Electrical Brainteaser - Help Please

[toggyvet]

Folks,

Whilst investigating my car's reluctance to start, I've found that the input voltage to the fuel pump relay (i.e. the connection from the battery to the relay) shows as 0.96V.

If I put a multimeter across the battery terminals (both cables connected), I get a reading of 11.86V.

The same meter where the -ve is connected to the negative terminal of the battery, and the +ve connected to the relay input connection shows 0.96V.

However, if I disconnect the negative cable from the battery, then measure across the -ve battery terminal to the relay input (+ve cable still connected to the battery), I get a reading of 11.86V.

This suggests that connecting the negative cable to the battery is having a dramatic effect. The car hasn't moved for over a week, but the battery has been recharged since.

Am I missing something obvious here?

 

[Ken]

I'm not the electrical wiz here, but vig will be checking in (he's the genius ); he'll be able to help you toggy.

_ken

 

[twiget]

I would suggest Ohm'ing out the cable going from the neg battery terminal to the relay. It could be going bad. There should not be a 11V+ drop accross a cable. The resistance of the battery cable should be 0 or pretty darn close to it (under 5 ohm's or so). If the cable is good, check the relay. It might have gone bad, and is shorting the power to ground.

Jason

 

[ironcity]

I agree with twiget the cable is taking the juice to ground if not through the relay itself.

 

[SPANISHVETTS]

If after checking all of the associated fuses, fuse links, etc., the problem persist, you probably have a high resistance connection somewhere in the circuit.
If so this is what is happening. With no current flowing the meter shows the full battery voltage at the relay. As soon as current starts flowing you have a voltage drop across the high resistance connection. (Ohms law E=I*R) In your case about 11 volts.
Usually the problem is on the negative side. Look for corrosion on all of the negative connections.
Another possibility is that a wire is broken or corroded inside the insulation.

 

[Top]

What does the meter say if you disconnect both cables and read just the battery. Under 12 volts is a bad battery no matter if it has been recharged or not, it is not holding a charge. Don't worry about the voltage down the line yet, make sure the source is good and a recharge does not prove it is good.

Cheers

 

[toggyvet]

Battery voltage reading

The reading across the battery with no cables attached is 12.4v, so hopefully it is OK.

 

[vigman]

Confused

If you measure @ the rear of the car the ground & the hot you get .96vdc

I'm assuming Key on

Now remember.. the fuel pump only runs for 2 seconds to prime the system ( controled by the ECM) and the pump will not come back on until the oil pressure switch CLOSES ( with over 4 psi of pressure in the system)

Jump the oil pressure switch and see if she runs!

Or the way you did it with heating up that pin on the ALDL connector!


YOU SHOULD NOT GET 12 VOLTS BACK THERE UNLESS THE CAR IS RUNNING OR YOU HAVE BYPASSED A SAFTEY!

Vig!

 

[toggyvet]

Your confused! - so am I

Vig,

Thanks for replying - I'm getting confused all over the place, so I'll try and be more clear as to where I've got to.

Problem - Car won't start - fuel pump not heard on ignition to ON

Tests performed as follows:

1) Battery connected as normal, reading about 11.96V across the terminals - attach new wire from +ve of battery to the 'G' ALDL pin, and the fuel pump bursts into life - I guess this proves the circuitry from the relay output (pin E) through the fuel pump fuse, through the pump and through the grounding back to the -ve terminal of the battery, it also shows that the relay switch is in the closed position - a voltmeter reading between the negative cable on the battery and the ALDL relay input pin (Pin C) shows 11.96V.

2) Take the relay 'lid' off to show the five metal input connectors - just to be sure, connect my 12V lead from the battery directly to pin E - fuel pump again bursts into life, all is OK.

3) Having proved the circuitry from the relay output pin, I thought I'd have a look at the 12V input to pin A (orange wire) - as I couldn't simulate the opening of the switch with the ECM pulse, I simply jumped pin A to pin E, thus closing the circuit - nothing happened - when I checked the voltage at pin A (i.e. a voltmeter reading bewteen the negative cable and pin E), the voltage was as 0.93V. - conclusion therefore was that the fuel pump wasn't running because of the huge voltage drop bewteen the battery and the relay.

4) Just by chance, I measured the voltage with the negative probe touching the negative terminal of the battery (the negative cable was NOT connected) and pin E (the positive cable was connected) - this time it read 11.96V, so the voltage drop must be due to the negative cable being connected and the effective completion of a circuit (although if the relay is in the closed position, is this circuit complete???).

5) Assumed that there must be a weak link or high resistance between the battery and the relay input Pin A - from wiring diagrams, it would seem that there is a fusible link 'H' (god knows where I find this) between the battery and the relay - there is also a fuel pump relay fuse somewhere in the equation.

I'm sure that the voltage drop is my problem - there isn't enough juice to run the fuel pump and it may well be the case that the relay is OK. However, I don't understand:

a) why the connection of the negative cable should be causing the voltage drop when that part of the circuitry has already been proved by test A above (the ALDL connection) - remember that the problem original started because of the car hitting a kerb, so would a dodgey grounding somewhere cause this problem?

b) how the hell I start checking for the cause of the high resistance when the cables all disappear in a huge harness under the engine block somewhere

c) where I should be looking for dodgey grounding points

d) my other post about the ECM was simply to confirm Code 54 (low fuel circuit voltage), but I've had the battery off so many times, the ECM memory will have been cleared anyway - the SES light has flashed a couple of times since I had the car, but will not light up now.


