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Electronics Need help wiring something up? Thinking of adding an electrical component to your car? Need help troubleshooting that wiring glitch?

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Old 07-22-2009, 08:30 PM   #1
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Fuel Sending unit

My car recently quit running. I confirmed my fuel pump is bad.
I have a 1989 RS 305 TBI.

My gas guage has been stuck on full since I bought the car. I am thinking now is a good time to replace the fuel sending unit. However, the quote for the fuel sending part from my local chevy dealer is $700.00

Does anyone know of other Fuel Sending units preferably cheaper?

THANKS!
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Old 07-22-2009, 11:45 PM   #2
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Re: Fuel Sending unit

1st - if you haven't tried these two things - before you tear the tank out ....

1) Check the fuse on the pass side of car, under hood, beside battery, tucked up in the fender - there is a single fuse there in a weatherpack - if it's blown, no fuel pump.

2) Check the Oil Pressure Sending Unit - should be located above the oil filter - unplug the connector to it, there are 3 wires, two are larger than the third. Jump the 2 larger ones with a piece of paperclip in the connector, then place ear on hump in back of car and listen for the whine of the fuel pump - if it runs, you know it's not a bad pump, but the OPSU.

Then, if still no pump, try CarQuest. They sell the pumps, as well as the sending units - I think my pump was $56 - it was an AC Delco part - much better than an Autozone/Advance pump, but cheaper than stealership prices.

If you have to replace the pump - some words of wisdom (I've done 2).

EMPTY THE TANK AS MUCH AS POSSIBLE - the sloshing fuel makes wiggling that thing out a bear!

1) Get a jack capable or lifting at least 20 inches - a $15 Walmart jack won't lift high enough. You'll need good jackstands that can hold trhe car up nice and high - the nose will be almost touching the ground when you have the rear of car high enough. Place jackstands under the car on the framerails just in front of the lower control arm attachments.
2) Plan on just taking the rear end out - it's much easier. The only thing holding it in is torque arm, sway bar end links, panhard bar, the bar above panhard (don't remember name), LCA's, and shocks.
3) To get the tank out, I had to jsut slightly (and carefully) bend on the filler tube - it come out so far, and then stops, that's where I put a tad of even, slight pressure on the tank to slightly bend the filler tube to wiggle it the rest of the way. Upon reinstall, put it back in there, attach the straps, and then crawl out, and using something long and strong (like a 2ft piece of metal conduit), gently bend the filler tube back to center of the hole, so your plastic filler neck trim will fit back correctly.
4) Don't forget the electrical connectors, pressure valve, etc. attached to the top of the tank - disconnect these.

Good Luck!
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Old 07-23-2009, 12:03 AM   #3
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Re: Fuel Sending unit

and dont forget to also move the exhaust too. or you could look here - http://www.thirdgen.org/techboard/fa...cess-hole.html
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Old 07-23-2009, 02:55 AM   #4
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Re: Fuel Sending unit

Quote:
Originally Posted by camaronewbie View Post
2) Check the Oil Pressure Sending Unit
Which has no relevance to weather the pump works or not.
It's is a redundant (fail safe for the relay) power source to the fuel pump and is not even needed for fuel pump operation.

I really wish people would quit using the oil pressure switch as a reason for the fuel pump not working because it's wrong


The gauge reading full may be indicating a break in the circuit rather than the sender being bad. That is a matter of testing the sending unit wire at a few points Or, if you're going to be changing the pump, the sender can be tested directly with a multi meter.
A grounded circuit will read empty.

Last edited by deadbird; 07-23-2009 at 03:04 AM.
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Old 07-23-2009, 12:33 PM   #5
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Re: Fuel Sending unit

Quote:
Originally Posted by deadbird View Post
I really wish people would quit using the oil pressure switch as a reason for the fuel pump not working because it's wrong
OK - not trying to really argue the point, but rather discover.

This is based on my experiences with 3 3rd gens, a 1990 RS TBI, a 1992 RS TBI, and a 1987 Bird CC Carb. In all 3 cases, it's been my experience that unless the OPSU isn't working properly, the pump won't run.

With you're scenerio, the OPSU is a 'redundant' secondary power source - meaning that if the primary source fails, the secondary 'redundant' source kicks in. In the interest of discovery, I ask ... what device in the vehicle 'decides' when the primary source has failed and how, and then how does it 'switch' to the secondary 'redundant' power source?

It is a fact that the OPSU can go bad in such a way that it reads oil pressure when there is none. When this happens, the fuel pump will run constantly, regardless of whether the key is in the ignition or in Timbuktu. The ONLY interruption between the source of power between the battery and the OPSU is the fuse in fenderwell beside the battery-unless that fuse is blown, the OPSU gets CONSTANT power regardless of the key position. If the OPSU was a secondary source, would this event not occur unless 'the decision' was made by the primary source?

