Other day the truck fried the fuel pump it had a faulty relay that left the pump going when I went to bed , me and my buddys lifted the bed up to change the bad pump it ended up frying the sending unit also now as a man on a tight budget I put a 12volt switch on the pump to engage it when I need fuel and turn it off when I get out of the truck ... then I started thr truck no problem but now my pedal for the brake goes down to the floor eithout me touching it ... any idea what might cause this ? I'm dumfounded on this ... any info would be helpful
does your truck have HYDRO BOOST where the assist for the power brakes is with hydraulic pressure from the power steering pump...or is it vacuum assist power brakes..either way.. you need to have or replace either. one..open the glove compartment...look at the label with all the numbers in rows.. look for J numbers it will be a 3 digit code..this label says JD7... it could be in any location. but in alphabetical order... Application: With JB8 brake system Notes: With hydroboost Application: With JB7 brake system Notes: With vacuum booster Application: With JB6 brake system Notes: With vacuum boosterif your truck is close to this applicationThis product is a fit for your:1989 GMC Truck C2500 3/4ton P/U 2WD 5.7L TBI 8cyl
PS. its probably the 2 or 3 wire OIL pressure switch thats keeping the fuel pump running.. the fuel pump relay is controlled by the ECM/PCMthe oil pressure switches outer terminals are controlled by oil pressure..both of these are in parallel.. so either can turn the pump on..i have a test that i perform on GM fuel injected trucks..engine running.. i pull the fuel pump relay.. if the engine continues to run.. then the oil pressure switch contacts are working..but if the fuel pump continues to run after shut down or key off.. and also runs with the fuel pump relay removed.. then the oil pressure switch contacts have failed..you can see the contacts inside the cap... and the end of the bar that pushes up to bridge them.. the center terminal ----\____ is for the oil pressure gauge..
if you get a replacement sending unit.. please verify that the threads are TAPERED .. NOT STRAIGHT.. i don't know why but some brands the threads on these are 1/2"-13 where the tapered thread should be 1/4-12 NPT male. they will fit.. they tend to send out strands of wire ripped from the threads.. the part in this image has the WRONG straight thread.. the cover was also NOT glued on.. so when i pulled the socket off.. the cover came up also.. this switch failed and kept the fuel pump running until the battery was discharged.. if the relay melted and the relay socket melted... its probably because the oil pressure switch failed and placed all the load on the relay that was not designed to handle all the amperage...i am hoping that you have also changed the fuel filter.. as blocked fuel filters increase the pumping pressure and amp draw of the pump causing relay and oil pressure switch failures... before the pump fails..
as for the fuel sending unit also failing.. burning up.. the fuel pump ground connection on the frame rail might be loose.. or you could have an issue at the front of the truck.. where the negative cable is attached to the engine block.. and from there to the cab firewall... from there .. from the passenger side forward cab mount to the frame rail... or there could be a ground from the engine block to the frame rails...i have a quick test...if you own a digital volt meter to prove the grounds are properly connected..engine running, headlights on.. digital volt meter set to 20 volts DC..1. Negative battery post to the positive battery post .. 14.1 to 14.8 volts DC..2. Negative battery post to the engine block.. 0.04 volts DC max..3. Negative battery post t the body.. 0.02 volts DC max..4. engine block to the body... 0.02 volts DC..and since this is a chevy pickup..5. Battery negative to the frame.. 0.02 volts DC MAX..6. firewall to the frame... 0.02 volts DC maxthis should take you under 2 minutes to run this test..please post results by number..184.108.40.206.5.6.