70s Vette Guy

Will do.

Thanks again.

Jebbysan

You have explained the Jebbysan way of vintage auto/bike wiring diagnoses........awesome.
Jump wires, a meter, test lights and knowing where to go and what to do......
I use the same method at work on our machines.....follow the path of the wiring.....find the breach or short.
On the starting system.....get a diagram (I use Doc Rebuilds huge laminated ones for my year). Diagrams don't lie....the problem is in there somewhere.
A lot of times it is the eyelet terminals at the starter itself.....bad connection...but the voltage drop will tell you this too.
Finding the problem yourself is very rewarding and can save you uber $$$$$.....
Most electrical problems can be fixed for under $10 unless the whole car needs rewiring.....


Jebby

OldCarBum

Check the wiring and especially the battery cables for continuity and resistance which could cause a low voltage condition.
A pinched, broken or corroded wire or cable could be the problem even though the connections are clean and tight and the wires and cables look to be in good condition.
Especially the grounds.
I had a similar hot start issue with my 73 and found that my battery ground cable looked to be in good condition, however it was corroded so badly under the insulation that there was only a couple of copper stands left.
The starter spun good when cold and dragged when hot.
I replaced the cable and no more problem.

wadenelson

Ok, OldCarBum, I gotta sacrifice you on the Altar of Voltage dropping to make an important point here:

You said: "Check the ... battery cables for continuity and resistance:

No, a thousand times no!

For starters, (or non-starters) continuity tells you nothing. Zero. Zip! Every single copper strand inside a battery cable, except for ONE, could be broken, and you would still have continuity.

And you cannot measure the resistance of a battery cable ACCURATELY with a Digital Ohmmeter (DVOM), DMM (digital multimeter, I use the two terms interchangeably) or even an old analog meter.

For starters, how hard you press the test leads against the work piece can vary the resistance reading by 1-10 ohms. As an EE, I don't even ATTEMPT to use to a meter to measure anything below 10 ohms because you simply can't get an accurate reading.

(BACKGROUND: OK TO SKIP

"Continuity" is a defined term. Most DMMs/DVOMs will beep indicating "continuity" if they measure less than 3-5 ohms. Others, 1 ohm or less. The whole point is a battery cable may be good at .1 ohm, and no good if it accurately measured two ohms! You can't distinguish with a continuity check!

p.s. You can test YOUR meter to see what amount of resistance it defines as continuity)

(END OF BACKGROUND INFO)

Since your DMM cannot accurately perform a resistance check on a very-low resistance wire, we substitute the technique of VOLTAGE DROPPING!

Remember, V = IR Volts equals amps times resistance?

If you know the Volts and the Amps going through a piece of wire, you can calculate the wire's resistance. R = V/I

It turns out a DMM can measure volts down to millivolts, so you can calculate a very accurate resistance #!

So Let's take a starter circuit. The big red positive cable from the battery positive terminal to the starter solenoid.

Let's just ASSUME the starter pulls 100 amps while cranking, ok?

If there is a 2.0 volt DROP from one side of that big red cable to the other, WHILE cranking, what's the resistance of that cable?

Turn the V = IR equation around V/I = R. 2.0/100 = .02 ohms. No way in hell your DMM can accurately measure .02 ohms

What is an acceptable amount of resistance for a five foot long red battery cable from the battery to the solenoid. .01? ohms? .001?

The answer is WHO CARES! If the solenoid is getting less than 9 volt WHILE CRANKING, there's a problem --- a bad battery cable, a corroded battery terminal, a loose cable stud on the solenoid. You've got an outrageous 3V drop in your circuit! Now go find it and fix it!

Just like you said, OldCarBum, a whole lotta times the cables and connections LOOK fine. It's not till you take them off the frame, terminal that you can see the corrosion and crud that needs to be cleaned up --- wire brush, grinder wheel, a touch of anti-seize, and re-assemble!

