Easier way to check pump continutity

[alang1990]

Can continuity at the pump be checked from a location other than the pump's connecter itself ?

Can this be checked from the fuel pump relay for instance ? Or the fuel pump's relay?

[moroza]

Yes and no. A check for 12V anywhere upstream of the pump itself won't reveal wiring problems between the pump and the check point. You can check resistance between the fuelpump ground and power (both under the RH rear seat, BRown and GReen/VIolet respectively), or between F23 or the relay and any other ground point, and compare to the resistance of the pump itself. I don't know what that resistance is.

[alang1990]

Yes and no. A check for 12V anywhere upstream of the pump itself won't reveal wiring problems between the pump and the check point. You can check resistance between the fuelpump ground and power (both under the RH rear seat, BRown and GReen/VIolet respectively), or between F23 or the relay and any other ground point, and compare to the resistance of the pump itself. I don't know what that resistance is.

Well as luck will have it, I have a spare pump that came with a fuel level sender I needed. Briefly fantasised about wiring up two pumps in the tank with spliced hoses and switchable wiring to provide a backup, decided to shelve the idea for another day.

I will check its resistance and then will check resistance at the fuse and relay and write back with figures and results. The objective of this thread was to figure out a simple way for noobs to check if the pump itself is dead (and for me to go through my routine annual checklist) so I won't bother with the back seat stuff.

[moroza]

I carried a spare fuelpump in my 534i, which fit easily along with a couple of coolant hoses, spare relays, a radiator cap, and maybe more inside the spare wheel turned upside-down. I had the same kit in my 544iT plus a whole spare heater valve & aux waterpump assembly, which all fit but not easily. 13 years of DDing an E34 and I never once needed anything in that kit except the wheel itself.

Neither the pump nor the important parts of the wiring are hard to access IMO. Seems noob-friendly enough? :dunno

[alang1990]

I carried a spare fuelpump in my 534i, which fit easily along with a couple of coolant hoses, spare relays, a radiator cap, and maybe more inside the spare wheel turned upside-down. I had the same kit in my 544iT plus a whole spare heater valve & aux waterpump assembly, which all fit but not easily. 13 years of DDing an E34 and I never once needed anything in that kit except the wheel itself.

Neither the pump nor the important parts of the wiring are hard to access IMO. Seems noob-friendly enough? :dunno


You have been fortunate good sir. Factory pumps seem to consistently fail in the 15-20 year mark. Since you have a backup and you're clearly capable of changing a pump out anywhere in 30 minutes that's ok. But I'm not and so I have a new oem pump (now 5 years old but still new comparatively) even though my old one seemed fine.

Anyway, I have news. I have checked a total of 3 working pumps, one of them new. All were retested and all are working and pumping strong. The resistance is beween 1 and 2 ohms.

Secondly, it is possible to check the pump's continuity from the fusebox ! Pull the pump's fuse, and put one multimeter lead on a shock tower stud, and the other into both connections of the fuse insert. You may need to clip a needle into your multimeter lead to make this possible. You will get continuity on one of the two fusebox fuse connections if your pump is not dead.

You can also check resistance in the exact same way. But beware, on my installed pump direct I got 0.8 ohms and when I checked through the fusebox, I got 1.6 ohms. It may be a problem with my digital multimeter, but don't be too surprised if the figures are different. Anyway, I unplugged the connector at the pump at the tank, and both continuity and resistance could not be read at the fusebox.

So there's no need to goto the tank to check on the pump directly. Just check at the fusebox.

[alang1990]

There is also no need to pull the fuel hose now and jump the relay junction to see if fuel spurts out and thus prove the pump is working fine. If there is continuity and normal resistance, checked at the fuel pump's fuse location on the fuse box itself, the pump is live and normal.

To test the fuel pump's relay, just swop in a known good relay (the adjacent o2 sensor relay or get another orange relay from the fuse box) and try.

[ross1]

The fuel pump's impedance, aside a dead short or open circuit, doesn't mean Jack. There is also the moderately important "pump" component of this assembly to be concerned with.
Plenty of bum pumps will "spurt fuel" which is a worthless indicator of the pump's health.

[alang1990]

Not really. Most pumps fail outright.

But you have identified something important, which is the obscure way that some devices fail. Own an old bmw long enough and you will not only encounter the common ways things go wrong, but even weird ways that things go wrong that you don't think to check for at first. So indeed there are pumps which spurt fuel when shorted but don't generate much usable pressure.

So what we've identified here is just a part of the diagnostic process, that previously involved getting to the pump and testing continuity at the pump directly, which has now been made much simpler.

Lets call this round 1 checks. Round 1 are the quick and dirty methods of testing the most probable known causes of a certain failure. These methods are simple and cheap enough for noobs to learn up and do without damaging anything going in or out, or even screwing up the tests themselves. People coming on here can be quickly guided to fairly accurate diagnosis. They are also simple enough for experts to do once every 6-12 months as part of a routine checks.

