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Username Post: Dual fuel tank selector        (Topic#230947)
bikernut 
Member
Posts: 86

Loc: NEW HAMPSHIRE
Reg: 02-04-03
12-03-09 06:08 PM - Post#1817292    

1987 K2500, 350 V/8 and 400 tranny
Can any one tell me if the above Selector Valve has power to it all the time or just for a short bit when you first turn on the key to pressurize the system and then after the engine starts the low oil pressure switch engages and powers it? I have one that seems to do that but I think the selector valve is junk as now the right tank won't work unless I disconnect the left side and jump to the right tank. I also can't find out where it gets it power to work the selector valve.
Can anyone give me a part # for the selector Valve or are they available from a parts store? any one know what they sell for? Do any of you guys have a good wiring diagram that you can copy and send me of this truck with dual tanks, I have purchased 2 books now and the wiring diagrams are junk, don't show much and I would like a more detailed look at the wiring system. thanks in advance for any help!
3500 Dually Member # 252 and Damn proud of it
2005 Chevy Dually Duramax w/Automatic with
enough power to pull this old guy from the P!ss Pot
1992 Chevy S-10 4X4 Blazer (wifes vehicle)
2003 Harley FLHTCUI (converted to a Lehman Trike)


 
Anonymous 

12-03-09 06:35 PM - Post#1817309    
    In response to bikernut

Too bad you don't have an 85...I just went all through that mess!!! I now know more about the fuel system of a dual tank truck then I ever wanted to know!!!

I suggest you look on Ebay for a shop manual for your truck!! That's what I did for my 85 Short/Wide 1/2 Ton...invaluable source of info!!!

 
Vaughn 
"13th Year" Gold Supporting Member
Posts: 15634

Loc: Colorado Springs, CO
Reg: 08-08-04
12-03-09 09:28 PM - Post#1817403    
    In response to Bobs85PU

It usually isn't the rocker switch inside the cab. Don't worry about whether there is power to the switch.

It is most often the selector valve under the cab of the truck. Yours has gone over center, which is why you have to reroute wiring and fuel lines.

Only way to fix it is to replace the selector valve underneath the cab.

 
Anonymous 

12-03-09 09:32 PM - Post#1817406    
    In response to Vaughn

  • Vaughn Said:
It usually isn't the rocker switch inside the cab. Don't worry about whether there is power to the switch.

It is most often the selector valve under the cab of the truck. Yours has gone over center, which is why you have to reroute wiring and fuel lines.

Only way to fix it is to replace the selector valve underneath the cab.



How different is the schematic for an 85 and an 87??? I could provide a bunch of info, if they're similar!!!

 
bikernut 
Member
Posts: 86

Loc: NEW HAMPSHIRE
Reg: 02-04-03
12-04-09 03:27 AM - Post#1817485    
    In response to Bobs85PU

Bobs85pu, the selector valve under on an 85 is the same as an 87. the only thing different is wire colors and the 87 has the electric fuel pumps in the tanks so that adds to the mess and the rocker switch is different. I could use any and all info that you have on this subject. thanks and you can ping me at this address;
bikernutgoofy@gmail.com
3500 Dually Member # 252 and Damn proud of it
2005 Chevy Dually Duramax w/Automatic with
enough power to pull this old guy from the P!ss Pot
1992 Chevy S-10 4X4 Blazer (wifes vehicle)
2003 Harley FLHTCUI (converted to a Lehman Trike)


 
Anonymous 

12-04-09 09:06 AM - Post#1817621    
    In response to bikernut

You have an E-mail with the schematic attached!!

 
bikernut 
Member
Posts: 86

Loc: NEW HAMPSHIRE
Reg: 02-04-03
12-04-09 11:56 AM - Post#1817704    
    In response to Bobs85PU

Bobs85PU, where is the "Fuel Module Circuit Card" located in the cab? thanks this will save me knee pain if I can locate it quick.
thanks bikernut
3500 Dually Member # 252 and Damn proud of it
2005 Chevy Dually Duramax w/Automatic with
enough power to pull this old guy from the P!ss Pot
1992 Chevy S-10 4X4 Blazer (wifes vehicle)
2003 Harley FLHTCUI (converted to a Lehman Trike)


 
Vaughn 
"13th Year" Gold Supporting Member
Posts: 15634

Loc: Colorado Springs, CO
Reg: 08-08-04
12-04-09 03:44 PM - Post#1817781    
    In response to bikernut

No, sorry 85 switching units were not the same.

