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Question on Wiring Diagram

Help, Advice and DIY Tutorials on Volvo's extremely popular car line -- Volvo's 1990s "bread and butter" cars -- powered by the ubiquitous and durable Volvo inline 5-cylinder engine.

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km427
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Question on Wiring Diagram

Post by km427 » Sun Feb 28, 2016 10:28 am

I'm missing something here. I'm simply trying to trace a wire off of the fuel pump relay.
In the diagram, the fuel pump relay is 2/23. I'm trying to trace the wire coming off of the 31b terminal of the relay, which should be a switched ground. The diagram sends it off to "C". My problem is trying to figure out just exactly what is "C".
I've attached a pic of the diagram, and here is a link to the entire wiring portion of the service manual for reference. http://www.volvowiringdiagrams.com/volv ... agrams.pdf

If I can just figure out what C is I'd be golden, but I'm lost and have gone through every single page of the PDF trying to figure it out with no luck.
I tracked down all of the Y-GR color wires as well and it's not helpful. One goes from the instrument panel to a connector and then to the fuel level sensor, but between the sensor and connector it is a W wire, not Y-GR like it is between the connector and instrument panel. The second one travels from 2/76B:3 (2/76 is listed as the Control Module Central Locking / Anti-theft Alarm) through connector 24/3C (which is listed as firewall cable harness - left door sill cable harness) on to 7/100:1. component type 7 is a sensor. But I can not find sensor 7/100 listed anywhere, so no idea what it is.

So to end with a couple of concise questions:
1. What is "C"?

2. What grounds 31b on fuel pump relay 2/23? Is it possibly grounded by the anti-theft system via 2/76?

3. The fuel pump relay is a 15/87/31/31b. 15 switched input (12v) from ignition. 87 is output to fuel pump. 31 is battery ground. 31b is switched ground (to what is the mystery). My question here is regarding the behavior of this fuel pump relay. Is the relay closed and providing 12v through 87 when 31b is grounded, or when it is not grounded? I'm not having any luck locating this info.
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Re: Question on Wiring Diagram

Post by Ozark Lee » Sun Feb 28, 2016 11:14 am

C directs the circuit to the next page. The corresponding C will be on the left hand side of the next page.

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Re: Question on Wiring Diagram

Post by sleddriver » Sun Feb 28, 2016 11:19 am

The diagram you posted in on page 86 of the manual, which is page 17/133 of the pdf . The missing page you are seeking is on page 87 of the manual, which is page 106/133 of the pdf. A bit confusing when ordered that way........

Hope that helps....
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Re: Question on Wiring Diagram

Post by km427 » Sun Feb 28, 2016 11:27 am

Thanks Ozark and sleddriver! Too simple! Much appreciated.

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Re: Question on Wiring Diagram

Post by km427 » Sun Feb 28, 2016 11:34 am

31b on the fuel pump relay runs to B45 on the ECM. So I would then assume that the ECM grounds this line to trigger the relay and start the fuel pump? And if I'm correct in that thinking, if the ECM is doing it's job here, I should have continuity from 31b to GND when key is in II, and an open circuit when the key is off?
EMS Diagram FULL pg2.png
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Re: Question on Wiring Diagram

Post by abscate » Sun Feb 28, 2016 12:29 pm

Welcome to the complexity of fly by wire cars....ugh

You can't always assume 31 is a battery ground, it's a ground function, but not necessarily a ground. The ECU is not just a simple switch to ground so you can't assume a simple check of that reminds to ground confirms operation.

Trace it from your fuel pump relay to C, and then , if I recall it actually gets ported to the throttle position switch where it probably ( total guess) gets processed as a throttle unloading or full throttle cutoff switch or something, which sends a signal to the ECU for actual grounding.

Be careful putting signals ( which includes measuring voltages and continuity ) on ECU connected stuff. It is sensitive to static discharge as well as voltages it isn't expecting.
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Re: Question on Wiring Diagram

Post by abscate » Sun Feb 28, 2016 12:42 pm

If 7/51 is the TPS my WAG was actually correct .....
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Re: Question on Wiring Diagram

Post by km427 » Sun Feb 28, 2016 1:07 pm

abscate wrote:Welcome to the complexity of fly by wire cars....ugh

You can't always assume 31 is a battery ground, it's a ground function, but not necessarily a ground. The ECU is not just a simple switch to ground so you can't assume a simple check of that reminds to ground confirms operation.

Trace it from your fuel pump relay to C, and then , if I recall it actually gets ported to the throttle position switch where it probably ( total guess) gets processed as a throttle unloading or full throttle cutoff switch or something, which sends a signal to the ECU for actual grounding.

Be careful putting signals ( which includes measuring voltages and continuity ) on ECU connected stuff. It is sensitive to static discharge as well as voltages it isn't expecting.
According to the diagrams it is just a straight connection (no connectors or anything between) from 31b right to the ECU. Any thoughts on how to check this?

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