Kind Regards,

Chris

 

[vigman]

Chris

Will scan pix in shop manual for physical location of "H"

Do you have AOL IM or Yahoo?

I really think the link is Buh BYE
( deceased--passed on... pushing up daisy's)

Will scan


BRB

Vig!

 

[vigman]

Pix of H 4U

TAda

 

[Corvette-Pilot]

However, if I disconnect the negative cable from the battery, then measure across the -ve battery terminal to the relay input (+ve cable still connected to the battery), I get a reading of 11.86V.
Am I missing something obvious here?

If you have removed the negative battery cable from the battery yet still measure a voltage down the line then you have a ground short someplace !

But then again??
If you don't have anything connected to the neg termianl except the meter then you shouldn't even get any sort of reading :confused :confused :confused

Are you sure of where you were connected in this case?

The other thing might be as others said, you have a bad cable or even a bad/dirty/loose connection that wont allow the current to flow thru. Double check all the connections are tight, pull them apart and clean all the contacts.

You should also pull the contact at the relay then set you meter to "Ohms" and measure from the + terminal to the end of the wire you just unplugged and it should be resonably close to 0 Ohms.

Just a few things to try.

 

[vigman]

His meter is still connected to the neg side of the battery!

That would fit the conditions of this test



Vig!

 

[Corvette-Pilot]

Re: His meter is still connected to the neg side of the battery!

That would fit the conditions of this test



Vig!

Hey Vig,
I assume you replyed to me ??

I read his second list of experiments and again he says he disconnected the negative battery terminal cable. Then put his neg meter lead on the neg battery post and measured a positive voltage near the relay.

I don't think this is possible unless I am missing something obvious?
What he has done is to make an open circiut with no way to complete back to ground (where his meter lead is).

At first I thought he was getting a ground from someplace else (shorted cable). So I am confused by this one issue.

 

[vigman]

You talkin to me?

"Then put his neg meter lead on the neg battery post "

Yes Pilot,

By this I assume ( OOPS ) that the neg lead was on the battery post ( with the neg black wire removed)

It's a guess at best.. but it would allow a positive reading on the meter...If he was measuring just on the cable (disconnected)

Their MIGHT be a back up battery for the alarm ( aftermarket ) which would give a reading.

Again... speculation on my part..

At best using the internet for car repairs is a good pointer....but nothing beats hands on work!


Vig!

 

[Corvette-Pilot]

Re: You talkin to me?

[BBy this I assume ( OOPS ) that the neg lead was on the battery post ( with the neg black wire removed)[/B]

Got ya,

In this case I think he may get battery voltage since there is no load (Open circiut).

Yes again, this is a hard way to troubleshoot.
But the info might help "toggyvet" or anyone else with some ideas !

Good luck

 

[toggyvet]

I guess we can forget the red herrings

Hi folks,

Thanks for all your input - I think the whole question of the negative terminal reading is probably leading us down the garden path as it were.

The fact remains that there is a hugh voltage drop between the +ve battery terminal and the input to the relay and this is where the investigations must be focussed.

I will try and locate the correct fusible link (H) and check just beyond this in the circuit (I think I counted at least 10 fusible links, all the same colour - thanks but your picture didn't add much Vig, as the wires are all over the place and bear no resemblance to the picture).

Failing that, I didn;t realise there were so many other components that were powered by this feed (ECM B1/C16, MAF Sensor Burn-Off Relay, Oil Pressure Switch, MAF power relay) - could it be the dodgey grounding of one of these components that is causing the problem? The best diagram appears to be Fig 70 in Chilton's manual, page 4-24 for the Mass Air Flow sensor circuit - VIN 8 engines.

Just to prove that the car will run if I solve the voltage drop, would it hurt to run the fuel pump via ALDL connections and then start her up? The fuel pump would be running continuously, but I could try it for a few seconds or more.

 

[Corvette-Pilot]

Try pulling fuses until the voltage drop goes away.
This might help point you to the correct circuit.

Sounds like a mess if the lead you are mesuring branches off all over the place
I work in electronics and those are the hardest to find.
Heavy load SOMEPLACE that is pulling the regulators down.
You need to pull out each circuit until the problem leaves then focus there (fuse trick)

It is starting to sound more like a component feed off another that went bad so it is a heavy load but not quite a short ?

No "check engine" light?
Fail codes??

All else fails
Call AAA

 

[vigman]

I disagree

If the power is not getting to the relay ( Pin A Orange line) it's a good bet the link has gone
and you need to test EACH one.


Im thinking not a bad ground.. but a bad power feed.

Since if you jump the ALDL pin G to power and the pump whizzes up... that appears to be fine....

But if Link "H" is dead.. no power to the ECM , no tickey... no laundry...no runny!

I would chase down the power feed problem first!


Vig!

 

[toggyvet]

Progress at last!

Ah ha,

Have now determined that it is definitely fusible link 'H' that is causing the voltage drop - the ohmeter determined no circuit between the +ve battery cable and the wire coming out of the black rubber thing at the end of the fusible link, yet showed a perfect circuit between the end of the fusible link and the input to the fuel pump relay.

So then, how do I know what the correct replacement is for the link - do they have ratings like normal fuses?

Can I replace the fusible link altogether with a more 'modern' approach?

Happy New Year!