First of all, 8.2 volts is too low, but the starter should at least try to turn slowly.
By "solenoid wire", do you mean the smaller wire that powers the solenoid or the really fat battery cable attached to the solenoid? The smaller one should have 0 volts until the ignition switch is turned to "crank', then it should have whatever full battery voltage is at the time.
If you're describing the fat battery cable at the starter, 6.85 volts subtracted from the 8.2 volts at the battery means you're dropping 1.35 volts across the two battery cables. That is also way too much. The acceptable standard is 0.2 volts across any single mechanical connection AND a maximum of 0.4 volts in either the positive cable circuit or the negative cable circuit.
Two things can cause excessive voltage drop. Excessive, undesirable resistance or excessive current draw. Excessive resistance is caused by frayed or corroded wires, loose or corroded mechanical connections, or cables that are too small. Excessive current is caused by a starter that is spinning too slowly or is partially shorted. A partially shorted starter can appear to work properly when bench-tested off the engine because it isn't being loaded down. The starter can spin too slowly due to rubbing rotating parts caused by worn bushings.
By testing between the cable on the starter and the engine block, what you have done is to determine there is a point of excessive voltage drop. Now we have to narrow down its location and cause. Start by measuring the voltage drop across a single mechanical connection while a helper turns the ignition switch to "crank". Put one meter probe on the positive battery cable clamp and the other probe on the positive battery post. (Yes, the two probes will be 1/2" apart). During cranking there must be less than 0.2 volts there. Ideal is 0.0 volts. Next, put one probe on the positive battery cable and the other one on the terminal end of that cable where it bolts to the solenoid, (not the threaded stud). It must be on the copper end that is crimped to the cable. Again, 0.2 volts or less is acceptable. You might have to hook one of the probes onto its test point with a clip lead unless you have really long arms! For the next test, leave the probe on the cable's terminal at the solenoid, and move the other one to the threaded stud that the terminal is bolted to. Chances are you'll find 0.0 volts there during cranking but up to 0.2 volts is acceptable. So there's three places to measure in the positive circuit. Any of them can have up to no more than 0.2 volts, but all three together can not add up to more than 0.4 volts. That's the industry standard. Common sense says that if you find a total of, ... Say 0.45 volts, that is not going to cause a no-crank problem.
Now do the same thing for the negative circuit. Start with the battery post and cable clamp. Next, measure from the cable clamp to the terminal on the engine block, then between that terminal and the block itself. None should read more than 0.2 volts and the total must not be more than 0.4 volts.
If you find excessive voltage drop across just one mechanical connection, take it apart and clean and tighten it. Based on the fact that your battery voltage is also being drawn down a lot, I suspect the problem is due to excessive current draw but there's a weird characteristic I'll address in a minute. If you find more than 0.2 volts dropped across all or most of the mechanical connections, we can temporarily rule out corrosion and dirt because all of them didn't get that way all at once.
When all voltage drops are high, including the drop in battery voltage from 12.27 to 8.2 volts, excessive current can be suspected. Once the starter is spinning, it actually acts like a small generator that produces a current that opposes the current leaving the battery. That's why a motor that's spinning draws less current than one that hasn't gotten up to speed yet. Said another way, as you load down or stall a motor, the current it's drawing will go up, possibly to the point of popping a fuse. (I'm thinking small motors now, like a heater fan motor, but a starter motor runs on the same principle). To add to the confusion, there are actually two interdependent circuits in the starter motor. Both are needed to develop magnetic fields of sufficient strength to spin the engine. When one circuit fails, the motor will draw half of its normal current so it will be too weak to spin the engine. Since it is locked up, it doesn't act as an opposing generator so actual current flow goes way up making the value appear normal. This is almost impossible to detect on a test bench. The real test is to replace the starter. Worn brushes are most common; a failure of one of the coils of wire are not real common. Since you replaced the starter already, I doubt excessive current draw is your problem.
I think you're going to find one connection with a real high voltage drop. By the way, the solenoid is on top of the starter motor, right? As such, it got replaced along with the starter motor. If yours is up on the fender like Fords used for many decades, that used to be the solenoid that did the switching of the 250 plus amps to the starter motor, but starting around the late '80s or early '90s, they started using a regular solenoid on the starter like everyone else. The confusing part is they kept using the same physical part as the starter relay and they put it in the same common location. That part, when used as just a relay is still capable of handling over 250 amps but in its current application is only being required to switch less than 20 amps. The clue is the size of the two cables on the two large studs. The positive battery cable attaches to one terminal in both applications, but with the older cars, the second large terminal is for the starter cable, so it has two large cables on the two large studs. On the newer cars, that second wire is very much smaller since it only has to handle 20 amps or less. So, when used as just a relay, there will be one fat cable and one small wire on the two large studs. If that description is confusing, I can wind up a different computer that will give me access to Ford's web site and diagrams.
Also don't overlook the fact that if your battery is original, it's about time for it to lose it cranking ability. Along with my story about excessive resistance, batteries have a characteristic called "internal resistance". It's that resistance that causes an internal voltage drop during cranking. It's why you found 12.27 volts under no load, and 8.2 volts during heavy load, (cranking). 4.07 volts is being dropped internally. Have you tried using a battery charger while cranking the engine? Internal resistance increases as a battery ages and as it "becomes weak". That's why an old flashlight battery might measure 1.5 volts with a voltmeter but it can't power a 1.5 volt light bulb. The voltmeter draws almost no current so no voltage is dropped, (lost) across the battery's internal resistance. That leaves the full 1.5 volts to be measured. The light bulb is low resistance so it draws a lot of current. That causes voltage to be dropped across the battery's internal resistance leaving not enough voltage to produce enough current to light the bulb.
Sorry if the story got too confusing. For now, look for any connection with excessive voltage drop.
Tuesday, September 14th, 2010 AT 10:16 PM