in tracking down a fuel problem I could not make heads or tails of the fuel pump relay; the document I had for it was not accurate. So I took it apart to understand if it was working correctly. Thought I would share what I gained since I had such terrible time finding this info. Sorry I didn't take pictures. The part is from 1979 Toyota Long bed, carbotrated, 20R, electronic fuel pump in tank. The Fuel Pump relay is in the cab to the right and above the steering column. Silver metal box with a tab for an attacment point. 28380-20010 (??) 056700-3640 (ND) I have attached a scematic I drew. Basically I lettered the tabs on the relay from A to E. Between B and D are a coil and 1k resistor (colors were hard to see, looked like 1K) in parallel. The coil activates the switch (CE CA) when the coil is unpowered the switch is set at CE when powered it is CA. The two inputs D and C are Battery (via Ignition) and ouput from some oil pressure sensor somewhere (if there is no/low oil pressure the fuel pump cuts out). I did not determine if the battery or the oil pressure sensor powers the coil (at D), since I determined the relay worked and 12V was available at both (D and C), it didn't matter to me. Testing the relay: Unpowered: CE - short (almost zero resistance). BD - ~74 ohms All others open (inifinte resistance) I did not test powered (I manually moved the switch and checked), but CE should become open and CA should be come short. Putting it in the car leaving the case off, I saw the switch change from CE to CA and since 12V was available at the sockets, I was happy the relay was working. This almost certianly means that A is the connection to the fuel pump. I would love to test between A and the top of the fuel pump, but I can not do this without lowering the gas tank (this weeks plan) as my head is too large. I knew there was a reason I should have had a child!
Good job, I,ve never sat down and tried to figure out the stock wiring on the relay, but I use the same relay on my rig rewired differently than stock to power my aftermarket pump, the way I wired it serves as a safety shutoff, plus I have the ability to prime the fuel system if its been parked for a time and I don't want to crank on it till it gets oil pressure to activate the pump.
Awesome post. Just used it to diagnose my 78 pickup. Looks like I have an intermittent electrical failure to the pump. Going to run a wire from the battery to the pump tomorrow to see if everything works.
Isn't his mechanical as well? Thought that's when they started. My 77 is in the tank. If it's in the tank, check for power to the pump there. I just went through replacing my fuel pump when really it was a bad connection at the tank.
Very useful info!! Thank you very much!! Does anyone know the location of the electric pump resistor?? That is next to test/inspect. Thank you again!!