The relay and the ECM can be taken out of the picture, and the fuel pump will run - that's how we all switch to carb, remove the ECM and relay, and still utilize the intank pump without any rewiring what-so-ever. My 1992 RS is currently setup this way - I have no ECM, and no relay, and my pump runs just fine with no added or changed wiring.

It's therefore my understanding that, the fuel pump relay is controlled by the ECM, and the process of a stock car is as follows ...

1) Turn key on. This provides power to the ECM, which sends a power signal to the fuel pump relay switching the relay ON, which runs the pump to send fuel up the line.

2) Two seconds later, the ECM cuts this power signal to the relay, cutting off the fuel pump so that it's not in a constant running state. By this time, the driver has started the car. And if driver didn't start the car yet, the pump is not running constantly wasting battery energy and/or creating a danger by having fuel pumped constantly. This 'decision' I can easily see happening, for it's not a 'decision' being made by any component, but rather a preprogrammed action on a timer - an easy task for these old computers.

The ECM will only send the 2 second 'priming' power signal to the relay once for each "key ON" sequence initiated. If the key is left in an ON state for say 8 hours, and the fuel that was 'primed' up to the engine has since drained back, or evaporated - the car may not start, unless there is another "key ON" sequence to reprime the pump.

3) If vehicle is started, then there is now oil pressure. Since the OPSU gets a constant supply of power directly from the battery, as soon as there is an oil pressure reading and as long as there continues to be an oil pressure reading, the pump will continue to run, which is where the pump gets it's power until the car is shut off, and there is no oil pressure reading.

4) As assumed by many, and may very well be the case, this design may well be a safety feature - in case of serious accident, hopefully no oil pressure will be present, thus the OPSU is no longer sending a constant supply of power to the fuel pump, keeping the pump from continuing to pump dangerous gas into the engine compartment where there may be issues to cause a fire.

It's my understanding that the ONLY reason for a relay at all in the fuel powering system is that the ECM can not directly power the fuel pump since all it's functions are low amperage with very large guage wiring (small wires), thus the ECM sends a small amp power to the relay to activate it, where the larger electical requirements are thus sent to the pump.

Anyway - that's been my experience with the 3 cars I've dealt with, and to me it makes perfect sense - it's a system that works MOST of the time. The one flaw in the system is the OPSU - when it goes bad either 1) it reads no oil pressure and thus no pump function (which really isn't a flaw, but leads many to think that their fuel pump is dead), or 2) it reads oil pressure whether any exists or not, and thus the pump runs constantly, which drains the battery after about 48-72 hours.

I might would have more insight if I ever saw a wiring diagram that shows the complete fuel pump wiring for these cars, but I have yet to see one that was complete. Haynes doesn't even have a wiring diagram for the fuel system on a TBI car in their book at all (they do have for TPI cars and carbed models), and neither does Chilton's that I ever saw, and after 2 hours of searching, I never found a complete diagram in ALLDATA either.

Again - all this is NOT to create some big argument, but rather to provide information for the basis of discovery and knowledge (and friendly debate). I'm more than happy to be wrong so that any fellow TGO member can gain the insight necessary to successfully fix, maintain, and modify their vehicle to obtain their desired effect.

PS - For anyone interested...

I have made use of the fuel pump relay in my car. Since I swapped to a carbed setup, I haven't been happy with the OPSU having total control of my fuel pump. I've been left twice because of this - once just this past June at Carlisle (how embarrassing).

So, I disconnected my OPSU, and replaced it with an older unit that didn't control the fuel pump, but only sends the signal to the guage (the earlier model cars had two seperate units, one that sent signal to the guage, and one that ran the pump - later models merged these functions into one unit). I took the smaller wire from my OPSU connector, and wired it directly to the single wire connector on the oil sending unit from the earlier models. Then I took the two remaining wires from the OPSU connector and rigged them into the fuel pump relay - the other two contacts on the relay are wired to a ground and to a run-in-hot wire.

This made it so that my fuel pump will NEVER run without the key in the ON position, since the fuel pump is now on a relay that requires power from the key on state to activate the relay. BUT ... this also means that as long as my key is ON, my pump will run - thus I have no "safety" feature left to deactivate the pump given a serious accident. I also plan on tapping into the relay with wires to run in-cabin, so that I can rig a kill switch as an anti-theft device.
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Old 07-23-2009, 01:37 PM   #6
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Re: Fuel Sending unit

I'm going thru the same thing right now. My fuel sending unit is all rusted and looks like crap. I googled it and am finding prices more in the 160 - 230 range for new aftermarket parts but haven't decided which way I'm going yet.
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Old 07-23-2009, 04:07 PM   #7
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Re: Fuel Sending unit

I fully understand how the switch works. There is no argument to be made.

