911SC only starts with alternator disconnected

[anthony]

I have a Fluke 73III multimeter. A few posts back I posted my results. How will a test light work differently? Are there settings on the Fluke that will simulate a test light?

[rick-l]

Originally Posted by anthony

I have a Fluke 73III multimeter. A few posts back I posted my results. How will a test light work differently? Are there settings on the Fluke that will simulate a test light?

Because you want to know that the resistance to ground through the alternator with the air plate switch closed is low enough to allow the relay that drives the fuel pump to pull in.

You didn't mention any numbers with the air flow switch test and I think there is supposed to be a ground closure when the plate is active.

With your meter you could go to pin 85 on the relay and see that it goes to ground (near 0 Volts) when you lift the plate.

[anthony]

For the air flow switch test I hooked up a volt meter. It read 12 volts and then when I lifted the plate it fell to zero. Would measuring the resistance with an ohm meter be helpful?

I'm more than happy to run out and buy a test light this morning if it gives me the right info to diagnose the issue.

[rick-l]

If that is the case what is this?

Originally Posted by anthony

Test #3
Relay installed
key turned to on position
86 --- 12v
87 --- .03v (should be zero)
87a --- 12v
85 --- .14v
30 --- .02v

Should be 12 Volts with the air flow plate at rest.

[anthony]

That is testing with the relay in the socket and the key on. I was just testing from ground with the volt meter and the positive terminal on the relay pulled out of the socket a little bit. (I admit and mentioned that I wasn't sure if I was performing test#3 correctly.)

For the air flow switch test I hooked up a volt meter to the battery (+) and hole 85 of the FP relay socket (no relay installed). It read 12 volts and then when I lifted the plate it fell to zero volts.

I'm more than happy to run out and buy a test light this morning if it gives me the right info to diagnose the issue.

[rick-l]

Originally Posted by anthony

For the air flow switch test I hooked up a volt meter to the battery (+) and hole 85 of the FP relay socket (no relay installed). It read 12 volts and then when I lifted the plate it fell to zero volts.

Any chance you have some small wire (like 30 ga wire wrap wire) you could wrap around pin 85, plug the relay in and measure the voltage? Plate at rest - 12 Volts, plate active 0 Volts, the relay clicks and the pump runs.


EDIT: and it seems the voltmeter readings should be opposite of what you describe

[T77911S]

Originally Posted by anthony

I have a Fluke 73III multimeter. A few posts back I posted my results. How will a test light work differently? Are there settings on the Fluke that will simulate a test light?

a test light provides a load for the circuit. a MM does not. where a MM can mislead you is if it reads 12v but there is a bad connection and there is not enough current to operate the load.

the other thing is a TL helps check for power with reference to ground since the clip should be on chasis ground.
if you need to check the ground side of a circuit, clip the TL to 12v.

[boyt911sc]

Originally Posted by anthony

For the air flow switch test I hooked up a volt meter. It read 12 volts and then when I lifted the plate it fell to zero. Would measuring the resistance with an ohm meter be helpful?

I'm more than happy to run out and buy a test light this morning if it gives me the right info to diagnose the issue.

Anthony,

This is a good example of a three-minute job that becomes a major work. For the FP relay and socket test, a test light works best because:
a). You want to see power @ 87a, 87, 86, and 30 terminals with the ignition switch at these positions (OFF or ON).
b). Terminal #85 (FP relay socket) is a ground wire. Use the multi-tester @ continuity setting.
c). AFM switch has two terminals. One goes to terminal #85 and the other one goes to terminal D- (alternator). Test continuity using the multi-tester.

Test your FP relay.
Terminals 87a-30 are NC (normally closed).
Terminals 87-30 are NO (normally open).

When you turn the ignition switch @ ON/RUN;
The NC terminals (87a-30) will be energized if terminal #85 is grounded (at the AFM switch) and NO terminals (87-30) will close.

This is another scenario if terminal #85 is not grounded (open). After you turned the ignition switch @ ON position (not start), the NC terminals 87a-30 will remain closed because the coil is not energized due to terminal #85 is not grounded. With the FP relay @ 87a-30, the FP will run.

This is the reason why you pull the AFM switch to make terminal #85 open (no ground). Follow the logic? Any question?

Tony

[anthony]

Ok, picked up a test light. Will run the tests again and report back. Thanks.

[rick-l]

Oooops this is a little more complicated than I thought.

Here is a good write up thanks to Timmy2
Fuel Pump Relay 1976 Pump runs when key in on position

What does D- on the alternator do. All the diagrams I see it just goes to ground.

[anthony]

I ran through the tests again. The thing that jumps out is no power at 87 of the socket (key on or off). To summarize:

key turned to on/run position
86 ...... power
87 ...... no power
87a ..... power
85 ...... no power
30 ....... no power

I get the same result with the relay plugged in (key on/run position)

I also tested 85 again back to the AFM switch. I put the test light on the battery + and at 85. The light glows. (key off, relay out). Lift the plate and the light goes out and breaks the ground.

According to what you said above, I should have power at 87 so I need to trace back from there.

a). You want to see power @ 87a, 87, 86, and 30 terminals with the ignition switch at these positions (OFF or ON).

BTW, you said something different in DKLever's topic. In his topic you said, 87 no power. Can you clarify?

