C4 1987 Throttle Body Coolant Bypass

[Vettech]

I take it all back - I lied!, there has been water on the floor, water level is low, but now not leaking, no indication of source underneath the car, but since it has stopped, its more than likely well up top.

After much looking it appears the "brass T piece" I made up to link the heater return water and KC4 return water back into the water pump side port is at fault. I'd like to say its just the hose clamps not tight enough, but since it required an in-ordinant amount of 15% Silver Solder to patch it together, I'll hazzard a guess it will be crook brazzing.

Bugga! Its under the A/C compressor, ac frame, tensioner  -  a job for Ron.

Maybe if I just get it out of the garage, I can then find that gallon of petrol !?!?!?!?!??......

[Vettech]

Took Bob three goes to fix what I had f#'fd up, just shows how much effort I put in.
This time we hydro pressure tested the part till we got it right.
As much as I'd like to take the accolades, its all down to years of experience, a keen eye and the deft hand of Bob.
Not to mention a nice hot Oxy tip and 45% S'solder.

Thank you Robert. Now ...   to stick it all back together, at least now I have the special T45 tool to remove THAT BOLT.

[87Z51]

Could have saved on 4 screws

[Vettech]

Ok - that looks the part.

[87Z51]

(Keep in mind that on C4's once started the fuel pumps power is supplied by the Oil Press Switch as a fail safe in the event of loss of engine oil pressure.)

Popular myth .
The OP switch runs the pump directly when cranking the engine over ( after it reaches a specified pressure )                                                                                               
 Once the engine fires up , the fuel pump is fed ( in parallel ) through the FP relay  controlled by the ECM .                                                                                     
 As you can see from the factory wiring diagram  , with the engine running you can disconnect the OP switch completely and the pump will still get power - supplied through the relay.
Purpose of switch is to let you start and maintain fuel pump operation should the relay fail

[87Z51]

Power goes from battery through BOTH FP relay and OP switch to pump at same time once engine is running

[Vettech]

I will be replying in full, but when I've found the info.
I must admit it took me awhile to find your cct pic in my books.

Stay Tuned.  My biggest issue is squeezing my pics small enough.

P.S plus now you have sown doubt, I will do a test, but the 87 is in bits, and I'm reluctant to use the 90.

[MY081]

Maybe 11PM is not the best time to fix the problems of the Corvette World. Uncle Doug. Ray

[Vettech]

Only 22.56.15 not 2300hrs, so there Ray - get it right.

I know that 2200hrs (10pm for interlectuals) is past you Old Farts bedtime, but some of us keep on keepng on.

 

[sirfixalot]

Fuel pump relay is shown in de energised position, relay is only energised  by ecm for a few seconds to get fuel pressure for start up circuit 465. Engine starts and allows 12V+ via oil pressure switch to fuel pump 12V + goes to diagnostic plug when fuel pump relay is de energised only. Both circuits are never energised at same time it is one or the other 12v back feeds to all points of both circuits. If the oil pressure switch is disconnected engine will stop, this is a safety feature that if engine stops ie in a front crash pump stops and doesn't poor fuel into engine bay if fuel line is damaged.

[Vettech]

I agree Sirfixalot, the text and even notations on the ccts indicate an ~ 2sec relay "on" time at ignition switched on so as to pressurize the fuel rail. The text is sparse and vague and does not specifically say much at all. See blowup of part of schematic.

HOWEVER ! Having said above, and placing a test lamp into the ALDL "G" plug - my 90 is either wired differently to the 87 schematic or I have a wiring problem, or incorrect relay being used, or wires in wrong connections. Why !?. I plug lamp into "G" (to earth), ignition on, and lamp comes on, start engine, the lamp stays on !?!?!?!  My understanding is the "G" is off relay NC contact "D" and upon relay activation it should open  - ummmm.?....

Could be:
Relay - wrong type or RS.
Oil pressure switch bypassed. (This depends on the +12v being swiched)

87 and earlier have issues with relays (MAF, BURNOFF & Fuel) and I see comments that they are the same.!?
THEY DEFINITELY ARE NOT THE SAME. They look the same, but internally the contactors are quiet different, and perform different functions. 

