1997 Pontiac Grand Prix Won't crank

Hi archemedes. Welcome to the forum. Can you describe the symptoms? Any sounds from the starter? Do any lights go out, get dim, or stay bright? Any other tests or voltage measurements taken already?

Caradiodoc

Hmm. 85 amps is too much if that was measured under no load on a test bench. An old heavy Chrysler V-8 starter from the '70s only draws around 50 amps. 85 amps under load in the car is not enough. That suggests a bad cable, cable connection, or one open circuit of the two in the starter. The most informative test would be to measure the voltage directly at the starter during cranking. You might want to hook up the voltmeter with clip leads so you don't have to try to hold the probes while the engine rocks back and forth. Put one probe of a cheap digital voltmeter right on a clean spot on the case of the starter. The other probe goes on the large battery cable terminal on the starter. The industry-recognized standard value is 9.6 volts during cranking. If you find less than that, there is excessive resistance in the ground or feed circuit that must be located with more measurements, and corrected. If you find more than 9.6 volts, move the probe from the battery cable terminal to the other heavy terminal and recheck the voltage. There will be 0 volts there until the starter engages. If the voltage is lower there than on the first test, the contact disc in the solenoid is pitted. That will cause low current and a big drop in voltage. Usually they cause a no-crank condition with a single kind of loud clunk from the starter. Slow cranking is generally a problem in the cables.

A less common problem is one of the two circuits in the starter is open. Worn brushes is the most likely cause, but often they cause an intermittent problem. This can be a hard one to figure out because the voltage measurements seem fine and current is near normal. Current flow actually goes way down once the starter motor gets up to speed. 150 amps is typical. When one of the starter circuits is open, only half of the normal current is drawn. Since the motor spins real slow, its current for the one working circuit goes way up, so high that it looks like the motor is drawing normal current. It continues to spin slowly because it is only building up half of the normal magnetic field. I don't know of a better way to determine if this is the problem other than to change the starter motor if all of the voltage measurements show no other problems.

Caradiodoc

I guess I can see the current being that low. They are tiny motors with a planetary gear reduction system so they do develop a lot of torque. What do you mean by "Voltage drops too low during cranking"? Where are you measuring?

Caradiodoc

Dandy! 7 volts is too low. Now we just have to figure out where it is being dropped.
Start right at the battery with the two probes right on the terminals. I assume you still have the miserable side post cables so go right on the two 5/16" bolt heads. You should see around 12.6 volts for a fully charged battery. Read the voltage while a helper cranks the engine. If it is less than 9.6 volts, the battery is defective or the cables aren't making good contact.

Next, if the voltage was ok in that last test, leave the probe on the positive bolt, and move the negative probe directly to the engine block and measure again during cranking. The reading should nearly match the first test. If it is lower, put one probe on the engine block and the other one one the negative battery bolt. Move the range switch on the voltmeter to one of the lowest ranges for better accuracy. Ideally you should see 0.0 volts during cranking. This checks for high resistance in the ground cable. It is acceptable to read up to 0.2 volts during cranking. Anything more indicates high resistance in that cable.

The same test can be done on the positive cable. You're looking for a voltage drop of more than 0.2 volts per mechanical connection or 0.4 volts in the entire positive or entire negative circuit.

Caradiodoc

Did you do the voltage tests? Where exactly were the voltmeter probes when you measured 7 volts?

Caradiodoc

Ok, that still doesn't say where the measurements were taken. I don't know how those lugs are set up physically or which order the parts are in. Starter current might not go through those lugs. Where were you measuring when you found the 7 volts? Was that right on the positive jump-starting lug, and where was the other probe, on the engine?

Caradiodoc

I'll be in and out for the next few hours so please be patient. I'll check back often.

I'm pretty sure those lugs are not part of the starter cables so they might have normal battery voltage on them while the starter suffers from low voltage. There has to be a high resistance connection someplace in those two cables. The thing to do is start at one of them, right at the battery, with one meter probe on the positive bolt, then follow that cable to the first accessible test point and put the other meter probe there. The meter will read 0 volts because both probes are in the same circuit. Next, when a helper cranks the engine, the high current will cause a voltage drop to occur across any abnormally high resistance. That resistance is way too small to measure, but you CAN measure the results of that resistance in the form of that voltage drop. Keep moving the probes of the meter to the next accessible point in the circuit. It won't take long to reach the large terminal on the starter. At each point, you're looking for a large increase in voltage during cranking. At a bare minimum, you must have 9.6 volts left at the starter after dropping some voltage along the way. The battery voltage will be drawn down a few volts too, but you must still be left with at least 9.6 at the starter. Let's guess the battery is drawn down to 11.0 volts during cranking. That's a typical value. Regardless where you measured 7 volts earlier, it's safe to assume you have that much or less at the starter. That means you're looking for a drop of 4.0 volts in total across all of the connections and cables in the starter circuit. It is acceptable to find 0.2 volts drop across any one connection. You can disregard those, but somewhere you're going to find a big voltage of a couple of volts. That's the connection that nust be cleaned up.

I'm going on the assumption there is a connection problem based on the stuff you said was already replaced. I can't read back far enough to double-check, but as I recall, you said the cables were already replaced. Do I remember correctly? If so, are they nice big fat cables at least as fat as the origineal ones?

Caradiodoc