Dougs951S

I'm making a new thread for this issue since it has now evolved. I'm getting really close to firing up my new N/A, I have gotten the injectors to pulse now but I have no power at the fuel pump. I tested this both cranking and with the relay jumped. I cleaned the ground beneath the carpet and I have continuity between the two pins of the fuel pump harness. I know terminal 30 of the DME where the pump gets it power has 12v since my ECU is getting power and trying to start. Sooooo.. how can I have power at the source, a good ground, continuity, and yet no power at the pump? Pump starts right up if jumped to the battery.

Toran

Did you check the fuse for the fuel pump?

Van

The DME relay is really 2 relays in one housing... The first relay switches on the DME, and then the second relay is triggered by the DME, through a ground connection, to send 12v to the fuel pump. So, it sounds like it's either this second part of the DME relay, the fuse, as Toran suggests, or the wire itself has a break.

Dougs951S

Van , I would agree but unless im not looking at it right, I dont see how. I jumped the relay board so that takes the (new) relay out of the equation. I checked the fuse. If I have continuity as the pump plug, doesnt that prove the wiring ia good and therefore I should have power? Unless the 12v wire is touching ground somewhere between the relay and the pump..

Van

So, you actually have 12v at the pump on the black/green wire when you put in your DME relay 3-way jumper?

konakat

Could be the pump ground. Try probing the two terminals of the pump with the relay jumpered. If you don't get any voltage, try probing between the chassis and each of the two terminals looking for voltage. If voltage is low or missing, you have a bad wire or connection somewhere along the way. If it's an early car the ICV runs off the fuel pump circuit, that may provide another data point and a more convenient place to check the circuit.

Dougs951S

No I do not have power even with the jumper in place and I have confirmed there is 12v at pin 30 of relay. I cleaned the ground right near the spare tire so im guessing the issue is in the power wire. My suspicion is that the 12v line is touching ground somewhere before the pump. I just dont want to start cutting into the harness to find out where.

Van

If the 12v is touching ground (shoring out) it will blow the fuse. If the line is cut (open) then the fuse will be good, but no voltage will be coming out.

joes

Take the DME/ fuel pump relay out. Check for 12V at terminal 30. Jumper terminal 30 and 87B. Now you have power to the fuel pump. If there is no power there you have a broken wire. If you have power, you have a bad ground.

Dougs951S

Boy I feel dumb, thanks van. I totally didnt think tha through, the ground is presumabely good (ill confirm if its a ground issue today by jumping battery - straight to the pump) and so is the fuse so it must be a break somewhere in the 12v line. I already have the rear carpet pulled so I guess ill start tracing and probing. Im guessing the issue is all the way up in the fuse box though, ive had some other weird electrical issues I traced all the way to the box, so I may be pulling and replacing that soon.

Dougs951S

More testing today yielded no answers, just more confusing questions. So without touching anything, I now have power at the pump. My explanation for this is last night I may not have had the jumper deep enough into the fuse block to make good contact. I think my pump is dead, it wont run now even when jumpered to a battery and its reading 1.7 ohms across the two terminals, when I believe it should be closer to 20 ohms. In any case, the weird thing is now that power is present at the plug, I wanted to test the wiring so I tried to get a resistance reading. There is no continuity, its reading an open circuit. This is the case even if I jump the negative probe of my fluke to the battery - while I have the positive probe on the hot pin of the pump harness. There is continuity between the battery - and the negative pin on the harness. How on earth can there be voltage present and be an open circuit? I tried it on every resistance setting on my meter, I can not pick up a resistance and it keeps showing open circuit. What. The. Hell?

So by jumping the probe to battery and removing the - side wiring, I have eliminated the ground as a source of possible issue. Yet...I have 12.5 volts across teh two pins? So what is going on? I believe the answer to this question will be what gets my car running. I just purchased a new HV (mercedes 450 SEL) pump to give me a bit extra headroom if I ever decide to supercharge this car.

mel_t_vin

What value(s) are you seeing/measuring via the different resistance scales?

Dougs951S

None. Open circuit. no continuity. Which is impossible because I have 12vdc (confirmed by arcing the power to a jumper cable attached to battery - and observing spark---plenty of amps there too, it was a HOT spark) and I'm using a really nice fluke, so it isnt like its a cheap meter. Works fine to measure pump resistance, I see 1.74 ohms across the pump pins on the pump its' self. did...did I just break physics? Hell for all I know maybe this is just an freak incident and now everything will be fine once I get my new pump in? It's just confusing, doesn't make any sense. I have power and ground....so I must have some kind of resistance, you'd think I'd be able to measure it..


Edit: just wanted to add another note about how I'm totally positive there is both good ground and power at the pump, if I short both pins to each other I get a nice hot spark, so plenty of voltage and amps (the stock pump draws 7 amps for the record), but no resistance....weird. At this stage I'm going to assume all is well wiring wise.

Dougs951S

Sorry to keep bumping this thread, I'm sort of talking to myself here and recording my progress. So, I tested the ground side of the circuit and now I wanted to test to the hot side. I jumped battery + directly to the ground pin of the pump harness for just a second, of course it sparked but didn't blow the 15A fuse. I then probed for resistance between battery+ and the negative pin on the pump harness. My fluke read 129.31 ohms, a totally valid value. Now..this is where it gets weird. Up to this point, the ONLY thing I've done between not being to get any resistance reading between the two harness pins and now was briefly short battery power directly to the ground on the harness. So then I checked again one last time for resistance between the two pins, to make sure I wasnt crazy. Sure enough, 132.34 ohms, basically the same as if I were connected straight to the battery. Hazzah! Both sides of the circuit magically work now! Also I think my battery is flattish in the sense that its not providing enough amps, if I jump my fuel pump to the battery it does not run, which lead me to believe it was dead. However to test that theory, I jumped the fuel pump with my 120v plug in battery jump starter/trickle charger and boom, it turned on. I have the battery on said charger right now overnight, I think now if I put the pump back in the car it ought to run! Still can't explain all the weirdness though, I'm no stranger to electrical testing (I built and installed my megasquirt no problem). I guess it was a "fluke" in my fluke? Any case, keep your fingers crossed for me folks. It looks like this is almost over.

konakat

A car battery shouldn't have any trouble powering a fuel pump. I think you still have an issue somewhere. Broken wires can be a bitch to diagnose because the insulation can be intact but the copper inside is broken. The two broken ends are so close together that sometimes they touch and sometimes they don't. I would suggest jumpering the relay so the pump runs (or should be running), then move the wiring harness around a lot, both up by the fuse box and back by the pump to see if you can cause the pump the start and stop.