No electrical power in truck-what to check?

[coyle13]

I could use a little direction on where to start.

First off. '91 Blazer, stock TBI, SM465, Scottsdale trim (no pwr windows, locks, cruise, etc)

I tired to start the truck after it was parked for about an hour after a 5 minute run. (So, engine wasn't to hot). It had ACC power for a second and then-poof-.....nothing. No horn, lights, etc. I didn't hear anything pop or see any smoke or smell anything burnt.

I checked the battery and I have 12.5v. Next, I checked the firewall junction box and I have power on both sides, although it is only about 10-10.5v. I don't have any power at the fuse panel. I didn't find any blown fuses on the fuse panel.

Questions-
1) Are there any fusible links from the firewall junction to the fusebox?
2) Is there anything I should check other than checking the continuity of the power wire from the junction box to the fuse panel?
3) Is there anything funky (ECM, dist. module, relay, etc.) that could cause this that I should check first?

I do have a full set of factory manuals and I do know the basics of diagnostics, but I would appreciate any direction I could get that would help narrow the search.

Thanks,

 

[Thunder]

There is or should be a wire that goes from the pos terminal to the big terminal on the alternator then feeds the fuse box. Check it for bad or corroded connections. I cant rember if there is a fuseable link in it or not. Some times that wire will corrode at the battery and just fall off. You will have no power to ign and truck wont start.

 

[4X4HIGH]

Sounds like you have a blown fuseable link which is down near the starter. The wire coming off the large starter lug that goes up to the junction block is where the fuseable link is at.

 

[coyle13]

But wouldn't the junction box on the firewall not have power if I had a blown fusable link at the starter? (i.e. the hot lead off the starter is good if I am getting power at the junction box?)

 

[dyeager535]

Look at your wiring diagrams, see how it's supposed to be wired. I know on the older setups, the fusible links down near the starter if blown, no the junction block won't get power.

But as Thunder mentioned, if there is something wrong with the wiring to the alternator on the newer trucks, it can cause issues with the electrical system. If the wiring is batt->starter->junction block then yes, your fusible links are fine if you've got voltage to the junction block.

 

[4X4HIGH]

But wouldn't the junction box on the firewall not have power if I had a blown fusable link at the starter? (i.e. the hot lead off the starter is good if I am getting power at the junction box?)

Whoops, i read to quickly, i thought you said that you had NO POWER at either side of the junction block. In this case the fuseable link is good. With low voltage you have a dirty or loose connection or bad ground somewhere.

Check to make sure the large nut is tight on the starter solenoid.

 

[coyle13]

Great.

Thanks for the suggestions.

I will look at the manual tonight and see where the power goes from the battery to the fuse box and see if the alternator wiring plays into it.

 

[Thunder]

Great.

Thanks for the suggestions.

I will look at the manual tonight and see where the power goes from the battery to the fuse box and see if the alternator wiring plays into it.

All the alternator hot 12V connection is used for is a junction block. Power goes thru a 10 gauge wire from batt to the large hot connection at the alternator, where another wire is connected that feeds the fuse block for Hot all the time circuits. There is a fuseable link located near the alternator. Check that wire before you do anything else. Especialy near the battery where corrosion may be. If your pos battery cable is still stock. The wire is black so it is hard to see if it has broken off the battery terminal.
Wires to the starter have absolutely nothing to do with your problem. That is for older trucks. TBI blazers hot 12V to the fuse box are fed thru the alternator.

 

[4X4HIGH]

The junction block on the firewall of my 89 blazer has the hot lead coming from the HOT starter lug. My truck is 100% stock as far as the wiring is concerned.

 

[Stomis]

I had the same problem with my scotts. Mine was a main starter terminal that was hanging on by 2 threads of a coroaded bolt.

 

[mikkelsent9584]

I had a similar problem with my 90 Suburban. No power at the fuse box and all links were ok . I pulled the steering column down and the , oh shi whats it called, wires in the ignition switch came loose. Plugged em back in no more issue everything works great again. Wierd problem really, hope this helps good luck.

 

[coyle13]

It looks like the hot lead to the junction box is coming off the starter on my 91, as well.

I can't figure out how the alternator would play into this. Shouldn't you be able to start a truck without an alternator? I have had a truck (mind you it was an old IH Scout) with smoked alternator and it still started and ran, just not for very long. Plus, I have NO POWER to anything in the truck. How would the charging system play into this?

 

[dyeager535]

If you've got no power anywhere, start with the battery cables, period. If you've got voltage at the "truck" side of the battery cables (and a good battery ground) then you need to look at what's going on between the junction block and starter/alternator, whatever.

Again dealing with my older truck, there are actually two fusible links, powering essentially half of the fuse panel each. If I lost all power, I'd probably not be suspecting the fusible links right off. Obviously there could be a huge problem that popped both of them, but it's more unlikely than a loose or corroded battery terminal or cable.

There isn't going to be a "switch" that cuts power off to the fuse panel, (kind of relating to your ignition system question, if it's making you doubt how things are routed) if there is no power at the fuse panel, somethings wrong with the wiring in the engine bay.

 

[Thunder]

It looks like the hot lead to the junction box is coming off the starter on my 91, as well.

I can't figure out how the alternator would play into this. Shouldn't you be able to start a truck without an alternator? I have had a truck (mind you it was an old IH Scout) with smoked alternator and it still started and ran, just not for very long. Plus, I have NO POWER to anything in the truck. How would the charging system play into this?

As I said before the alternator has nothing to do with it. It is simply used as a junction for a hot 12V wire. Your truck will not start if there is something wrong with that wire. However the starter and lights will work.
That wire feeds parts of the Ignition, ECM, radio, FPR, and is used as control voltage for many relays. It is pretty hard if not impossible to start a TBI truck without ant power to the ECM and fuel pump relay. The low voltage electronic circuts are fed from this wire to avoid voltage surges and interferance from the starter which could fry the ECM.
It is not the only 12V feed. There is the wire from the starter too. The starter wire feeds the high amperage draw circuts like lights, heater, AC, and starter solonoid. That is the wire that has the junction on the firewall.
I am not saying that the wire to the alternator is your problem. Just to check it to make sure it was connected. To elminate it as a source of the problem(s), there could be more than one thing wrong.
If you have a full 12 V at the alternator. Then the wireing to the starter and connections. may be your problem.

If you have no power to anything or low voltage as you said. Check your battery cables/ Connections, battery Ground wire. ECT.

 

[coyle13]

I understand.

I will just check everything, starting with the battery cables/grounds and work my way back.

I was just wondering if there was some "common" problem with TBI/GM trucks and no power (similar to bad steering/cracked frame or intermittent ignition/bad module-both of which I have experienced). I was pretty sure there wasn't a magic bullet, because I did search for a while before posting.
I mostly lurk and search, as you can see by the # of posts I have had in the last 4 years.

Thanks for the suggestions and I will keep you all posted.