1990 Q45 FPCU and Pump relay

[bwyck]

Hi to all, This is my first time posting and after reading so many post's I figure this is the best place to go.

I have read as many posts as I could find on here about The Fuel pump, FPCU, and relay.

I'll try not to confuse anyone with my questions so bear with me.To start this is a friends car I was trying to help him save some money.

I first repalced the fuel pump as the old one was 100% bad. I checked the FPCU which seemed to be in good order, no burnt or heated up spots or solders points. I put it all back together and no start. ( Car turns over fine )I checked power to everything and all seems good. Relay would not switch so I put in new relay and still no switch. I Checked the relay straight from the battery and it switched just fine but will not work in relay harness.

Here's were I'll try not to confuse anyone.

A: The hot wires are Black/White and Blue/Black and both are getting voltage to relay harness.B: With the relay in-- the Orange/Blue wire should be hot and sending volts to the FPCU and Pump-- but is not.C: There is also a Pink/Blue wire that I'm guessing is the wire that should be switching the relay. This wire runs from the ECU and with the key in the on position and the relay hooked up the wire has voltage. D: Here's the tricky part, If I ground that particular wire with the relay hooked up it will switch over like its supposed to and the car will start. I'm guessing that the ECU should be making that switch but is not. ( I"m hoping I'm wrong!)

Now with relay out,A: I can jump from Black/White or Blue/Black to the Orange/Blue( wire to both the FPCU and Pump) and the car will start right up and run great.I know this would be a sure sign that the relay is bad but both relays tested good. ( Read previous section)

Last Question for now,When the car is running and I check voltage at the FPCU the White wire ( The wire I thought was ground ) is sending out 8 Volts. Now please correct me if I'm wrong but shouldn't ground be 0 volts or is the white wire not ground for FPCU to Pump.

Thanks in advance for any help.

[Q45tech]

The FPCU unit acts like a large variable resistor except it is made up of switching transistors.

Reading the non +12 lead from the fuel pump to ground with a vom will show a voltage at idle and the voltage will decrease as the pump goes into higher speed modes.

The pump is never truely grounded sense there is always some resistance [effect] in FPCU.

If the ecu does not engage relay [thru a switch transistor that acts like a ground] the transistor may have been blown open by applying +12 volts to that wire or a natural failure from a shorted relay.

One must be careful in trouble shooting any lead that goes to ecu because it is full of switching transistors that ground external circuits [8 for coils, 8 for injectors, and one for every relay it controls.

Accidently putting +12v on any of these leads will immediately blow open the switching transistor making ecu worthless.

Common when some send +12 on a ecu injector control line.

Every ecu made for every car works like this ecu supplies grounds to coils,injectors, relays.

[qsiguy]

Don't have time to run through it with you but I'll check the thread later. If you dont' have the FSM this should give some assistance. This is the Fuel Pump system for a '94 Q45 which I think is the same. Someone correct me if I'm wrong. I saved the fuel pump pages and posted it on my web site in the tech directory.

http://c4caraudio.com/tech/Fue...5.pdf

[maxnix]

Did you replace the fuel pump fuse (I guess you must have)? Fuel filter?

[qsiguy]

He says he can ground the Pink/blue wire (you sure it's not pink/black per FSM?) and the relay switches on and it will start. Fuses are ok, the output from the ECU is inop or not making it from the ECU to the relay.

Take some ohm readings per the FSM section I gave you the link to above between the pink/blue wire at the relay to the ECU harness at the ECU to make sure there is not a break in the wire somewhere.

If you need an ECU you can find them pretty cheap so don't panic if you need a new one. I just picked up one on ebay for $30 plus shipping. I have three of them now. You can get them all day long for around $50-80 used from salvage yards and from members on this forum.

[bwyck]

Thank you all very much. Its nice to be able to talk to people who know there stuff.

Thanks for the info on the FPCU, That solved that Question.

As far as the relay goes I tested between the relay harness and ECU harness and no broken wire, evan tried 3 different relays none that worked correctly so I'd have to say that the transistor is blown and I need a different ECU. I will be looking here on Nico for one.(By the way, my bad, the wire is pink /black, sorry.)Don't really know If I accidently did or what but regardless its not working.

How long can the car be driven with the relay bypassed?

(Quote from Q45tech): Accidently putting +12v on any of these leads will immediately blow open the switching transistor making ECU worthless.Question: When I hook in the relay the (pink/black) wire has +12v being sent to the ECU via the relay unless I manually ground it. I'm confused as to why.

I do not have a FSM so thank you for the diagram, that helped tremendouly. The disgram matched exactly the 90 Q45 fuell system.Any chance you could send fuse panel description as to what fuse is for what. The fuse cover is gone and I have no idea what fuse or relay is for what in this car and I'll be doing alot of work on in the near future and diagrams would be helpful. I'll try to get the guy that owns the car to buy one!

Hopefully I can get some more answers from you guys cause it makes my life much easier with someone that knows the Q's.

Thanks

[mxr662]

http://q45.org/gallery/v/mxr66....html

The fuel pump fuse is the yellow one(15 amp) 7th one down on right side on my 90q.

Andy

[DrewQ45]

I have FPCU and even fuel pump if needed...

zerothread/241278

[qsiguy]

Question: When I hook in the relay the (pink/black) wire has +12v being sent to the ECU via the relay unless I manually ground it. I'm confused as to why.

The relay coil is just that, a coil of wire. When you plug in the relay and there is no signal being sent to the relay on the pink/black wire the 12volts that is on the other side of the relay coil will pass through the relay and show up on the pink/black wire. It is extremely low current so will not damage the ECU output.

If it were mine and I was waiting to get a new ECU I would probably wire up a relay to switched ignition (one that stays on while the key is in the crank position) and switch a ground to the fuel pump relay on the pink/black wire. The FPCU will still function properly to alter the fuel pressure (assuming the ECU output for that is ok). You should be able to run like that without problems for a long time. I just wouldn't turn the key on and let it sit without the engine running. I'm not sure if the FPCU will turn off the pump when there is no demand. Shouldn't really matter anyway as the fuel will just be returned to the tank on the return line and just sit there and circulate.

Let me know if you need a relay diagram to accomplish that.

By the way, that $30 ECU I picked up doesn't work properly. I need to do some diagnostics on it but I think there may be an injector output out or something, it will start but runs rough with that ECU installed. Moral of the story, never know what you will get from E-Bay. I picked up one from a salvage yard it it's been great. I got the E-Bay one to experiment with anyway so I'm going to see if I can fix it.