A visual inspection alone is no substitute for a valid voltage drop test on a suspect engine ground. Ultimately, visual checks may waste more time and cause more grief than you would imagine.
This case history builds upon my story in the May 2021 issue of GEARS. For one thing, I hope it reminds you to test grounds instead of guessing at their condition. For another, the example may convince you to test essentials sooner rather than later – especially during a difficult diagnosis.
A 1999 Honda Civic with a 1.6-liter engine and automatic transmission came into a bustling general repair shop. The owner explained that the car surged and bucked while cruising at 40 to 45 miles per hour. He also said that the charging system seemed to be failing.
Unfortunately, this job began with two strikes against it. First, the service writer failed to get a detailed vehicle history on the Civic. Second, he struggled to contact the customer later on. The gentleman was out of town and having cellular telephone trouble during the trip.
When a technician road tested the Civic, the car surged during steady-throttle driving at 45 miles per hour. Another tech observed the same symptom during a second road test. Both guys recognized the surging/bucking sensation; they had encountered it on some domestic cars. The techs suspected torque converter clutch trouble.
But the self-diagnostic results from the Civic’s PCM were puzzling. As you know, a PCM is a computer that manages both the engine and automatic transmission. This one had not stored any trouble codes – although the shift indicator’s D4 light was flashing. Usually, a blinking D4 light on a Honda instrument panel meant a transmission-related issue of some kind. But although the car had highly suspicious symptoms (surging during cruise and had a flashing D4 light), the PCM still had not set any codes.
The charging system’s behavior intensified the car’s mystery. The alternator charged normally after a tech started the engine. But afterward, charging voltage steadily decreased for no obvious reason. In fact, the longer the Civic idled, the lower the charging voltage dropped. After shutting off the ignition switch and restarting the engine, the alternator charged okay at first. Then charging voltage began dropping again – slowly but steadily.
The techs had seen situations where a failing alternator had hampered the operation of an ECM or PCM. So they replaced the alternator with a trusted brand of remanufactured alternator. The fresh alternator made no difference. In desperation, they tried another replacement alternator, but it didn’t fix the car, either. The charging system symptom and the surging condition persisted. Recognizing the techs’ frustration with the Civic, the shop foreman said he would personally check the car.
LOOSE GROUND CABLE
The next morning, the shop foreman looked over the Civic’s engine compartment before reaching for his test equipment. Too often, he had found, frustrated techs would overlook something fairly basic and waste time. That basic problem would often spawn the strangest symptoms. First, he tried to wiggle the battery cable terminal on the positive battery post; it was tight. Then he tried to wiggle the terminal on the negative battery post. It also was tight.
Next, he traced the negative battery cable over to the right side of the engine compartment (Figure 1). The negative cable’s eyelet terminal was bolted firmly to the inner fender panel. For the moment, however, he could not see where and how the engine was grounded to the passenger side of the car. The foreman removed the air cleaner’s large inlet snorkel because it obstructed his view of the transmission and part of the engine.
Now he could see a ground cable connected between the right front frame and a bracket on the bellhousing (figure 2). The ground cable’s eyelet terminal felt tight against the frame. But the opposite end of that cable was very loose. In fact, the bolt holding the cable eyelet to the bellhousing bracket almost fell out when he wiggled the cable!
Closer inspection revealed that arcing had occurred between the loose bolt and the threads in the bellhousing bracket. The foreman chased the threads, installed a new bolt, and tightened it. Finally, he performed a cranking voltage drop test. The voltage drop measured less than 0.30 volt.
The fact that the customer was out of town was a mixed blessing. The service writer was unable to get the vehicle history from him. But his absence gave the shop foreman time to road test the car extensively. After a few days, he was reasonably confident that the car was fine. The Civic performed well, the D4 light stayed off, and no trouble codes appeared.
PRACTICE ON HEALTHY VEHICLES
Sadly, the root cause of the loose ground remained a mystery. The customer professed total ignorance – had no idea why that critical bolt loosened up. Thankfully, the car continued running okay long after the foreman fixed it.
This Civic episode occurred nearly 600 miles from me. Luckily, a friend owned a well-maintained 1999 Civic. Therefore, I shot the accompanying photographs of the car and recreated the foreman’s voltage drop test. Remember, the best way to learn normal test values is to practice those tests on known-good vehicles.
Note that I connected my digital voltmeter between the negative battery post and the transmission case (figure 3). I used a large alligator clip to secure one of the test leads to the transmission case. The cranking voltage drop on my friend’s Civic measured a mere 0.2068 volt (figure 4). A good working limit for this cranking voltage drop test is 0.50 volt.
CONCLUSIONS AND RECOMMENDATIONS
Okay, let’s recap and expand upon the details I discussed in the May issue of GEARS. First, experience repeatedly has shown that a bad engine ground may cause a wide array of goofy, unpredictable and/or illogical symptoms. It also may prevent an onboard computer from storing any trouble codes, let alone accurate, meaningful codes.
Second, experience has proven that a cranking voltage drop test is faster, simpler, and more accurate than visually searching for potential engine ground problems. Cramped engine compartments make meaningful visual inspections more difficult – if not impossible – to do. In all fairness to our discussion, the foreman didn’t spot the loose ground on the ’99 Civic until he removed the air cleaner snorkel. And by that point, his techs had been fighting this diagnosis for more than a day. That included two needless alternator replacements.
When in doubt, I urge you to do a cranking voltage drop test first. Where necessary, repair the main engine ground and retest. Then see if this repair has solved the vehicle’s problems. This ’99 Civic episode is a classic example of focusing on a main engine ground first.
Third, never assume that a bad main engine ground will cause an obvious, audible change in the engine’s cranking speed. In my previous article, in the May 2021 issue of GEARS, I explained that a restricted engine ground might force starter current (amps) to seek another route back to the negative battery terminal. That alternative return path may go through components such as a clutch cable, throttle cable, transmission shift cable, etc. If that alternative return path is adequate, you may not hear an abnormal cranking speed. Burned, blistered and/or seized cables – as well as repeated cable failures – strongly suggest that starter current caused the trouble.
Fourth, note a small difference between two basic but reliable cranking voltage drop checks. The first one, which I discussed in the May 2021 issue of GEARS, measures the voltage drop from the negative battery post to the starter housing. This procedure checks the entire ground path from the starter itself back to the battery – including the contact area between the starter housing and the engine or transmission case.
On the one hand, excessive voltage drops between the starter housing and the engine or transmission are rare. On the other hand, some techs misunderstand what actually constitutes a bad connection between the starter and an engine or transmission. (To briefly recap, the cranking voltage drop between them typically measures less than 0.10 volt.)
The second cranking voltage drop test, which I emphasized here, often is called a main-ground or engine-ground test. This procedure measures the cranking voltage drop from the negative battery post to a secure spot somewhere on the engine or transmission. Once again, a good working limit is approximately 0.50 volt or less. Over the years, performing this simple test has saved untold time and aggravation – especially on those vehicles with strange symptoms.
Finally, keep this perspective. It’s one thing to search for a potential engine ground problem on a hunch or a guess. It’s an entirely different situation when an abnormal cranking voltage drop test confirms that an engine ground is faulty. Simply put, test results justify such a search.