belgiumbarry

what should be "normal" voltage drop at the fuel pump ?

Not running i see 13V at the hot post .
Idling 12V at the hot post ( perhaps alternator not yet charging at that low rpm ? ) ... but only between 10.5V and max 11V at the fuel pump. Average more 10.5 then 11....

PS as we had bad contact at the fuel pump fuse the first regularity rally ( fuse not blown , but plastic melting and as result no more tension on it ) , i already placed a extern modern fuse with connections to the "oem" fuse terminals.

Second regularity we had to abandon due bad running , so now i mounted a ,new oem Bosch, fuel pump to test ... car idles good, pity could not test drive it today , but made me measure the voltage as extra check....with mentioned results.

Thanks for replys !

FredR

Your hot post values sound reasonable to me. The battery on no load produces about 12.5 volts, at idle the alternator is not producing enough juice to match the demand so the voltage drops a little as the battery takes up the slack. Power has to get to the fuel pump from whatever voltage the supply sits at so there will be a finite voltage drop in that line but 1.5 volts sounds a bit suspect to me.

Suggest you get someone to rev the engine until the voltage reaches the usual 13.5 volts and then measure what you get at the pump itself. If it is more than 1 volt I would want to know why.

Mrmerlin

check the grounds in the hatch area you need to have the engine running at 1500 RPM for valid voltage testing

belgiumbarry

thanks Fred , was also my idea... but i was "home alone" ...
test will be for next week .... perhaps drive it first and see if everything is back to normal .... or not.

If the fuse is "good" , can the pump relay be the source of voltage drop ? i assume the amps pass there though contacts ?

belgiumbarry

Thanks Stan , will do .
I already assumed that at idling rpm the alternator doesn't charge much....if any at all.

In the end my problem could be something else then the fuel pump.... i just assumed that as it sure drove "better" after filling up compared 1/2 tank when the problem came .

FredR

The relay is an electromagnetic contactor and generally speaking they either work or they do not. Anything that has excessive resistance will create an unwelcome voltage drop for a given ampage be it a bad earth, bad cables or a pump problem.

Billu

Any electrical connection, if burnt or corroded, can drop voltage across it.
Yes, a relay with burnt/corroded contacts can create resistance and drop voltage.

dr bob

CIS cars have a fancy fuel pump relay that includes some electronics to detect engine running based on tachometer pulses. The relay itself, integrated in the case, is more often a problem that the later cars which use a standard 53b relay. Over the several decades that I've played with CIS cars, I've seen more than a few weak relays cause low fuel pressure.

Easiest diagnostic is with the same switched jumper that all 928 owners carry in their spares/tools bag. Yours has about about a meter of wire attached, with the switch at one end and a pair of spade contacts at the other. Remove the relay, verify that the switch is in the open position, and carefully plug the wires into the socket terminals corresponding to pins 30 and 87. At this point, the pump will run whenever the switch is closed, regardless of ignition switch position. So close the switch and start the engine, retest the voltage at the pump terminals. If it's closer to battery voltage, the relay is a little tired and needs replacement.

The wiring diagram set I have is for US cars, so I don't know where fuel pump power is sourced for your Euro model. US CIS cars get fuel pump power from the CE 30 bus, supplied via the jump post. Later US cars have a feeder from the battery positive dedicated to fuel pump(s) and the exhaust oxygen sensor heater. Take a look at the diagrams for your car, tracing the current flow back from the 30 pin on the relay to source. Check voltage at each connection along the way including the fuseholder, and from relay terminal 87 through all connections on the way to the pump. Check the ground point for the pump too. If the jumpered the relay socket doesn't restore correct voltage, the terminal testing will tell you where the circuit has resistance. Don't forget the fuse and holder especially if you have the little bullet fuses. Original fuses have just copper elements, and the little cone terminals will corrode over time. Next-gen bullet fuses have tin coating and look more silver rather than brown; they still sit in copper-alloy holders that deserve cleaning once in a while.

GregBBRD

More information needed before you can get an answer, from me.

Need to start this conversation with the knowledge of which Bosch fuel pump you are running.

While the wiring, relay, and fuse are adequate for a stock fuel pump, if you changed over to one of the higher volume Bosch pumps (like an 044), the wiring is too small for the current draw and you will continue to "overheat" stock relays and fuses, until you correct this problem.

dr bob

Following on Greg's comments and those before, it's worthwhile testing the actual current that the pump is demanding. Tired/worn pumps work harder and harder over time, often with high current demand a telltale before failure. IIRC the total pump demand should be less than 8 Amps or so. Since you'll be testing with that switched jumper in the relay socket, add your Amp meter into the circuit for a look. Put a 8A fuse in series with your test circuit too just in case -- might help protect the meter if current is too high.

Reminder that the pump draws the same running current regardless of engine running or stopped; current is related to pressure and flow, and with a bypass style regulator and no vacuum bias with your CIS, the only worry with running the pump without the engine is accidentally getting fuel into the cylinders. Else, battery maintainer on the battery, you can test to your heart's content without starting the engine. Fuel should only flow when airflow lifts the plate, but if the idle mixture screw is set a little aggressively there may be some fuel flow with no airflow. Use appropriate caution.

belgiumbarry

thanks everybody for the tips & idea's !

I installed a Bosch pump 0 580 254 053

I thought that was a correct pump for a Euro '82 S ? ( at least among those still available ? )

belgiumbarry

cleaned the ground at the back , but seemed OK .... wanted to do the same with the wires at the hot post .... to see that bolt was "loose" ! omg, stupid me....never checked it...
Cleaned those connections and torqued the bold.
Let de car idle and see now approx 1V less at the fuel pump . Was 12.5 at hot post , 11.5 at pump.
Hangs much better on the gas now , no "hesitations" like yesterday... maybe solved ! .
(No test drive as it is raining and i have to move 3 perfect dry cars in it to clear the lift.... o well , tomorrow. )

So, if that was "it" , it wasn't the fuel pump ... but ok, i have a new one now and more, the old pump in the car as i bought it , wasn't a Bosch oem one , but worked also perfect before the problems began.

FredR

That you gained a volt by cleaning that connection tells its own story. By the time the alternator is driving and reaches its cut off voltage of 13.5 volts or maybe a bit higher the pump will be seeing at least 12.5 volts so no issue.

If you do not know the age of the critical relays it is wise to change them out and together with your new pump you should get a notional improvement on long term reliability

Strosek Ultra

The Bosch 0 580 254 053 fuel pump, 175L/h @ 5 bar. The 044 pump 260L/h @ 5 bar.
https://www.bosch-motorsport-shop.co...fuel-pump~4342
Åke

belgiumbarry

Ake, so again i assume to have the correct pump.. 11A instead of 14A for the 044 pump. Even the oem fuse is "only" 15A.
Now i understand Greg , the 044 pump would be rather heavy for the oem wiring/contacts.