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Low Voltage at Fuel Pump
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Mark Lewalski
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PostPosted: Sun May 17, 2020 2:57 pm    Post subject: Low Voltage at Fuel Pump Reply with quote

I realized that my 89 fuel pump wasn't vibrating when I would turn the ignition on. I feel the relay in the relay box in the engine compartment is clicking as the ignition is turned on.

I hooked up a multimeter to the fuel pump posts while everything was still connected and it momentarily reads 7.8 vdc as the ignition is turned on.

I'm a little rusty with working on my Vanagons since some medical issues arose and put a stop my tinkering. I could use some troubleshooting help here please.

Thanks,
Mark
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PostPosted: Sun May 17, 2020 3:09 pm    Post subject: Re: Low Voltage at Fuel Pump Reply with quote

The fuel pump wiring has a 2 pin connector behind the air cleaner and you might check the voltages there as well.

When checking in either place, do another test with the negative meter probe on clean body metal instead of on the ground wire. This can help determine whether the voltage is low or the ground is weak.

Mark
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Mark Lewalski
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PostPosted: Mon May 18, 2020 4:03 am    Post subject: Re: Low Voltage at Fuel Pump Reply with quote

I was also thinking of grounds...

I read a previous thread that details (w/photos) all the ground locations but the ground shown for the '86 fuel pump example is right above the pump itself. My wires both come from somewhere else - anyone know where that fuel pump ground would be on an '89?

I know that I can trace them out (and normally I would do that) but I guess I'm looking for a bit less time under the vehicle or bent over the engine bay if possible - my back just can't take that much anymore.

If someone knows where that ground point is, please let me know.

Thanks,
Mark
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PostPosted: Mon May 18, 2020 1:08 pm    Post subject: Re: Low Voltage at Fuel Pump Reply with quote

maybe a dumb simple question but are the two relays above the ign coil in the box the same (interchangble?) im having similar issues and this will help me a little on diagnosis
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PostPosted: Mon May 18, 2020 10:38 pm    Post subject: Re: Low Voltage at Fuel Pump Reply with quote

Look up how to test for voltage drop and find where your electrons all got used up.
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PostPosted: Mon May 18, 2020 10:46 pm    Post subject: Re: Low Voltage at Fuel Pump Reply with quote

Mark Lewalski wrote:
I was also thinking of grounds...

I read a previous thread that details (w/photos) all the ground locations but the ground shown for the '86 fuel pump example is right above the pump itself. My wires both come from somewhere else - anyone know where that fuel pump ground would be on an '89?


Below the coil: https://www.thesamba.com/vw/forum/viewtopic.php?t=548492

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Mark Lewalski
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PostPosted: Wed May 20, 2020 3:54 pm    Post subject: Re: Low Voltage at Fuel Pump Reply with quote

So I checked the connector behind the air filter housing. Good voltage on the relay side but only 7 vdc on the pump side while the connector was still connected together.

I was able to separate the two halves and when I did, a small cloud of dust and dirt popped out! I removed the blade connectors and they were VERY dirty - not corroded, just dirty. I cleaned them up and then tried to remove the female connectors but just couldn't figure out how to release them from the plastic connector. Tried my best to clean them out and then inserted/removed the blades a few times to try and get a good connection surface.

After that, I reattached everything and had only gained 2.5 volts through the connector. I then took matters into my own hands and replaced the connector with two sets of male/female bullet crimp connectors. Now I finally have 12 volts at the pump.

But it doesn't work.

I tapped on it a bit just in case it was locked up but no go.

So I'm now getting ready to purchase a new fuel pump.

I'm wondering if the poor running of my 89 was possibly due to intermittent (or low) voltage to the fuel pump along with an intermittent (or failing) pump.

Next step in the evolution of things I guess... that's the way my VWs roll.

I'll keep you posted.

Mark
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PostPosted: Tue Jun 15, 2021 3:32 pm    Post subject: Re: Low Voltage at Fuel Pump Reply with quote

Any update on this? Going through something similar. Did a reseal of my tank and hoses, but Im afraid I maybe ran the pump dry and it gave up the ghost. Ran fine the day I put everything back together, but the next morning it doesnt wanna run. Voltage at my fuel pump is about 9.5 volts. Which seems low. Trying to cross out electrical gremlins before throwing parts at it. New pump is ordered, just preparing for the possibility that the issue is farther up the line than the pump itself.
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PostPosted: Wed Jun 16, 2021 7:10 am    Post subject: Re: Low Voltage at Fuel Pump Reply with quote

Blake;

FuPu is a 12VDC part!...any less voltage and it might not start, or run more highly stressed (I was going to first say "hotter", but that won't be the case, since it is a "wet" pump constantly being cooled by fuel!), or maybe not make the required pressure for the FI sys (which is pretty important!).