STORYTIME: I diagnosed a brand-new Sienna minivan that sometimes wouldn't start and wouldn't charge the battery properly. All that was wrong was a loose battery terminal, which allowed a layer of white "rust" to form between the battery post and the terminal. I only found it when I went to replace the battery, it was completely invisible until I removed the battery terminal. 30 seconds with a terminal brush and was done. END OF STORYTIME

Because a starter pulls so many amps even a tiny amount of resistance in either the positive cable, or ground circuit (battery to frame wire or connection, frame to engine ground strap, strap to motor) gets AMPLIFIED into a big Voltage drop.

A keyswitch circuit pulls a whole lot fewer amps. But the technique of VOLTAGE DROPPING still applies and works well!

Finding where a VOLTAGE DROP is ocurring help you identify a bad wire (strands broken internally), bad connection (bolted onto corrosion, grease, paiint), burned contacts (switch or relay), whatever the cause of the voltage drop is.

It helps to know some practical numbers: Headlights, you want less than a .5V drop or they won't be as bright. Starter circuit, less than .3V drop to function properly. ECMs, computers, less than .05V drop so your sensors get measured accurately.

All wires have resistance, there will always be SOME voltage drop through the wires, switches, connectors, ground connections, the problem comes when there is TOO much and the circuit doesn't function properly. For example:

Dim headlights
car won't crank hot
Engine temp sensor reads wrong
power window motor slow

Bottom line: You can find the cause of circuits that malfunction a LOT FASTER by voltage dropping circuits, looking for the "lost volts" than by attempting to check "continuity and resistance" particularly on very low resistance components like battery cables.

Now, that didn't hurt too bad, did it? Cheers!

wadenelson

bad info! Deleted! Thanks Big2Bird!

biackbenz

Vette Guy and Wade - I have the exact same problem with my 78. Been messing with it on and off for a long time. As the clutch switch is jumped out I'm guessing the PO had issues with this as well. For an interim fix (bubbaesque) I have connected a momentary contact switch to start the car when the starter won't work. When I need to use this the car starts right up. So as stated above it would appear that I have an issue with the secondary circuit between the key and the solenoid. Vette Guy, would you prefer I start my own thread or would you like me to add my testing to this thread. Will not hijack but I am sure looking forward to anything you find. Jim

Big2Bird

np.

70s Vette Guy

You can add here or create your own thread. I don't have a problem with either.

Still chasing down my problem.

derekderek

you can get really bubba and run a wire from S post into the car. open battery door and touch wire to pos post. car will start right up. the S circuit is bad. now, a circuit can test good to a voltmeter and not pass enough current to do the job. the solenoid takes quite a few amps. not like starter motor or cig lighter, but more than a headlight. switch terminals or a spot in the wire that got stressed could be the issue.

biackbenz

WadeNelson - Hope you can help me / us some more! I have the same problem as the OP. Sometimes the starter will not turn. When that happens the ampmeter does drop as I turn the key but nothing happens. As a temporary fix, and to keep from being stranded, I fixed a remote starter in the engine bay. Now when the starter will not turn, I press the button and the car starts right up. I have been using your test above and this is what I have so far:

1) I connected the positive probe of the meter to the starter ground and the negative to the battery negative. While cranking I had 210MV +/-. I understand this to mean that I have a good ground and this is not a problem.

2) I connected the meter positive to the main battery feed terminal on the solenoid. I connected the meter negative to the ground lug on the starter. 12.64 volts drops to 9.6V while I am cranking. This indicates that the primary circuit is OK, correct?

3) I connected the Positive meter probe to the "S" terminal on the solenoid and the meter negative to the battery. While cranking the voltage drops to 8.66. I should really have something more like 10V so this indicates a voltage drop in the starting circuit?

Note - the PO has jumped out the clutch switch. My car is a 4 speed. During cranking I have 9.6 volts at the yellow wire at the jumper.

The red wires from the solenoid travel to a plug located just above the starter on the firewall. These connections seem to be good. However, I did clean the up.

4) I put the positive lead on the S terminal and the negative probe into the socket of one of the red wires located on the start switch. 12.6 volts drops down to 1 volt while cranking. Does this mean anything?

Look forward to your help and hope this adds to the discussion.

biackbenz

I should have stated in my above post that my starter was cranking during all of my tests. Distributor disconnected and a battery charger connected to maintain full battery power.

