Usage:
All 3208 Truck Engines
- SUPPLEMENT: 06/29/83
TRUCK ENGINE NEWS, JUNE 23, 1982, PAGE 8. The article, "Engine Ground Strap Necessary To Prevent Electrical Discharge Damage To Crankshaft Bearings, " gives an explanation of what electrical discharge damage is and how to prevent it. Caterpillar now has available a Supplement to Section 3 of "Engine Bearings And Crankshafts," Form SEBD0531. The Supplement, "Analyzing Electrical Discharge Damage," Form SEBD0610, gives a detailed description and more examples of electrical discharge damage. The Supplement also gives a procedure that the serviceman can use to check the effectiveness of the ground on a truck engine. Make a reference to the Supplement in the Truck Engine News article.
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Caterpillar's specifications say that engine-to-frame ground straps must be used on all truck engine installations. Engine component damage is possible if an acceptable engine-to-frame ground stap is not part of the vehicle's electrical system.
On some installations, one of the alternator's electrical connections is made directly to an engine component such as a cylinder head or the cylinder block. If that component does not have a ground strap to the vehicle frame, current will flow through the engine as part of the electrical system. This current can damage engine components.
Electrical Discharge Damage
Diagnoses of some main bearing failures on 3208 Truck Engines have shown that the failures were the result of electrical discharge damage.
When current flows through the main bearings and the crankshaft main journals, electrical damage (pitting) is done to the bearing and journal surfaces. This damage, which can be seen only through a microscope, makes the journal surface rough. The rough surfaces slowly remove main bearing material until the bearings are destroyed.
The electrical damage (pitting) can not be seen, but the bearing material removed by them can be seen as white lines (streaks) on the main journals. See Illustrations 1 and 2. When electrical discharge damage first starts, the white streaks are generally short and are not parallel. In time, the white lines become parallel and extend all around the main journals.
If the white streaks shown in Illustrations 1 and 2 are present on the main journals, it is an indication of electrical discharge damage.
Damaged main bearings will be completely worn, but normally the wear will be heavier on the rear main bearings. There will be no indications of heat either in a bearing or on a main journal unless a bearing has turned in its bore.
Rod journals and bearings generally show no damage unless a main bearing has turned in its bore and caused debris.
Illustration 1. Early electrical discharge damage.
Illustration 2. Early electrical discharge damage, but more damage than shown in Illustration 1.
Illustration 3. Bearings destroyed by electrical discharge damage. Bearings are in sequence from left to right.
Inspection Of Electrical System For Ground Strap
To prevent electrical discharge damage, check to make sure the vehicle's electrical system has an engine-to-frame ground strap. For vehicles which have the alternator connected to an engine component, the ground strap must connect that component to the frame. Many trucks have been seen without engine ground straps.
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Some vehicles have starter-to-frame ground straps. But, many of these starters are not electrically grounded to the engine. They have electrical insulation systems. For this reason, the starter-to-frame ground strap may not be acceptable ground.
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Caterpillar's 3208 Truck Engines installed in vehicles without engine-to-frame ground straps can be damaged by electrical discharge. Electrical discharge damage is most probable on vehicles which have attachments such as power take-offs directly connected to the crankshaft.
The recommendation is that an engine-to-frame ground strap be installed in any vehicle equipped with a 3208 Truck Engine, if the vehicle does not have an existing ground strap.