Starter relay doesn't energize

I have a 2002 tundra 4x4 4.7 V8. Replaced starter relay with new Denso one. I can bypass the relay, jump pins 5 and 3 and truck starts and runs. Voltage reading at pins 1 and 5 are 12.7 volts. Checked to make sure relay operated by applying 12 volts and gnd on pins 1 and 2 of relay. Replace ST2 5a fuse in engine bay fuse box. Now here's where my brain hurts, the 12v at pin 1 of the relay when relay is connected drops to .3 volts when trying to start. I also jumped from pin 5 to pin 1 and the same thing happens. I have also direct grounded pin 2 of the relay when trying to start and relay doesn't energize. I have traced wires and can't find an open circuit, there is something causing a voltage drop and i can find it. I'm hoping its not the ECU or could it be the ECU. Thanks for any help.

 

Uh, you know the relay has two power lines coming off it, right? One passes direct to the pump. The other passes thru the finned fuel resistor (opn.c, IIRC) that's sitting on the fender in this location (on the newer years it moved to passenger side, next to the PS reservoir), which drops the voltage on the line going to the pump. This is to toggle between low-flow/hi-flow modes.

Sorry if I'm misunderstanding what you're saying and you know this already.

 

This is the schematic that I'm using and did not know or shows the finned fuel resistor in the circuit. I guess I'm using the wrong or missing schematic parts.

 

Download your apporpriate Service Manual. Get the link in the 4th or 5th paragraph here.

Meanwhile, see this post for the fuel circuit PDF for the '03 which should be generally similar if not identical to your '02.

 

Ok I downloaded (2002 Tundra EWD - EWD468U) from @RainMan_PNW posts and the service manual. I cannot find where you refer to it having "two power lines coming off it". From the wiring diagram I sent it does not show any connection to the fuel pump. If it does please advise of its location on the wiring diagram. Thanks!

 

Did you download the doc I shared with the fuel circuit in my last reply? If not, you really should. Two diff't feeds to the pump, unless I'm seriously suffering from Friday brainfry.

 

Check these

 

2 is your ground, 3 should be 12v measured to ground. 1 should have voltage when the key is turned to the start position. 5 is current out to starter when relay is energized.

 

Unfortunately we all suffer some "brainfry" periodically. Since the OP is dealing with "Starter relay", fuel pump and any of it's circuitry have nada to do with starter circuit.

re-reading OP first post before commenting about the results being seen

 

If your truck starts when you jump 3-5 the you know the starter is good. You then need to verify your ohms to ground on pin 2. If that is good you need to verify your ignition and neutral safety switch.

 

Please do not bypass any fuses for testing. You could burn up your ecu.

 

Yep, it's friday brainfry.

Pulled a 12hr yesterday of BS that was mentally fatiguing. I've been a mess most of the day.

 

Oh and sorry @growupoman

 

I really appreicate all of your replys. What is throwing me for a loop is when i jump pins 5 to 3 the truck starts and run just fine. When I place ground on the starter relay side to by pass any other ground path the voltage drops to .3v and no relay operation. When I tested ground on pin 2 in the on and start position no solid gnd however, from the start position to on position the meter flashes 14 megs. Since it starts with the jumper would that rule out a bad ignition switch or is there a ghost in there. I did replace the neutral sw also. Hey @shifty we all have been down that BS work me to death days i appreicate your help.

 

* Focus on pin 2 at the moment to eliminate this before proceeding.

Get your meter set on continuity to ground and first test the pin 2 off the relay, then from J2 and see if that is all good. If not then go back to J19 and do the same. Ensure the wiring does have path to ground. You're try to eliminate sections of wiring since you said you didn't have a good ground.

 

You cannot test ohms with voltage applied. Test pin 2 ohms to ground with key off.

 

In theory, when I ground pin 2 of the starter relay, the relay should operate when the ignition is in the start position. Correct? Mine does not. @TX-TRD1stGEN your schematic you posted shows a 2003, mine is a 2002 which is the schematic shown here. According to this schematic there should not be 12v on pin 1 of the starter relay until its in the start position. I do get 12v in the on/run position, then it drops to .3v in the start position. In this schematic I don't see how that's possible to have 12v at pin 1, must have wires crossed between starter relay and igniton sw? At the same time when I jumper pins 5 to 3 the truck will start and run.

