Fuse block power supply

[71K1500]

Hi.
Joined a while ago but never posted.
My son bought a 71 GMC 4X4 woth a 5.7 tbi mitor from a 92. It's having random shut off issues.
Im wondering what wire/s supply power to the fuse block in the key on pisition.
It shuts off as if the key is switched off but only when the throttle is released such as when slowing down. Im thinkjng a loose wire or plug.
Thanks
Jeff

[franken]

If I recall, it's a large red wire that comes from the battery. There's a junction bloack on the fender next to the battery that should have a fusible link on the battery side. It goes over the radiator to the connector by the master cylinder.
There's a color wiring diagram on this site that might help...

I'd check if say the horn or dome light don't work when the thing shuts down, since that wire powers everything but the starter motor.

[71K1500]

Thanks Franken. I'll check that wire for a break.
Trying to figure this out has been frustrating.

Jeff

[randy500]

That same wire continues into the truck to the key switch and also powers the fuse panel cig lighter and maybe something else with key off, ignition switch then takes over to transfer power.
Purple, starter to solenoid during start only
Brown, accessories, acc position and on, no power during start
Pink switched power when on and during start
Red power in from battery

Check, but those are the main colors and the colors correspond to key position/function.

[kwmech]

What are the shut off issues??

[dmjlambert]

Yes, a agree with randy500, except one very minor thing: on my truck pink has power with key on and during start. I think the trucks may vary slightly between '67 and '72. Mine is a 1969.

Always-on power from the battery goes to the ignition switch on the red wire, and also to the top section of the fuse panel to power the brake lights, CIG lighter, and hazard blinker.

With they key in the on position, power comes out of the ignition switch on the pink wire to power the ignition coil or the HEI distributor's ignition coil and also to power the IGN UNFUSED terminals on the fuse panel.

Also with the key in the on position, power comes out of the ignition switch on the brown wire to go to the fuse panel lower section for radio, wipers, and AC blower motor.

Is it the engine that loses power and shuts off when throttle is released, or other things turning off such as blower motor or radio? I suggest hook up a test light or volt meter at the coil or HEI distributor verify you're actually losing electricity, and that you don't have a problem with the idle on your carburetor. You can use alligator clip test wires and a volt meter or test light to connect to the various terminals on the fuse panel to see where exactly you are losing power.

Here are some diagrams and links:

from this thread: http://67-72chevytrucks.com/vboard/s...98#post8004798

Ignition switch diagram:

which is a snippet from main electric diagram: http://67-72chevytrucks.com/vboard/s...27#post9186727

[71K1500]

Thanks for the response everyone.
The engine shuts off as if the ignition is shut off. Just like turning the key off. Everything else seems to work fine. My son bought the truck like this with a 5.7 tbi and 700r4. The farmer who had this changed it over years ago. Ive changed out and tested iac, cap ,rotor, plugs, pump relay, etc.
Im sure its electrical but cant duplicate it in the shop. It only happens when throttle is released such as slowing down and ocassionally at an idle. It will crank over and start right back up but sometimes have to crank 2 or 3 times. It never loses power or shuts off when on throttle or cruising speed.
I dont think its fuel related as it never stumbles. Its as if the key is turned off. Im thinking of replacing the ignition switch and trace from there.
Ive also posted on Chevytalk site and have suggestions on the pickup coil and icm.
Jeff

[71K1500]

Not sure pics are upside down
Sorry

[dmjlambert]

I suggest use a thin wire, such as a 28 or 30 gauge wire, strip the end and put it over the terminal on the distributor coil and push the connector onto it. Then run that wire into the cab and connect it to a voltmeter that you can view while driving. You can find out if you lose voltage right at the point where the electricity goes into the coil. If it dies while driving, look at the meter and see if you still have electricity there.

It sounds like you may have an electric fuel pump. Repeat the test for the fuel pump by checking that you have electricity at the fuel pump when it dies. If the fuel pump has a separate wire going to it for ground, connect the volt meter across both wires of the fuel pump, so you can know your pump is powered. There may be an oil pressure switch that allows the fuel pump to run only if the engine is turning, or another type of safety shut off that is causing the problem.

[RichardJ]

Some interesting responses.

I thought kwmech's question "What are the shut off issues??" was the most useful.

Recently had a problem with Son-in-Law's '94 TBI350. It would die at idle, after warmed up. Similar to what you did, I checked fuel pressure, TPS and IAC. Changed really old, crusty plugs and cap/rotor.
HEI module had been installed dry. Put heat transfer grease under it and no change.
Replaced HEI module with a used one that I thought was good.

I didn't die at idle anymore, but then after two mile warm up would die slowing to a stop as you described.

Changed HEI module for a new one and problem gone.

A heat transfer compound is not the same thing as a dielectric grease. Be sure you use the correct heat transfer product as many people will sell you the wrong thing.