So before you (maybe unnecessarily!) buy a replacement, I recommend you follow the entire power supply and current path (so this includes return path to chassis!) and locate the VDrop (and note this only occurs while load current is flowing, so tests need to be performed while engine is running OR to be a bit safer, FuPu Relay (which is controlled by FI ECU, and which times out when engine is not sensed to be running) can be dummied up to remain ON continuously during troubleshooting, and measure V along the entire path, until you find those 3V!

When you locate the place where 12V come in and 9V go out, that will typically give a manner to proceed with remedies...terminals get loose or corroded, contacts get carbonized, etc. Shiny clean metal and tight are the general recommendation, and long-term protected with ACZP will keep them that way, supplying a full 12V to the FuPu (including submerged, for the Synchro drivers reading this!) See: http://www.sw-em.com/anti_corrosive_paste.htm

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PostPosted: Wed Jun 16, 2021 8:57 am    Post subject: Re: Low Voltage at Fuel Pump Reply with quote

Ok so this morning I made fresh new connectors at the little junction behind the air cleaner box, cleaned up the grounds by the pump and Im getting about 11.8v to the pump with the relay out and being jumped. So I got a little more juice, but not the full 12.5v seen everywhere else.

I notice some people talk about restricting the voltage to the pump, so Im wondering if this is plenty voltage. The pump gets hot, and I can feel some slight erratic vibration in the pump and fuel filter, so something is going on in there. Its just not nearly as loud as it was before I did the fuel tank reseal.

Thanks for all the help! I didnt try starting it with a jumper in place of the relay. Would a bad relay cause a drop in voltage?
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PostPosted: Wed Jun 16, 2021 10:39 am    Post subject: Re: Low Voltage at Fuel Pump Reply with quote

Blakesinclair wrote:
Ok so this morning I made fresh new connectors at the little junction behind the air cleaner box, cleaned up the grounds by the pump and Im getting about 11.8v to the pump with the relay out and being jumped. So I got a little more juice, but not the full 12.5v seen everywhere else.

I notice some people talk about restricting the voltage to the pump, so Im wondering if this is plenty voltage. The pump gets hot, and I can feel some slight erratic vibration in the pump and fuel filter, so something is going on in there. Its just not nearly as loud as it was before I did the fuel tank reseal.

Thanks for all the help! I didnt try starting it with a jumper in place of the relay. Would a bad relay cause a drop in voltage?


Fuel pump relay removed, no jumper wire installed, did you see the 12.5 Volts at fuel pump relay socket 30?

Since the ground current path to fuel pump goes through vehicle metal, that metal could easily account for that voltage drop. If it was me, for testing & learning purposes, I'd run a new wire from battery negative to fuel pump negative then measure voltage at fuel pump wiring.

I can't say for sure but I'd guess the 0.8 volt difference is acceptable.

In my limited experience, I think that since the electrical path in/out of the relay switch portion is short and even if the switch contacts were slightly pitted due to normal wear/tear, I can't see the relay as being a significant voltage drop point if all else inside it is ok.

Assuming your pump was noisy due to cavitation (before tank reseal job), noise you hear/feel now is likely cavitation noise. If the fuel tank is not vented properly, that might account for the noise.

Neil.
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PostPosted: Wed Jun 16, 2021 12:03 pm    Post subject: Re: Low Voltage at Fuel Pump Reply with quote

With the meter probes right on the + and - terminals of the pump with pump running, that 11.8v is fine. (engine off)

It is important to measure right at the pump terminals. Then move the negative probe to clean bare metal on the body to compare the voltage that way. That will show if the ground is a weak link.

Mark
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PostPosted: Wed Jun 16, 2021 12:23 pm    Post subject: Re: Low Voltage at Fuel Pump Reply with quote

Good to know. I'll start with a new pump, and start digging around the ventilation to avoid any cavitation issues. Worst case, I have a backup if my original is fine.

The old hoses/grommets were a disaster, so the tank probably had no problem venting, albeit improperly. So now I'm a little worried I did a disservice by fixing all those busted hoses and creating more back pressure for the fuel system.
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PostPosted: Wed Jun 16, 2021 12:30 pm    Post subject: Re: Low Voltage at Fuel Pump Reply with quote

Vanagon Nut wrote:
Blakesinclair wrote:
... Im getting about 11.8v to the pump with the relay out and being jumped. So I got a little more juice, but not the full 12.5v seen everywhere else.