I think my issue lies in the start switch. Any ideas as to what I can do next is greatly appreciated.

OldCarBum

Are you running the OEM points distributor or an aftermarket electronic upgrade?
The OEM ignition circuit runs a resistor wire (pink wire) to the coil which reduces the 12vdc to about 9.6 vdc when cranking the engine to protect the points.
If you upgraded your distributor to electronic, you need a minimum of around 10.6 vdc to fire the electronic ignition.
You need to replace the resistance (pink) wire with a dedicated 12 vdc wire from the fuse box under the dash.
Others have found other sources in the engine compartment to run the dedicated 12vdc wire but it’s my understanding that it is best to run it directly from the fuse box to the coil and connect it to the same connector at the coil and delete the pink resistor wire.

lionelhutz

This is usually considered threadjacking, but 70s Vette Guy posted he's OK with your problem being in his thread so I'll respond. Besides, it could help him too.

The readings quoted above are likely where your problem comes from.

The solenoid is likely just getting enough voltage when cold. Then, when hot it needs even more voltage to energize and won't pull-in. The coils of wire in the solenoid get hot which increases there resistance. The magnetic field to pull-in the solenoid relies on the current in the coils. More resistance means more voltage is required to get it to pull-in.

The power for all the electrical stuff in the car is fed from the main battery lug on the solenoid. You are dropping the voltage there to 9.6V while cranking which tends to indicate you have a positive battery cable issue.

Try measuring;
1. between the battery posts while cranking.
2. from the solenoid battery post to the battery positive post. You should get a similar voltage level as you did in your test #1.

stellar

If you ever see smoke coming from the wiring you will need this to make a proper repair.

Wrecked82

I had this identical problem. Remember lying on the ground at a gas station to start my car with a screw driver also - yikes! Anyway, I survived. Put a Protorque high torque starter, wrapped also on and 15 years later, still working.

Too Many Vettes

In response to GM starter heat-soak problem as written in Hemmings Muscle Machine by Ray Bohacz : Back around 1981 I bought a 1970 El Camino SS 454. I put a set of headers and an ordinary starter assembly on it. Soon I found it would not start after a heat soak. I discovered a NAPA "motorhome" solenoid and installed it. The solenoid had a brown Bakelite cap. Ever since I installed that solenoid no more hot start problem for 38 years .As best as I can tell, it is now sold as Echlin ST-124 for about $25. This seems to be a forgotten solution to many heat soak startup failures. Rays response: The correct application for 1955-75 was Echlin p/n ST-107 (or Delco p/nD-978) with a black phenolic end cap: however, when GM went to a higher -torque starter in the late 1970s, it made a new shorter solenoid to address the relocation of the armature connection on the top of the high-torque starter motor. The new unit was made with a higher temperature brown phenolic cap to address this high-heat isse. The Echlin ST-124 (ir DekciD-981} you refer to is physically shorter; however, the solenoid includes an extra longer copper bushing to fit the early armature connection on the starter, as well as a short bushing, so it will fit both high-torque and low torque applications.
In mid 70's I bought a big block 69 corvette I had heat soak problems. I installed heavy ground cable from battery to the engine block. No problems since.

augiedoggy

I had this issue on my 74 with sidepipes. My uncle also had this issue on his 76 with headers... After replacing the starter for my uncle I learned he had lied to me and told me he had a new battery and it didnt fix the issue... It was the battery in his case. In my case It was the starter.. I replaced with a newer S10 style starter and put heat wrap around it and that was 6 years ago. I also replaced the original ground cable from the engine to chassis because it looked questionable but I dont remember that being the issue.

Jetglow

I installed this starter hoping it would solve the "heat soak" problem......but it reared its head again yesterday. Acted like it barely wanted to turn over after driving 20 min. I shut it down and 2 min later went to start when it acted like a low battery ...so I waited until engine cool and started right up not problem.
https://www.summitracing.com/parts/p...+5ba295df391ba

I was hoping it was simply a starter heat soak problem...

I guess im going to go down the Ground to frame road now ......