What I said is it is not required and it isn't.

Quote:
In the interest of discovery, I ask ... what device in the vehicle 'decides' when the primary source has failed and how, and then how does it 'switch' to the secondary 'redundant' power source?
There is no 'device' or 'switching'.

It is a simple pressure switch. When oil pressure rises above 4(ish) psi, the switch closes. It is wired to the hot side of the relay and the pump side of the relay, bypassing it.

The ECM switches the relay, prime, crank and run, regardless of the OPS there or not.

The ECM will will run the pump the instant it receives a pulse from the ignition, the same thing makes the injectors pulse. The OPS again, not needed.

When the engine is running both the relay and OPS are supplying power. If the relay fails, the OPS keeps the pump running.

It's a much less complex circuit than you're making it out to be.


I do apologize if I came off as an *** in my first reply though.


Quote:
I might would have more insight if I ever saw a wiring diagram that shows the complete fuel pump wiring for these cars, but I have yet to see one that was complete.
Circuit is basically the same for any of thirdgen with a fuel pump...
Attached Images
File Type: gif OPS.gif (14.3 KB, 61 views)

Last edited by deadbird; 07-23-2009 at 04:38 PM.
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Old 07-23-2009, 05:29 PM   #8
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Re: Fuel Sending unit

Quote:
Originally Posted by deadbird View Post
The ECM switches the relay, prime, crank and run, regardless of the OPS there or not.

The ECM will will run the pump the instant it receives a pulse from the ignition, the same thing makes the injectors pulse. The OPS again, not needed.
Could very well be - I was always under the impression that if the OPSU didn't sense oil pressure, the pump would not run - making the OPSU necessary in order for the pump to run at all after the initial 2-3 second prime. I guess maybe I made that assumption from several sources

1) being statements from many on TGO that this was built in as a safety feature

2) that my factory fuel pump will run without the ECM and without the relay(it just won't prime), but it will NOT run without the OPSU

3) that my factory pump would not run without the OPSU even when the ECM and relay were connected - it would only prime.

My totally stock 1990 TBI had a bad OPSU, and I would only get a 2-second prime until I replaced my OPSU - and I made the mistake of changing out the pump when it wasn't necessary. This was hilarious, although not at the time - I was 8 miles from nowhere at night, and could get the car cranked and running for about 200 yards before it died (off the 'priming' fuel) - so I started the car, jerked it into drive, mashed the gas to get as much speed as possible, then it died and I coasted until it stopped rolling, and repeat - 8 miles of this, until I got to a phone at least I made it After replacing the pump, the new one didn't work either - I thought I would explode until I found out about the OPSU - I felt like

Quote:
Originally Posted by deadbird View Post
I do apologize if I came off as an *** in my first reply though.
Oh no - not at all! Like I said, I've made assumptions based on experience and reading through TGO posts like everyone else here, and they may very well be wrong. And, whether anyone else likes to admit it or not, I will boldly admit that every car has it's own personal Gremlins, which makes my problems slightly different than anyone elses, even if we're talking identical year models/motors/etc.

Like I said, in the interest of discovery, and getting info to folks with problems, whether it be correct or incorrect, any and all info is good info in my book! As with anything else, info has to ultimately be dissected as to it's worth by the one seeking the info.

And, I'd much rather discuss and dissect and debate info with a fellow TGO brother (or sister) all day long rather than drop another gas tank on a 3rd gen! This has been fun for me, as I hope for you - it's nice to rack my brain about something other than work!

Hope any and all of this helps a fellow member!
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Old 08-28-2009, 12:19 PM   #9
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Re: Fuel Sending unit

You helped me out!! I actually thought that the OPSU served as both a backup and 'safety' switch all in one unit. It's funny because I've heard 2 sides of the story regarding the OPSU's function: 'The unit will not allow the fuel pump to prime (and then crank) when it senses low or no oil pressure. That way you won't blow up your engine' (for safety). And also 'The unit acts as a secondary power source should the fuel pump relay fail and upon cranking, when the switch senses a certain amount of pressure, it primes the pump allowing start-up (for backup). Since seeing this so much, I guess I just got to thinking that it did both tasks, not!
Thanks for clearing that up.

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Old 09-20-2009, 06:42 AM   #10
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Re: Fuel Sending unit

I just bought this one and going to install this week.

http://www.rockauto.com/dbphp/prt,22,FG20A

It is a perfect match for the one in my 89 IROC.
I paid $126 with tax and shipping. Really good deal considering what the dealer wants,
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Old 09-20-2009, 06:42 AM
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