Thread: Low voltage at solenoid mid-70's SBC

Tweet

My two-year-old battery produces a steady 12.24v, also measured at the cable end when disconnected from the solenoid, but the reading is approx. 3v when connected. That drops to less than 1v when I turn on the ignition switch, the opposite of what I expected. None of the gauges even respond.

The car is garaged and covered during the winter months (I live in PA), so I routinely start the engine every three weeks, running it until the upper radiator hose is hot. I cleaned all electrical connections last spring (2021), with the result being excellent and consistent gauge readings, the headlights and running lights were brighter than before and the voltage drop was slight when the headlights were switched on. The volt meter never dropped below 12v, even at idle (approx. 550rmp). Life was good.

This February (2022), I turned on the ignition switch and all the gauges responded as expected. However, the instant I tried to start the engine, everything went dead and has stayed dead as I described in the opening paragraph.

I have a Ron Francis wiring kit since 1992, so all the known fuses are within relatively easy reach and none are blown.

Any thoughts as to what might have happened when I tried to start the engine in February? As far as I know, nothing happened to the car in the three weeks prior to February. After some research, I'm leaning towards a problem with the ignition wire or the solenoid.

If it's running and your reading 12 volts. The charging system is not working. Even at idle, a good charging system will output between 13.8 & 14.2 volts. Recharge the battery out of the car or at least disconnected cables in the car. Give it a slow charge so the voltage reading ( after charged is +/- 12.6 volts) Then retest while running. Does you meter have an OHM reading? If so verify each of the battery cables for high resistance readings.

It's not running, but the last time it was, January 2022, the charging system was 100%. As an engineer, I my immediate reaction is to look for "what changed" that cause the problem, so my focus was on anything that could be different in the three weeks since the last engine start, such as dead battery or a "hidden" fuse that suddenly blew when I hit the starter. I'm not averse to replacing parts until the problem is solved, but that can get expensive (maybe not in this case) and I'd like to actually diagnose the problem to the extent possible before doing so.

Check your battery cables. Could be an internally damaged cable (had that happen due to hidden, internal corrosion) or bad connections at each end. Was the previous cleaning with something corrosive? If so, there may have been quick corrosion at the connection points.

Sounds like you had a marginal connection that allowed low amp draw components (like gauges) to work, but the instant you applied real load, the connection gave out.

Also test each cell in the battery. It can still test at 12 volts but have no amps behind it.

Probably goes without saying.....

Most electrical problems are the result of bad connections - especially poor ground. Negative cable(s) needs to be connected to bright metal on the engine block. Grounding strap(s) need to be installed to chassis from engine - again to bright metal. Star washers are your friend as well. I recommend a bonding the case of the altenator directly to the chassis as well.

When cars sit, the process of galvanic corrosion still occurs.

And "bright metal" means No Paint! sand or grind away any coating(s). I was just a young squirt freshman in high school, auto shop class, we learned about batteries and connections. A few days later my mom is having a new battery installed in the family car. At the Sears Auto center, I'm watching the guy replace the battery and before he re-installed the terminals he sprayed some corrosion product onto the posts. Out of my mouth comes, "Well, that is stupid" LOL.. the guy spins and tells me to shut up, he does this every day for a living, he knows what is best! I smirk and tell my mom something like it'll never start again! LOL.. Of course once he was done.. it wouldn't crank.. well duh!!! I just sat there smirking at him.. Never said a word.

Thanks for all of your contributions. I was hoping the problem was anything but the battery cables because the battery is very difficult to access as it's hung below the car and also requires removing an exhaust pipe. I finally spent the hour plus this morning pulling out the battery and all the cables (including those to a shut-off switch) and found that the positive cable to the shut-off switch had no connectivity via my VOM and zero resistance. Bingo! All the others had connectivity and very low resistance.

Since I had all the cables off, and they're at least 30 years old, I replaced them with new and now the gauges all turn on, the fan spins and the engine cranks.

In my admittedly limited experience, every electrical problem prior to this one was due to a poor ground. Shiny metal-to-metal contact is your friend.

Thanks for the update. Glad you found the problem.

times 2, always nice to know what the end result was.

Good Job Dorsey!

Ya gotta feel pretty satisfied that the problem has been identified, addressed and your action plan produced the desired results.

Glenn