87 was the first year for fuel injection, so in addition to switching between tanks, the in-tank pumps were switched (no provision for this on carb units) - so the switching valves are not the same. The fuel injection units were designed to operate under pressure, while the mechanical pump carb style trucks switching valve were designed to operate under vacuum.

If you attempt to operate a carb switching unit in place of a fuel injection switching valve, the carb switching valve will fail because of the added pressure.

 
bikernut 
Member
Posts: 86

Loc: NEW HAMPSHIRE
Reg: 02-04-03
12-04-09 03:58 PM - Post#1817795    
    In response to Vaughn

The switching valve is the same from 81-87 whether it is carbed or TBI I found this out when I called my local GM dealer and started asking some questions about them.
3500 Dually Member # 252 and Damn proud of it
2005 Chevy Dually Duramax w/Automatic with
enough power to pull this old guy from the P!ss Pot
1992 Chevy S-10 4X4 Blazer (wifes vehicle)
2003 Harley FLHTCUI (converted to a Lehman Trike)


 
Anonymous 

12-04-09 05:00 PM - Post#1817823    
    In response to Vaughn

  • Vaughn Said:
No, sorry 85 switching units were not the same.

87 was the first year for fuel injection, so in addition to switching between tanks, the in-tank pumps were switched (no provision for this on carb units) - so the switching valves are not the same. The fuel injection units were designed to operate under pressure, while the mechanical pump carb style trucks switching valve were designed to operate under vacuum.

If you attempt to operate a carb switching unit in place of a fuel injection switching valve, the carb switching valve will fail because of the added pressure.



Were all 87s TBI?

I have no knowledge of an 87 truck, I can only pass on that the LMC catalog shows two different switches...one for a carb engine (which fits 81-87, Part #36-0732), the other is for the TBI engine (which fits 87 only, Part #36-0743)

According to the O'Reilly Auto Parts site, Carb and TBI 87 dual tank trucks take the same selector valve (part #U7001).

 
Anonymous 

12-04-09 05:05 PM - Post#1817827    
    In response to bikernut

  • bikernut Said:
Bobs85PU, where is the "Fuel Module Circuit Card" located in the cab? thanks this will save me knee pain if I can locate it quick.
thanks bikernut



Can't help!! Remember, I have an 85 Short/Wide C10.

 
bikernut 
Member
Posts: 86

Loc: NEW HAMPSHIRE
Reg: 02-04-03
12-05-09 05:52 AM - Post#1818097    
    In response to Bobs85PU

yes the dash switches are different but I was referring to the switching valve under the truck that does all the work not the dash switch, sorry for the confusion. Check out page 134 in LMCTruck and it shows this. thanks
3500 Dually Member # 252 and Damn proud of it
2005 Chevy Dually Duramax w/Automatic with
enough power to pull this old guy from the P!ss Pot
1992 Chevy S-10 4X4 Blazer (wifes vehicle)
2003 Harley FLHTCUI (converted to a Lehman Trike)


 
Viny 
Forum Newbie
Posts: 5

Reg: 01-25-10
01-25-10 09:58 AM - Post#1849138    
    In response to bikernut

Ok, Here's a little bit I can add to this. I have a 1987 K20 PU with a 350. I had the engine rebuilt and it has an edelbrock carb (unsure of which one) and dual tanks. I'm in the process of replacing the fuel sending units as they are shot...hence my surfing and discovery of this forum and post.

The drivers side tank is factory and has a standard FSU with 3 tubes and the passenger side is aftermarket and only has a fuel out and vapor back (I guess?). The fuel level guage is different than factory and I'm currently in the process of obtaining that.

So, anyway, this truck doesn't have fuel injection, the fuel pump is mounted on the block near the front of the passenger side and the fuel selector valve is indeed located under the cab, kinda on the passenger side. My inspection of it shows several fuel lines in and out of it and 1 wire connecting to it only. Therefore, I surmise it works to choose between the two tanks whether there is power applied or not. (I am happy to be corrected on this if this is wrong.)

I don't use this truck often and have had a nagging problem of having to prime it after it sits for some period of time. I assume the fuel in the bowl evaporates and the fuel in the line drains back into the tank. For this reason, I'm toying with inserting a check valve into the fuel line to attempt to keep fuel in the line. I would definitely appreciate any feedback/opinions as to this theory and possible solution.