If round 1 does not reveal the answer to a situation (and it usually does 95% of the time), then we have to goto round 2. That involves the more difficult, time consuming, expensive, and error prone checks of each device concerning a situation.

For example, lets take a low fuel pressure situation that first shows up as a no start. Round 1 reveals that the pump has continuity and stable impedance, the cps and dme are working, the engine coolant sensor is alright, fuel relay is fine, no ews issues to worry about, current is getting to the coils. Problem is still there. Then we goto round 2. Buy a fuel pressure gauge and T it in. Replace fuel filter. Clamp the outgoing fuel lines from rail. Clamp the incoming lines to the fuel rail. Check voltage at the pump's connectors. Check for voltage drops at various points in the circuit. Much more complicated, costly and requires more skill and cost investment. And that's just the fuel pressure side of the equation. But unfortunately necessary at times.

[ross1]

So condescending. I see you've already seized on "noobs" to preach to and selected someone to patronize.
How many times now?

[whiskychaser]

Love the avatar, Ross. My wife is convinced that is what you get if the brake fluid is low

Btw If you are compiling ways of finding out if your pump isn't working, you might add a healthy slap to the list.

[moroza]

Not really. Most pumps fail outright.

What I've seen is that they start failing intermittently, revivable with a smack, before they crap out altogether. Sudden failure out of the blue does happen but not quite as often (I don't have actual statistics either way). E34 pumps seem to lose their check valves sometime before the actual pump goes, resulting in prolonged cranking after it's sat for a day or two, running healthy otherwise.



So what we've identified here is just a part of the diagnostic process, that previously involved getting to the pump and testing continuity at the pump directly, which has now been made much simpler.

Agreed, more or less. It's a "get what you pay for" test - trivial to do, but apart from a reading of 0 or >~5ohm, useless. Pump draw being a lot higher than DMM draw, it won't reveal damaged wiring like in that other current thread.


So condescending. I see you've already seized on "noobs" to preach to and selected someone to patronize.
How many times now?

I don't find it condescending or patronizing, though the description is a bit verbose. And you might note that the fuse (#23) suggestion was already made in post 2.

I never needed my emergency pump but I did have two fail on my cars, losing their check valves.

I also witnessed a Tacoma apparently blow its engine from too much detonation caused by low fuel pressure. Nobody tested continuity or resistance, but those would've failed to diagnose it. Hell, they even had a certified Toyota mechanic with a fancy-$chmancy scanner sit there for half an hour trying to interpret misfire codes. He got nowhere and left, though not before peering down his nose and referring to me a shadetree so-and-so when I mentioned I was also a mechanic. Talk about patronizing and condescending... after that pompous ass and his expensive tools left, having delivered nothing and paid accordingly, this so-and-so got involved. Took five minutes to hook up a free Autozone rental pressure tester that showed about half the pressure it was supposed to put out. New pump cured it.

The pumpkin is indeed brilliant.

[alang1990]

Agreed, more or less. It's a "get what you pay for" test - trivial to do, but apart from a reading of 0 or >~5ohm, useless. Pump draw being a lot higher than DMM draw, it won't reveal damaged wiring like in that other current thread.


Wiring is not a common problem with the fuel system. I know if we keep seeing a few such threads and because they are harder to solve those threads go on much longer - the tendency is to assume its common. It isn't. Wiring falls into the 5% obscure category. So ways to easily test the 95% box are very useful and that's the first place to start.

But as I pointed our earlier, own an old bmw long enough and you'll encounter the obscure versions of problems, which includes wiring.




I don't find it condescending or patronizing, though the description is a bit verbose.


I would prefer to say it was detailed. :) And the only thing condescending is the idea that it is somehow personally inferior to be a noob, cunningly inserted by suggesting that calling someone a noob is condescending.





And you might note that the fuse (#23) suggestion was already made in post 2.


Yes and that was confirmed. Thank you Moroza you've made an important step possible. Pardon me for not emphasising your contribution sufficiently.





I also witnessed a Tacoma apparently blow its engine from too much detonation caused by low fuel pressure. Nobody tested continuity or resistance, but those would've failed to diagnose it. Hell, they even had a certified Toyota mechanic with a fancy-$chmancy scanner sit there for half an hour trying to interpret misfire codes. He got nowhere and left, though not before peering down his nose and referring to me a shadetree so-and-so when I mentioned I was also a mechanic.



Yup. Code reading and quick and dirty tests have their value but where they fail to reveal the cause, one must immediately switch to the longer more time/effort consuming direct tests. When round 1 testing doesn't provide the answer, switch to round 2 immediately without struggling to find the problem with round 1 testing.