1984 Big Red Electrical Problem

[Jim Dunn]

Just getting my Big Red back together following a stuck exhaust valve problem. In checking things out, I find that I can't get the starter to crank the engine. Check of the system, I find that I have 12V through the solenoid and to the end of the starter cable, when the cable is disconnected from the starter. However, when I make connection to the starter, I only read 6V. The battery is fully charged and connections seem tight. You ever been there and done that? Thanks. Jim

 

[phucheneh]

6v when you're not even cranking?

A low voltage reading at the starter, with 12v+ at the terminals of the battery, would generally indicate a bad cable. The cable is becoming a load in the circuit and 'using' voltage. But it shouldn't happen when the starter is not being triggered...your B+ cable should have no path to ground, and thusly there is no current flowing. And you can't have voltage drop with no current flow...you will always read battery voltage to the point of an open circuit.

When you turn the key, the 12v wire to the solenoid gets switched on (should have no voltage with the key off), and that completes the connection between the big B+ terminal and the guts of the starter. Current then flows through them and ground is found through the starter housing and the engine block.

Are you sure you have the starter connected right? You didn't accidentally connect the solenoid wire to ground, did you? I suspect not, as you would have either immediately blown a fuse when you turned the key...or set the wire on fire (if the circuit is not properly fused).

 

[tortillasoup]

Bad cables, bad grounds, bad everything. Car is so old that you need to disconnect some cables and do some ohms testing. Bet everything is horrifically corroded. Sand every metal part until shiny, don't scrape any paint off.

 

[LTC8K6]

How did you determine a full charge on the battery?

 

[phucheneh]

Bad cables, bad grounds, bad everything. Car is so old that you need to disconnect some cables and do some ohms testing. Bet everything is horrifically corroded. Sand every metal part until shiny, don't scrape any paint off.

Read more. Seriously.

If his post is accurately expressing what is happening, it's not as simple as excessive resistance in the cable. You should not ever see voltage drop across a positive connection that is not currently powering anything. A starter cable should have zero current flow when it's not turning the starter. Something else is up. You will someday kill someone just because you were unable to admit that something is over your head.

Look at it this way: If you had a garden hose hooked up to a spigot with 40psi of pressure, and at the end of the hose you have a closed spray nozzle with 20psi of pressure...your problem is not a clog; it's a freaking leak.

OP needs to be careful. A bad cable could actually be keeping his truck from burning to the ground. The first thing I would do is make absolutely sure that there is no continuity between the positive cable and ground ('continuity' and 'resistance' are the proper words for that 'ohms test' ).

 

[tortillasoup]

Well if OP has everything turned off (not trying to crank) and there is a strong enough load by merely attaching the battery cable to the battery to bring a HEALTHY 12V battery down to 6V @ the terminals, then there is a serious problem. Now if he is saying that when he makes the connection to the starter, does that mean that there is a strong current draw even when not cranking? I'm kinda curious from where he is doing this voltage measurement. Also I think it's funny that you think my advice is "dangerous" considering the fact that there is nothing dangerous about inspecting his electrical wiring harnesses and cleaning up his electrical grounds.

OP can do an ohms test of the battery cable on both ends and also of the starter cable of both ends, should really be 0 ohms reading and any more could be damaged cable or dirty connections.

 

[hanoverphist]

sounds like a short in the line to me

 

[RLGL]

Just getting my Big Red back together following a stuck exhaust valve problem. In checking things out, I find that I can't get the starter to crank the engine. Check of the system, I find that I have 12V through the solenoid and to the end of the starter cable, when the cable is disconnected from the starter. However, when I make connection to the starter, I only read 6V. The battery is fully charged and connections seem tight. You ever been there and done that? Thanks. Jim

With everything connected, do a voltage drop test on the leads to the starter, the drop should be no more than .5 volt. Also check from the engine block back to the negative post. Battery connections need to be clean, tight and dry