87 GMC sierra electrical issues

[CrimsonLion86]

Just bought my first Squarebody. runs good with a 305, own by an older fellow who took pretty good care of it. the only issues I am having are electrical. When I punch the gas, the radio cuts out, then returns when I let off the gas. the only needles that move in the dash are the mph and temp. There is also an issue with the passenger side tank not activating when the switch is flipped. I had to replace a stoplight fuse the second day I had it. I am new to working on vehicles, and frankly I don't know where to start. If anyone can direct me to some literature or offer advice on where to begin it would be much appreciated. Thank you in advance,

C.L.

[Bandit130]

First...Welcome to the forum. There is a world of information here.
I would first check all around the accelerator pedal mechanism to see if any wires have gotten in it pinching and causing radio to go off and on.

Next, I would disconnect battery cable, remove each fuse from fusebox, physically check and with a multimeter to see if you have a bad one.
That is where I would start, then go from there.

[Big Port Jimmy 6]

Well I've heard of speed controlled volume, but not like this. There are several diagrams floating around about the fuel valves, first you need to see if you have the six wire valve or one of the older style. Does you switch say depress fully? We will all be glad to give you some help to get you going. Jay

[wilkin250r]

"New to working on vehicles" is a big red flag, so let's cover ALL the options so we can determine the problems you're facing.

First off, what year is this truck?

When you say the passenger side tank doesn't activate, that could be a couple different causes, obviously each with different solutions. It is VERY common on these older trucks that the tank, valves, and switch all work just fine and the tank switches properly, but you just can't see the fuel level in one or both.

So when you try to switch tanks, what does it DO? Does the gauge needle just peg all the way to the right?

[CrimsonLion86]

Well I certainly appreciate all of the quick responses! It is an '87 gmc sierra 1500 4x2. I did go through and check the fuses and had to replace two which solved a few of the problems. The radio issue is still present and I believe the switch or the tanks may not be the issue, I'm betting it is the fuel pump. when I return the switch to the driver side tank I can here the fuel pump engage but the case is not so for the passenger side. I lucked out on this truck, got it for 1200$. I went and purchased a Chilton manual for it today and started reading... I think that thing is going to be my bible for this truck.

[CrimsonLion86]

Originally Posted by wilkin250r

"New to working on vehicles" is a big red flag, so let's cover ALL the options so we can determine the problems you're facing.

First off, what year is this truck?

When you say the passenger side tank doesn't activate, that could be a couple different causes, obviously each with different solutions. It is VERY common on these older trucks that the tank, valves, and switch all work just fine and the tank switches properly, but you just can't see the fuel level in one or both.

So when you try to switch tanks, what does it DO? Does the gauge needle just peg all the way to the right?

I noticed when I switch tank the fuel gauge actually doesn't move at all. I think its the fuel pump not the switch itself

[wilkin250r]

I'm not as familiar with the TBI setup as other guys, but it sounds to me like you might have a bad valve at the tank, or bad wiring.

The switch on your dashboard activates a valve underneath the truck near the gas tanks. The valve does two jobs, it switches the fuel lines, and also switches the electrical connections. So either the valve isn't getting power, or IS getting power but still not working correctly.

[CrimsonLion86]

the chilton manual is probably going to answer this, but what is the difference between the valve and the pump?

[hatzie]

In 1987 The valve selects the plumbing. The pump is inside the tank on the sender.
There are several threads on troubleshooting and repairing the motorized 1981-1991 tank valve system. They are not difficult to troubleshoot or repair.

When you switch the ignition on;

• Where does the fuel needle settle? The gauge signal is controlled by the fuel valve.
• Where does the oil pressure gauge needle settle?
• What does the VOLT gauge do?

Does the radio cut out with the ignition on engine off and throttle pedal on the floor?

The GM service manuals tend to have a lot more year specific information than the one-size-fits-all Chilton. Look in my signature.

[CrimsonLion86]

Originally Posted by hatzie

In 1987 The valve selects the plumbing. The pump is inside the tank on the sender.
There are several threads on troubleshooting and repairing the motorized 1981-1991 tank valve system. They are not difficult to troubleshoot or repair.