 

Have you worked on or replaced the ignition?

 

Ok i have replaced the igniton sw, replaced fusses, starter and starter relay. It is still doing the same thing. My next step is to trace wires to see what is open, shorted or grounded cause a voltage drop. Unless someone has a better and a more logical explanation of what might be the problem. Thanks for any and all help. With my many years in electronic from the military and building hot rods ('32, '52, '54', '55, '56 chevy trucks) this one is whooping my butt.

 

I forgot, I also replaced the neutral switch. I guess the other thing to do is replace the ECU after tracing wires.

 

If you remove the relay is there voltage in the #1 location? With key off or with key on start pos.

 

Ok after tracing and verifying continuity of the pink wire to ECU I do not get gnd when ignition is turned on or in start position. Pin from ECU for Pink wire reads 2.3 megs either on or start position. @TX-TRD1stGEN i get voltage in the start position only on #1 of the start relay. I think my only choice is to get another ECU. I did gnd pin #2 of start relay and it does operate in the start position. I will update after new ECU is installed.

 

If you're willing to do used, pull your glovebox and check for the ####-##### part number off the ECU. Flagship One is a great source for reasonable pricing: https://www.fs1inc.com/

 

You don't have any "starter interrupt" devices on your truck do you? Security system or otherwise? This problem has not come up on here before that I recall related to ECU. All of your ground wires are good? not by appearance but by testing them? Starter ground? Chassis ground? ECU Ground? This reeks of Neutral safety switch or other interrupt device. The switch you replaced was OEM?

 

The only device I found was the entry remote box. I don't know of any other reason what would cause the ECU from not providing ground on the pink wire coming from the ECU in the on or start position. I there another way to check the ECU for ground?

 

Just a quick update, I have replaced all components in the starter relay circuit to include the ecu. Something is causing the ECU to not send gnd to the relay. I can bypass the relay (jumper pins 3 and 5) or manually ground pin 2 for relay operation, the truck will start and run great. Not having an ECU schematic or what if scenarios I'm at a loss.

 

Changing out the ECU will in no way fix your lack of ground at pin 2 of starter relay. It's not even part of the sequence of the starter circuit.

The Park Neutral switch IS part of starter circuit for applying a ground to starter relay when in Park. Since you have replaced the neutral switch, I suggest you re-visit your work in case the loss of ground is in that area.
Follow the Ground and the red arrows. With the EWD you can locate any and all junction points in order to check for loss of gnd.




Even tho I am an 04, the reading of pin 2 to batt gnd should still be about 1 ohm while in Park. No need to worry about putting a key in ignition for that check.

 

Exactly right. Neutral safety switch controls ground. Ecm just reads ground to verify it can fire.

the IF ground point used for this circuit is shown here (left kick panel).

 

Bypass p/n switch as a test? Not sure what's in the guts of it, hence the "?"

I remember also @bfunke posting this recently about how the p/n switch being improperly installed/adjusted can cause failed starts. (link to thread)

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Just wanted to say problem solved!! Special thanks goes out to @BubbaW for his directional drawing, had no idea that was the path for ground. Seriously thought it was the ECU providing groound. What was happening in my continunity test, I would disconnect the neutral switch connector to check pin 9 to starter relay, tested good. Then by moving the harness back to the connector I would lose continunity because the pink wire would moved inside of the harness. The only way I found this was I moved the harness to the driver side (to test the pink wire in the harness) and when I did (as my picture shows) the pink wire came out of the harness. This portion of the harness is on top of the transmission with no way to see if there had been any damages to it without disconnecting it from the passenger side and moving it to the driver side. What caused it to break, I'm not sure. The wire loom on top was torn a little but not bad.

I would like to say this forum saved my butt and me not pulling my hair out, was driving me crazy!! Thanks to all who providing information leading me on my journey to find the problem. I hope this will help the next person who thinks they have checked and rechecked their tests and thinks its all good like I did.