[7tee]

Originally Posted by RichardJ

Some interesting responses.

I thought kwmech's question "What are the shut off issues??" was the most useful.

Recently had a problem with Son-in-Law's '94 TBI350. It would die at idle, after warmed up. Similar to what you did, I checked fuel pressure, TPS and IAC. Changed really old, crusty plugs and cap/rotor.
HEI module had been installed dry. Put heat transfer grease under it and no change.
Replaced HEI module with a used one that I thought was good.

I didn't die at idle anymore, but then after two mile warm up would die slowing to a stop as you described.

Changed HEI module for a new one and problem gone.

A heat transfer compound is not the same thing as a dielectric grease. Be sure you use the correct heat transfer product as many people will sell you the wrong thing.

I had the same problem and did the module replacement and all was good from then on...

[toolboxchev]

I am in the process of rebuilding a used good panel due most likely to the same issue.

It is a bear to remove, clean and hopefully tin all the connectors with solder.

Most of the guys are spot on about the red wire coming in. This area is suseptable to galvanic corrosion due in part to the different metals used in construction. Brass, Copper, Steel.

There is also a Horn Relay up in there that has an always on position. The connections in there can be problematic also.

[71K1500]

Great suggestions, thanks.
On my way to pick up a new ignition switch and change that out.
As far as the coil, its not a coil in cap. Its separate. And it has shut off simply idling while warming up. Its cold here, -17 at the moment. There is no hesitation when it shuts off. Immediate as if the key is turned off which is why im thinking ignition, and will spin over but not alwsys start unless the key is left off for a few seconds.
As for the fuel pump it puts out 12 psi. It always restarts within a few seconds so its hard to isolate the trouble. If it wss a no start situation I think it would be easier.

Jeff

[Andy4639]

Have you looked at the back side of the key switch where the wires all connect to it. I had one in my 71 get hot and melted the plastic and the wires would come disconnected when it got hot enough for the plastic to deform. Took me several days to figure it out. Replaced the plastic connector on it and all has been fine since.

It was doing the same thing as yours.

[RichardJ]

Coil in cap?

>>As far as the coil, its not a coil in cap. Its separate. And it has shut off simply idling while warming up.<<

Is that in reference to my post?

The TBI still uses an HEI distributor, but with a 8-pin HEI module instead of the old 4-pin. It's halfway under the cap.

[71K1500]

Thanks for the suggestions.
I put a new ignition switch and checked the plugs and wires. They were all good Andy and nothing melted. Didnt fix the issue. I picked up a new hei and coil so will change those tomorrow. Thanks Richard.
My snapon guy had an old MT2500 reader so I plugged it in and tried to figure it out. Seems like the iac (idle air control) is reading 0- 2 at idle (about 575 rpm). To my way of thinking this would indicate an air leak somewhere. Ive checked with no luck. Im beginning to think the tbi base gasket or egr may leak randomly.
Oh well....tomorrows a new day eh.

Keep the suggestions coming
Thanks
Jeff

[Andy4639]

While it's running spray WD-40 all around the base plate and vacuum hoses. I know years ago the vacuum lines where bad about cracking and it would cause all kinds of problems.

[71K1500]

Ive done that several times. wd40, carb clean etc. Ive moved wires, tested components. Thats why im pulling my hair out and throwing parts at it lol. My underststanding is the tbi system is pretty simple. Basically its like the old carb system but updated with a few sensors.
Theres got to be something im missing but havent found it yet. Ill try again tomorrow with wd40 but i think I'll chsnge the base gasket anyway since I have one.
I think I'm just venting a bit.
Thanks guys

[franken]

Replacing parts until randomly replacing the right one isn't a wise approach.
You might have a look at the addition to post #2 at the end if you missed it.

[71K1500]

Thanks Franken didnt notice that. I'll check next time it happens. Did it twice this morning then fine rest of the day.
I know that throwing parts at it is wrong. I own several hwy trucks and that approach gets expensive really fast. I cant duplicate the problem and it gets frusyrating.
Appreciate all the input.

Jeff

[71K1500]

Well its been 4 days with no issues. Runs like a champ.
I went through everything again. Testing, tracing wires, vacuum lines etc. I finally pulled the egr valve again to bench test. Held vacuum perfectly. Plunger was fairly clean of carbon but I figured to clean it anyway. I found out that the plunger valve that closes off exhaust gas would occasionally leak. I made a block off plate and gasket and installed them. I changed the pcv as well. Hasnt skipped a beat since.
I think what was hsppening is that when coming off throttle it would randomly stick open a bit which caused both a vacuum leak and exhaust gas in the cylinders which couldnt be compensated for.
Time will tell if my theory is correct. I'll keep you updated if it changes but for now I just want to thank all of you for your help.
The people on this site are great.

Jeff