I can't say for sure but I'd guess the 0.8 volt difference is acceptable.


Jeepers. I love math SO much....

Meant: ... the 0.7 volt difference....

Neil.
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PostPosted: Sat Jun 26, 2021 3:53 pm    Post subject: Re: Low Voltage at Fuel Pump Reply with quote

Update. After bench testing the old pump and getting nothing, I ordered a new pump. Installed it today and it works! Gonna do some researching because the pump is loud. I got the cheaper option GoWesty had on offer, so maybe that has something to do with it. The old pump was quiet as a mouse, but OEM. So Ill run this one for a bit and hope I didnt mess up any venting causing the noise. But I have a feeling its just a crappy cheap fuel pump.
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PostPosted: Sat Jun 26, 2021 6:33 pm    Post subject: Re: Low Voltage at Fuel Pump Reply with quote

You know? My first impulse was to criticize the choice of getting the cheaper fuel pump.

But honestly? The only fuel pump that I had die was a new Bosch "last of the quality German made pumps". I was in nowhereville Idaho.
A friend from Boise picked up a "lesser" made in Yugoslavia pump which operates to this day without fail.
I also carry a spare pump now.

Bosch products ain't what they used to be. It may not be worth the double price plus premium.

Are the rubber mounts in place to dampen vibration?

Dave
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PostPosted: Sat Jun 26, 2021 7:11 pm    Post subject: Re: Low Voltage at Fuel Pump Reply with quote

The original ones were busted, so I fixed all that when I replaced the pump. Ill double check nothing is loose or vibrating against the frame, but before I installed the new one I did a real quick test on the bench. It seemed loud, but figured it was just because it had no fuel in it. This was like a 1 second zap test, but it was pretty buzzy right there in my hand.

Ill try digging around for one of those Yugoslavian ones. Probably not a bad idea to be driving with a working spare anyways.
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PostPosted: Sun Jun 27, 2021 8:09 am    Post subject: Re: Low Voltage at Fuel Pump Reply with quote

FWIW, the OP posted this a few days after asking about low voltage at the fuel pump. I'm not sure if it's directly related to the fuel pump problem but it's worth considering.

https://www.thesamba.com/vw/forum/viewtopic.php?t=...highlight=

Mark Lewalski wrote:
I finally got around to inspect and change out ground cables on my 89. tencentlife has a good article on his website that covers some of this as well as the post here in the Forum that covers all the ground locations.

I mainly want to show folks that the concern is real about dirty/corroded ground points on our vehicles.

I removed the negative battery cable. It's not corroded much at all but here is the connection at the body:

Image may have been reduced in size. Click image to view fullscreen.


Obviously, the cable will have no metal to metal contact other than the threads of the fastener.

Here is the fastener:

underside:
Image may have been reduced in size. Click image to view fullscreen.


topside and threads:
Image may have been reduced in size. Click image to view fullscreen.




Next up is the transmission to frame ground strap. I didn't take photos of the frame and transmission attachment points but they were very dirty under the strap eyelets.

Here is one end of the ground strap:

Image may have been reduced in size. Click image to view fullscreen.


Again, the bolt (frame side) and nut (transmission mount side) were dirty and corroded on the threads and contact surfaces.

Here is the engine to body ground strap connection at the body. You can see that the connection point is painted with no bare metal contact surface:

Image may have been reduced in size. Click image to view fullscreen.


Here is the engine block connection point:

Image may have been reduced in size. Click image to view fullscreen.


Dirty but not corroded.

However, here are the fasteners from both ends:

Image may have been reduced in size. Click image to view fullscreen.


and

Image may have been reduced in size. Click image to view fullscreen.


Only the threads would be serving as the metal to metal connection on the body side and they don't look too good.


So, like I said, I just wanted to give some visual evidence of what we all probably are dealing with. My 89 is pretty clean and I believe it was garage kept for many years before I purchased it. Other vehicles may see much worse conditions of their grounds.

Thanks,
Mark

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PostPosted: Sun Jun 27, 2021 11:58 am    Post subject: Re: Low Voltage at Fuel Pump Reply with quote

Those images remind me of the note in Bentley regarding the transaxle to body ground strap. It occurred to me there might be a same or similar note re: battery ground strap. There is:

P. 27.6
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