either the fuel pump is bad, the fuel pump ground is disconnected, the connector on the top of the gas tank is bad, or the wire from the inertia switch to the pump is open. To determine which terminal in the inertia switch connector is which, connect the test light from each terminal (one at a time) to ground while energizing the pump relay. One terminal will light the light up. This is the power feed from the relay. If neither terminal lights the light, the pink / black wire from the IRCM to the inertia switch is open. Assuming one terminal lights the light when the relay is energized, turn your attention to the other terminal. If it reads infinite (or vary large) resistance to ground, you will have to drop the gas tank to inspect the connector on top of the tank and / or the pump. If the terminal reads near one ohm, run one final test. Run a long jumper from the battery positive terminal to the connector terminal. The pump should run if it is OK. If the pump doesn’t run, drop the gas tank to inspect / replace the pump. On 83-88, Remove the relay from its socket, and measure resistance of the pink / black wire to ground. It should be about one ohm. If it reads infinite (or very large) resistance, either the fuel pump is bad, the fuel pump ground is disconnected, the connector on the top of the gas tank is bad, or the pink / black wire from the relay to the pump is open. If the terminal reads near one ohm, run one final test. Run a long jumper from the battery positive terminal to the connector terminal. The pump should run if it is OK. If the pump doesn’t run, drop the gas tank to inspect / replace the pump.