If there is 0V on the red wire going into the ign switch then there cannot be any power further down the line towards the start relay coil such as on either side of the crank fuse. before the red power input to the ignition switch there is only the hot at all times input to the fuse box under the hood follwed by the 20A ignition fuse then the orange wire, D5 connector and red wire to the ignition switch. The problem would be in this chain.

Don't know how there can be power to the downstream crank fuse if there is no power to the ignition switch.

George

 

I am looking at a slightly different wiring diagram and it shows:

battery junction block> fusible link (16ga black)>red>d5 connector>red >ign sw>start>yellow>d1 connector>yellow>crank fuse>purple>neutral/park switch>yellow>starter relay coil>black>gnd

No orange and no purple/white but the input to the ignition switch is still red and should be always on. upstream of that is the d5 connector another red, a fusible link and an always on battery junction block. Given no power to the ign sw this should be where your problem is.

George

 

I think I measured the crank fuse incorrectly, I measured across it, i.e. one probe on one side of the fuse contact and the other probe on the other one - the fuse continuity reads good.

I will double check the measurements i made at the ignition sw, access to the contacts is not great, so I could have screwed up there.

 

Since you indicated no power at the red wire input to the ingnition switch why not check the always hot red wire on the battery side of the ingition switch connector?

George

 

So to test the hot red wire on the battery side of the ignition switch I test from the red wire coming from the battery to one side of the crank fuse?

Just to clarity - the crank fuse is 10A (says 10A on the diagram sticker inside the fusebox) not 20A, not that should be an issue any way, and as far as I can see I have no fuse box under the
hood, mine is on the left side of the drivers dash.

I took the measurements again. Start

Relay
Purple - Start=0v Run=0v Off=0v
Yellow - Start=0v Run=0v Off=0v
Red - Start=12v Run=12v Off=12v
Black Start=0v Run=0v Off=0v

Fuse 10A crank fuse
Fuse removed then measured across fuse connector
Run=0v Start=12v

Measured from each fuse connector to ground, 0v each time

Ignition switch
Red - Start=0v Run=0v Off=0v
Yellow - Start=0v Run=0v Off=0v

I'm measuring from the wires going into the multi-wire plug that plugs into the ignition switch, so red probe stuck in there, definitely making contact, and my black probe going to a good ground.

Where is the always on battery junction block located? And the d5 connector red, fusible link?

I swapped out the ignition switch for the old one (which also looked fairly new) just for giggles, still had the same issue.

I'm trying to get at the starter motor and solenoid to measure, but it is difficult to get my probes on it. I can that the casing to the solenoid is pretty rusty though.

 

I think that at this point picking apart anything after the ignition switch is the wrong approach. You are not getting always on power to the ignition switch which is what gets sent down stream for functions like start. No power here and the truck will not start. I don't have your truck series but if the wiring diagram that I have is accurate then the way that always on power gets to the ignition switch is:

Battery>12 gauge blue fusible link/red wire>alternator positive post>12 gauge blue fusible link/red wire>battery junction block>16 gauge black fusible link/red wire>D5 connector>red wire>ignition switch

The battery junction block will probably be in the engine compartment, possibly with a cover over it. It will be a series of threaded posts in a row distributing 12V power from the battery to various subsystems. You'll see some heavy red wires among others. A fusible link is a wire with a fat section somewhere built into the wire. It blows open if amperage is exceeded. I'm betting that this link is blown. Locate that junction block and make sure that the entire block is at 12V. Check for tight not corroded connections. Then find the D5 connector which is probably the connector on the steering column where the ignition switch pigtail is connected to the harness and measure the red wire contact from the junction block. If the connector is hard to seperate then probe through the wire with a sharp small probe or needle. Sometimes you can also probe into the back of the connector where the wire enters. If your hot at the block and not at the D5 connector then the fusible link wire has to be replaced.

While there are times where you measure voltage across something like a fuse but in most cases just keep your black meter probe grounded and probe each point with the positive probe wire.

George

 

Thanks George, I really appreciate your patience with me.

I have found out one more thing, the starter relay is shot, it had recently been replaced, it smelt wrong, slight burnt smell, I then measured and tested it and fails on both counts. I swapped it out for another new one, but as I suspected, it didn't make any difference as I'm not getting 12v on the yellow wire to the relay. I also jumpered out the red and purple wires and the starter cranks, just to rule that out

I've found the connection box, as you face the front of the car it is under the front right wing (drivers side), it looks ok, but have more testing to do.

So, what with the relay being toasted, this may point to a high current short someplace, and also with your guess that a fusable link is most likely blown, is hopefully getting me in the right direction. The black wire right next to the large red wire in the photo below, has a fusable link, it measured fine, I made a slight cut one side and then continuity tested from the other side. The other black wires in the photo have fusable links, so I need to check them. For reference they have 'fusable link 1mm sq' printed on them.

I tested from the large red wire (shown in photo) to the red connector on the ignition switch, and there is 0v, I then measured voltage from the same red connector to one side of the 10A crank fuse, there is 12v, I then measured the voltage from the red wire while attached to the terminal (shown in photo), with my ground test probe to the negative on the battery, all of the terminals in battery terminal box have 12v.











97 Chevy Blazer LS 4WD, battery terminals, as you face the car, under right wing (drivers side)

 

As I mentioned before, we need to leave the negative probe of your volt meter grounded and use the positive probe to check individual voltage levels as suggested here. There are times to check the voltage drop across something or to check continuity but not at this time. Continuity can be very misleading because the very light load from the meter may show continuity but the item being measured may not perform properly under an operating load. That why we just probe the voltage level before and during cranking to find out what is going on.

The large red wire at the connection block is most likely the voltage source from the battery/alternator post. It should measure 12v at rest and around 10-11 volts when cranking the engine. It is always on. You need to find the fusible link that goes from those posts to the ignition switch. It should be a black link and it should then become a red wire on its way to the ignition switch. You need to measure the voltage on this wire after the fusible link both at rest and during an attempt to crank the starter. You may have to open some of those wire loom coverings to find it.

George

 

Every terminal block post shown in the photo has 12v with reference to the negative on the battery.

Every fusible link has 12v with reference to the negative on the battery.

The large red wire does indeed come from the battery/alternator.

I now have to repeat the the above measurements when I can get somebody to turn the ignition key for me, a friend of the family is coming over who is more knowledgeable than me on such matters as this.

Just for giggles, I cut an opening into the red wire that goes into the ignition switch, after the large multi block connector, so about 5 inches from the ignition switch, then jumpered (I put a fuse inline) from the constant 12v at the battery terminal block, so the red wire at the ignition switch now has 12v, and it made no difference.

 

When you say every fusible link has 12V do you mean where the fusible link attaches at the terminal block or on the other side of the fuse link from the terminal block (on the load side)?

So there is normally no 12v on the red wire going into the ignition switch but when you introduce 12v at that point near the switch, correcting that problem - the truck still won't start? Does that red wire go directly into the switch or does it go through a connector first? If there is no connector then what I would do next is rule out a disfunctional ignition switch by jumpering 12v again right before the switch on the red wire and see if 12v shows up at the output of the ignition switch on the yellow wire when you turn the switch to start.