When you switch the ignition on;

• Where does the fuel needle settle? The gauge signal is controlled by the fuel valve.
• Where does the oil pressure gauge needle settle?
• What does the VOLT gauge do?

Does the radio cut out with the ignition on engine off and throttle pedal on the floor?

The GM service manuals tend to have a lot more year specific information than the one-size-fits-all Chilton. Look in my signature.

To answer the questions regarding the needles I replaced a 30 amp fuse for the dash gauges and the needles all seem to be working fine... for now. I suspect a short may be why the fuses are tripping but I have only had the truck less than a week and don't think I can rely on the small amount of information I have on how it performed for the previous owner. as far as the radio issue I have only tested it while driving the truck but will test it without the engine running to see if it still has the same issue. again I appreciate the help, I feel ashamed for being a 30 yr old and not knowing more but at the same time I excited to be starting this project.



C.L.

[CrimsonLion86]

Originally Posted by CrimsonLion86

To answer the questions regarding the needles I replaced a 30 amp fuse for the dash gauges and the needles all seem to be working fine... for now. I suspect a short may be why the fuses are tripping but I have only had the truck less than a week and don't think I can rely on the small amount of information I have on how it performed for the previous owner. as far as the radio issue I have only tested it while driving the truck but will test it without the engine running to see if it still has the same issue. again I appreciate the help, I feel ashamed for being a 30 yr old and not knowing more but at the same time I excited to be starting this project.



C.L.

Actually I just realized our questions regarding the needles pertain to the fuel tank issue, and on that note I noticed when I switched tanks the needles actually do not move from the position the were in on the driver side tank. I did find some information regarding testing the electrical for the fuel system in the tank so I'm going to try those tips out this week. I will keep everone updated. Thanks again

C.L.

[hatzie]

Originally Posted by CrimsonLion86

To answer the questions regarding the needles I replaced a 30 amp fuse for the dash gauges and the needles all seem to be working fine... for now. I suspect a short may be why the fuses are tripping but I have only had the truck less than a week and don't think I can rely on the small amount of information I have on how it performed for the previous owner. as far as the radio issue I have only tested it while driving the truck but will test it without the engine running to see if it still has the same issue. again I appreciate the help, I feel ashamed for being a 30 yr old and not knowing more but at the same time I excited to be starting this project.



C.L.

If the fuse in question is the one marked GAGE/IDLE it should be a YELLOW 20A fuse.
If you had to put a GREEN 30A fuse in that slot to keep it from blowing you need to find the short and fix it before you burn the truck down. The offending wire will probably be pretty warm.
If you used 30A because that was all you had... get some 20A fuses. They are the most common size in the panel.

Fuel gauge needle is pegged at 3:00 or about that?

[CrimsonLion86]

The needle is actually sitting at just above the half full mark. I will look at the diagram under the dash again just to make sure I am referencing the correct fuse amperage and location.

[CrimsonLion86]

good call on the 30 not being correct... the only reason I replaced it with that is because that was what was in that slot to begin with. probably why the previous owner had problems.... just replaced it with a 20. thank you very much for pointing that out!

[CrimsonLion86]

ok so on break at work today I got to finish looking over all of the fuses. There were five incorrectly placed fuses... no wonder the previous owner was having problems. I replaced them all with the correct ones and have eliminated all previously mentioned issues. Radio works without a problem, all gauges are working fine. The only issue now is the parking brake light wont go off.....

[wilkin250r]

My parking brake light is due to a faulty cable, it has a spring on it for tension/return, and the spring is partially broken. When I release the parking brake, the spring pulls the tension off the cable so the brakes aren't engaged anymore, but it doesn't have enough "spring" to fully extend the pedal and trigger the switch.

First test I would suggest: Pull the parking brake release, and put your foot under your pedal and pull the pedal out. See if you have the same problem I do.

[CrimsonLion86]

Originally Posted by wilkin250r

My parking brake light is due to a faulty cable, it has a spring on it for tension/return, and the spring is partially broken. When I release the parking brake, the spring pulls the tension off the cable so the brakes aren't engaged anymore, but it doesn't have enough "spring" to fully extend the pedal and trigger the switch.

First test I would suggest: Pull the parking brake release, and put your foot under your pedal and pull the pedal out. See if you have the same problem I do.

I will do thank you!