Any other specifics I could provide, I sure will. Thanks in advance....

 
bikernut 
Member
Posts: 86

Loc: NEW HAMPSHIRE
Reg: 02-04-03
01-25-10 10:47 AM - Post#1849169    
    In response to Viny

Are you sure the vehicle is an 1987 as all Gm turned to TBI in 1987, I have never seen one without TBI. Yes the vehicles with the engine mounted fuel pumps had 3 lines and the TBI's just have 2 lines. The one wire is for the fuel gauge and is supplied with about 4 volts so a test light will glow very dim when checking for voltage. Your fuel swap switch has nothing to do with that one wire at the fuel tank, it is routed to the transfer valve located under the cab on the passenger side under the frame crossover and it will have 5 wires to it and 6 fuel lines I believe. you can put a check valve but that won't stop the fuel from vaporizing out of the bowl when not used. good luck
3500 Dually Member # 252 and Damn proud of it
2005 Chevy Dually Duramax w/Automatic with
enough power to pull this old guy from the P!ss Pot
1992 Chevy S-10 4X4 Blazer (wifes vehicle)
2003 Harley FLHTCUI (converted to a Lehman Trike)


Edited by bikernut on 01-25-10 10:47 AM. Reason for edit: No reason given.

 
Vaughn 
"13th Year" Gold Supporting Member
Posts: 15634

Loc: Colorado Springs, CO
Reg: 08-08-04
01-26-10 01:33 PM - Post#1849983    
    In response to bikernut

Check valve will do nothing. The plugs in the base of the float bowl on the carb are leaking. These are usually epoxied in place during a carb rebuild to keep it from draining out.

If it sits for a long time, you will still have to prime it to get it to start - whether you epoxy the plugs or not.

Get a couple RV electric fuel booster pumps, and install them just ahead of the tanks. Anytime you want to start it, you will run the booster pumps for a few seconds before starting.

They HAVE TO BE the electric RV boost pumps. These style of pumps have a one way valve in them and are DRAW-THROUGH style of electric pump. No standard electric pump is draw-through, so it will not work with your stock mechanical pump. These are commonly used on RV's that have a little trouble with vapor lock.

 
Viny 
Forum Newbie
Posts: 5

Reg: 01-25-10
01-26-10 04:49 PM - Post#1850101    
    In response to Vaughn

OK, I do stand corrected. I had a guy at the auto parts store tell me the same thing so I dug out the title and it indeed is an '86. I wonder how long I've had that a year off in my mind??? Good thing I don't take myself too seriously about this stuff...

Anyway, I do appreciate the information about the check valve and its futility. I also appreciate the advice about adding the RV booster pump.

Would you mind posting a link of the type you are referring to? While we're at it, I'm going to redo the mounting of the add-on tank on the passenger side. It was mounted using metal straps that ran from the frame to the bottom edge of the fender. I am considering obtaining a pair of Original style L brackets and elongating the straps that go up and over. What do you think of that approach and do you have a better idea for this? Again, I appreciate the guidance.

 
75gmck25 
Contributor
Posts: 345

Loc: Alexandria, VA
Reg: 12-25-07
01-27-10 05:34 AM - Post#1850492    
    In response to Viny

My truck had an add-on aux tank suspended from the bed, and I just went to the junkyard and got a complete factory aux tank with brackets and a sending unit. It think they charged about $25. Just make sure the sending unit has the same number of lines as your original.

Bruce

 
FERG 
Frequent Contributor
Posts: 1115
FERG
Loc: Wylie, Texas
Reg: 05-22-03
01-27-10 07:11 AM - Post#1850540    
    In response to 75gmck25

Sorry to hijack...

Does anyone have a schematic that shows the wiring for the fuel sending untits and the fuel gauge on the dash. Mine keeps reading weird and I need to track down grounds.....


Mine is an 82 with dual tanks.


Edited by FERG on 01-27-10 07:11 AM. Reason for edit: No reason given.

 
Viny 
Forum Newbie
Posts: 5

Reg: 01-25-10
01-27-10 07:51 AM - Post#1850567    
    In response to Vaughn

Is this what you are talking about?

http://www.amazon.com/Mr-Gasket-42S-Electri c-Fuel/...

Thanks

 
Anonymous 

01-27-10 05:45 PM - Post#1850976    
    In response to FERG

  • FERG Said:
Sorry to hijack...

Does anyone have a schematic that shows the wiring for the fuel sending untits and the fuel gauge on the dash. Mine keeps reading weird and I need to track down grounds.....


Mine is an 82 with dual tanks.



