Author Topic: C4 1987 Throttle Body Coolant Bypass  (Read 1369 times)

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Offline 87Z51

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Re: C4 1987 Throttle Body Coolant Bypass
« Reply #24 on: May 03, 2021, 04:22:30 PM »
(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

Offline 87Z51

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Re: C4 1987 Throttle Body Coolant Bypass
« Reply #25 on: May 03, 2021, 04:23:46 PM »
Power goes from battery through BOTH FP relay and OP switch to pump at same time once engine is running

Offline Vettech

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Re: C4 1987 Throttle Body Coolant Bypass
« Reply #26 on: May 04, 2021, 10:56:15 PM »
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.

Vettech.

Offline MY081

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Re: C4 1987 Throttle Body Coolant Bypass
« Reply #27 on: May 05, 2021, 09:08:27 AM »
Maybe 11PM is not the best time to fix the problems of the Corvette World. Uncle Doug. Ray

Offline Vettech

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Re: C4 1987 Throttle Body Coolant Bypass
« Reply #28 on: May 05, 2021, 07:29:13 PM »
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.

 :therethere:
Vettech.

Online sirfixalot

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Re: C4 1987 Throttle Body Coolant Bypass
« Reply #29 on: May 05, 2021, 08:31:16 PM »
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.

Offline Vettech

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Re: C4 1987 Throttle Body Coolant Bypass
« Reply #30 on: Today at 01:55:50 PM »
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.  :banghead:

Vettech.