[sirfixalot]

By the top diagram test light at G test point on diagnostic plug should be off until ECM turns off Fuel Pump Relay, Engine starts & builds oil pressure which should then put 12V onto G test point, Test light should then stay on via oil pressure switch until engine is turned off

[87Z51]

relay is only energised  by ecm for a few seconds to get fuel pressure for start up circuit 465. Engine starts and allows 12V+ via oil pressure switch to fuel pump 12V + goes to diagnostic plug when fuel pump relay is de energised only. Both circuits are never energised at same time it is one or the other

Cut and pasted from that other forum because I can't be bothered to rewrite it ( posted 20 years ago and still argued today )

The fuel pump has two, independent sources of power.  The oil pressure switch is a N.O. pressure switch with an approx 4 psi trigger,                                     
  When the oil pressure at the switch is ~4 psi or more, the switch is closed and 12 volts is fed to the fuel pump.
Besides the initial 2 seconds after the key is turned to the "ON" position, when the
ECM receives pulses from the distributor that indicate the engine is turning, the ECM closes the primary circuit of the fuel pump relay, sending 12 volts to the fuel pump.                     

IF the engine stops turning AND the oil pressure drops below ~4 psi, the fuel pump should stop running. The engine stopping turning or low oil pressure ALONE won't remove the power from the fuel pump.
It takes both.                                                                                                                                                                                                                                     

The Oil Pressure Switch acts as a redundant power source for the Fuel Pump . If the Fuel Pump Relay went bad and the OP Switch were working and the ,            the car would still start and run, but the start time would be extended because the Fuel Rail wouldn't be primed without the relay operation and the pump wouldn't run until the engine cranked enough to build oil pressure and close the OP Switch and power the pump.

[Vettech]

No arguement - a discussion.

And I've seen said same arg☆☆☆ discussion on several US sites.

I love material proof, so I'll find the fuel relay, since its all hanging down at the moment, and with car running, I'll pull the relay.
(This test will be on the 90, I assume same cct's as 87, but with relay located above passengers foot well)

And I've also read the ECM gets pulses, but I read this as the ECM interprets this as indication engine is running.

[Vettech]

TIME TO END THIS DISCUSSION

Keep in mind this test I just performed was on a 90 C4. To the best of my knowledge and from what I could see, the wiring, wire colours are virtually the same as my 2x 87 GMSM's. The majour difference is that five relays including FUEL are now located above the LH passenger footwell, plse see pic below. It took a while to identify them as the mounting bar has vanished and they just hang about.

The five relays are: Fuel, Fog Lamps, Dome Lights, Rear Demister and Horn, all now marked in white painter pen. All 5 are now common garden variety Bosch/Lucas/Hella style auto relays.

The vehicle was started, ran for a few minutes to warm, the relay was pulled out and the car continued to run, obviously on the Oil Pressure Switch, this when considering my test lamp test which gave the lamp "ON" after the engine started, and the only way the test lamp in ALDL "G" can be on is if the relay is un-operated (idle), the "G" is connected to the "NC" contactor, my original observations were no doubt a bit haphazzard trying fumble plugs, wires, lamps, ign key etc

I have no doubt anyone looking at the first cct would come to the "double supply" conclusion. Worth noting that the dome light relay was quite hot to the touch, since the doors etc were open and its operated whilst I fumbled under the dash, if the fuel relay was on whilst ever the engine was running - I'd imagine it would get red hot.

[Vettech]

Further, the ECM is a computer consisting of amongst other things GATES - "OR" Gates, "NOR" Gates "AND" Gates to name a few.
It is not alive and therefore cant tell if the engine is running or not, unless it can see certain conditions or inputs on defined terminals.
Therefore; if it sees a fat +12v on terminal B2 ("Fuel Pump ON Input")"AND" pulses from the Dizzy on maybe terminal B5, then it outputs the signal that engine is running, and enables all systems accordingly.
Since Fuel Pump on "AND" Dizzy pulses = Engine Running.
This would roughly equate to one line of program code within the central processor.