I also did another check 87 gets power when the key is turned to the STARTING position and power switches off when the key is turned back to the ON/RUN position.

I'm studying the Bentley manual and this seems correct. 87 is supposed to be a yellow wire to 50 on the ignition switch but it is only energized when the key is turned to START. (assuming I'm reading Bentley correctly)

[rick-l]

87 goes to the start position on the ignition switch

EDIT: so it is working correctly as you tested it

[anthony]

I see that on the schematic so it shouldn't have power with the key in the OFF or ON position. I think my 87 is behaving correctly.

[anthony]

I did the rest of the suggested tests.

b). Terminal #85 (FP relay socket) is a ground wire. Use the multi-tester @ continuity setting

Yes, continuity from 85 to ground.

c). AFM switch has two terminals. One goes to terminal #85 and the other one goes to terminal D- (alternator). Test continuity using the multi-tester.

I pulled the AFM plug and tested one terminal for continuity to ground (YES GOOD) and the other for continuity to 85 (YES GOOD). Both check out.

Test your FP relay.
Terminals 87a-30 are NC (normally closed).
Terminals 87-30 are NO (normally open).

Yes, both show the correct close/open as indicated using the ohm meter to test. I also tested all my other relays and they are all the same. Any there any other test to do on the relay?

[anthony]

I've read timmy2's description of the way the circuit works and it acts like the switch in the relay doesn't close back so that 87a-30 powers the fuel pump. I need a hint as to where to look next.

[boyt911sc]

[QUOTE=rick-l;8693092]Because you want to know that the resistance to ground through the alternator with the air plate switch closed is low enough to allow the relay that drives the fuel pump to pull in.

You didn't mention any numbers with the air flow switch test and I think there is supposed to be a ground closure when the plate is active.

With your meter you could go to pin 85 on the relay and see that it goes to ground (near 0 Volts) when you lift the plate.[/QUOTE]




Rick,

Are you familiar with CIS air flow meter (AFM) switch? Your post above is very misleading and would not be helpful to the OP. Have you actually done such investigation before or you simply read about it? This is a technical forum and we could agree and disagree about things and still be friends at the end of the day.

Sharing our experiences with others is good and we learned from each others mistakes too. And your information is contrary to the fact about how the switch works.

Tony

[anthony]

With your meter you could go to pin 85 on the relay and see that it goes to ground (near 0 Volts) when you lift the plate.

Pin 85 goes from 12.13v to .08v.

Because you want to know that the resistance to ground through the alternator with the air plate switch closed is low enough to allow the relay that drives the fuel pump to pull in.

How would it be best to perform this test? Do I pull the alternator and test right from the alternator minus to 85? And by switch closed you mean air plate lifted, correct?

[boyt911sc]

Originally Posted by anthony

Pin 85 goes from 12.13v to .08v.



How would it be best to perform this test? Do I pull the alternator and test right from the alternator minus to 85? And by switch closed you mean air plate lifted, correct?

Anthony,

There are a few important things you need to know:
a). Is the FP relay good?
b). Is the FP relay socket good?
c). Is the FP electrical circuity good?

Items a & b could be done under under 5 mins. Contact or PM LJ (ossiblue). He has more patient and articulate than me so I refer him. Read my post #57. You are doing fine just need some guidance and direction. Keep us posted.

Tony

[anthony]

Yes, I need some guidance and direction. :-) And I have read #57 and I'm not sure what to do next.

a). Is the FP relay good?

I did the tests you described and the relay checks out. I also tried all the black relays I have with no luck.

b). Is the FP relay socket good?

Is there some other test I need to do to determine this that I haven't done?

c). Is the FP electrical circuity good?

Previously, I checked the fuel pump. It runs from the relay socket if I jumper 87a and 30. If I do that the car runs great!

[boyt911sc]

Originally Posted by anthony

I ran through the tests again. The thing that jumps out is no power at 87 of the socket (key on or off). To summarize:

key turned to on/run position
86 ...... power
87 ...... no power
87a ..... power
85 ...... no power
30 ....... no power

I get the same result with the relay plugged in (key on/run position)

I also tested 85 again back to the AFM switch. I put the test light on the battery + and at 85. The light glows. (key off, relay out). Lift the plate and the light goes out and breaks the ground.

According to what you said above, I should have power at 87 so I need to trace back from there.



BTW, you said something different in DKLever's topic. In his topic you said, 87 no power. Can you clarify?

I also did another check 87 gets power when the key is turned to the STARTING position and power switches off when the key is turned back to the ON/RUN position.

I'm studying the Bentley manual and this seems correct. 87 is supposed to be a yellow wire to 50 on the ignition switch but it is only energized when the key is turned to START. (assuming I'm reading Bentley correctly)

Anthony,

In reference to DKlever48's post, there was a subsequent post I inadvertently mentioned about power to 87 and nobody noticed it. Then posted to correct the blunder. There should be no power to terminal #87 with the ignition switch @ ON/Run position. Terminal #87 only gets power when you turn the ignition switch to START position.

I went over to review your test results and everything seems right. But I did not find any test in particular about the FP relay itself. Have you tested and confirmed that the FPR's (FP relay) coil is switching from 87a-30 to 87-30 when energized? The relay could be defective or the contact/s at the socket is compromised. Keep us posted.

Tony