This is for an 85 C-10:



 
Vaughn 
"13th Year" Gold Supporting Member
Posts: 15634

Loc: Colorado Springs, CO
Reg: 08-08-04
01-28-10 01:49 PM - Post#1851444    
    In response to Bobs85PU

It might work, but I would not buy a Mr. Gasket fuel pump - the quality is pretty low. Look at the feedback at the bottom of the page from people who bought it.

I went looking for the RV booster pumps yesterday, and couldn't find the pump online (RV dealerships used to have these on their shelves). It may be that they replaced them with vane pumps, which can be used in a draw through fashion and are self priming. Vane pumps are a lot more reliable that solenoid pumps anyway.

An example:
http://cgi.ebay.com/ebaymotors/Carter-P60 504-Inlin...

Since this is self priming, you MIGHT be able to get away with using only one pump, installed after the fuel tank selector switch.

 
Viny 
Forum Newbie
Posts: 5

Reg: 01-25-10
01-29-10 10:17 AM - Post#1852081    
    In response to Vaughn

Vaughn, Thanks for the info. I will look into it.

Man,the reviews for the Mr. Gasket pump were bad. Ok, when I get back to this project, I'll let you know progress....

 
Viny 
Forum Newbie
Posts: 5

Reg: 01-25-10
01-10-11 09:55 AM - Post#2029819    
    In response to Viny

OK, So, I thought I'd post a "last word" kind of follow-up in case anyone could benefit from it. So, in review, I had a problem with my '86 Chevy PU 350 e/w an Edelbrock carb. When the truck sat for any length of time, I would have to prime the carb by pouring gas into the throat to start it up. I did 2 things that appear to have fixed this.

After consulting with Edelbrock, I installed a thermal isolation gasket between the carb and manifold that is supposed to keep the heat from the engine boiling off the gas in the carb after shut-off. This was occurring and the gasket stopped it (or at least made it much better).

I also installed a check valve in the fuel line between the fuel pump and the filter. This has had the added benefit of not letting gas drain back into the tank as the truck sits for an extended period of time.

So, now I can jump in the truck, and after no more than about 5 seconds of cranking, the truck starts right up. I think the longest time it has sat is on the order of 3-4 weeks at a time.

So, I hope this helps someone who has been wrestling with this very annoying situation. Any questions, let me know.,,

Thanks

 
gchemist 
Senior Chevytalk Moderator
Posts: 21993
gchemist
Loc: Austin, TX 78748
Reg: 05-09-00
01-10-11 05:28 PM - Post#2030028    
    In response to Viny

Push the pedal before starting. Setting the choke helps at startup on cold days!
'83 Silverardo XST - ZZ4 powered
'96 GMC Jimmy LS Ret. @236651 miles


Edited by gchemist on 01-10-11 05:29 PM. Reason for edit: No reason given.

 
badbob85037 
Forum Newbie
Posts: 54

Loc: Arizona
Reg: 03-13-09
01-31-11 02:48 PM - Post#2040127    
    In response to gchemist

That did help along with the wiring schematic. Thanks

 
badbob85037 
Forum Newbie
Posts: 54

Loc: Arizona
Reg: 03-13-09
01-31-11 07:00 PM - Post#2040245    
    In response to badbob85037

I'm back. You wouldn't have a schematic for the two tank set up for a 1974 k-10. I lost a tank a couple years ago from a clown without insurance pulling out on me. I'm just now getting around to mounting a new one and the wires look like they need replaced. The switch valve has only one light green wire but the switch in the cab has 5 or 6. I think the three tan are to change the fuel gauge. It's the others I'm not sure of. Thanks.

 
gchemist 
Senior Chevytalk Moderator
Posts: 21993
gchemist
Loc: Austin, TX 78748
Reg: 05-09-00
01-31-11 08:02 PM - Post#2040281    
    In response to badbob85037

You can find all the wiring diagrams here: http://forum.73-87chevytrucks.com/smforum/index.ph...
'83 Silverardo XST - ZZ4 powered
'96 GMC Jimmy LS Ret. @236651 miles


 
muddin_wolverine 
Forum Newbie
Posts: 47
muddin_wolverine
Reg: 06-16-08
02-14-11 10:24 AM - Post#2046475    
    In response to gchemist

autozone also has a bunch of wire diagrams on their website, I use it frequently

I HATED my dual tanks far too much to fix them both ha so I dropped them and put a 32gal cell in the bed, but it's an offroad rig, so not really feasible for most

'87 chevy k10, 454, 10in susp, 3in bod, 42in tsls, 4speed/np205
'05 R6
'90 s10, P.O.S.


 
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