Vehicle - Vibration Diagnosis Charts, Reed Tachometer
Bulletin No. T 86-4File in Group 0
Number 24
Corp. Ref. No 805 0117
Date Sept. '85
SUBJECT: VIBRATION DIAGNOSIS USING REED TACHOMETERS
MODELS AFFECTED: ALL
FIGURE 1 - Reed Tachometer:
Diagnosing automotive vibrations and isolating the responsible component or components can be both difficult and time-consuming. To aid in diagnosing vibrations, Cadillac has developed car line Rotational Order charts. These charts, combined with a resonant Reed Tachometer (Figure 1), provide a systematic method for diagnosing vibration conditions.
1. Resonant Reed Tachometer The Reed Tachometer consists of two rows of reeds, each reed is designed to vibrate at a particular frequency. The scale on the Reed Tachometer measures frequency in Hertz (Hz). The Rotational Order charts are designed to associate a particular frequency vibration with the responsible component. Each component on a vehicle will vibrate at a natural frequency. This frequency acts like a fingerprint and will identify a component. For a particular component, this frequency will vary with speed and/or engine RPM as shown in the charts.
FIGURE 1 - Reed Tachometer:
Our experience has shown the Reed Tachometer should be placed on the passenger door armrest or top of the instrument panel for ease of viewing and for effective pick-up of a vibration. Figure 1 shows a Reed Tachometer which is picking up a 13 Hz vibration.
Biddle FRAHM Resonant Reed Tachometers P/N 99181100, called "Autotach" by the manufacturer cost approximately $440.00, and can be ordered directly from:
Biddle Instruments 510 Township Line Road Blue Bell, Pennsylvania 19422 (215) 646-9200
11. Rotational Order Charts
Rotational Order charts are attached for:
1983 C-Cars and 1984-85 D-Cars w/4.1L V-8 Engine and 3.42:1 Ratio Rear Axle 1984-85 J-Cars w/2.0L L-4 Engine and 125C Model CA or CB Transaxle 1985 J-Cars w/2.8L V-6 Engine and Automatic Transaxle 1984-85 E/K-Cars w/4.1L V-8 Engine and 3.15:1 Ratio Final Drive 1985 C-Cars w/4.1 L V-8 Engine and 440-T4 Model AY Transaxle
These charts will also be included in Section OA of all future Cadillac Service Information Manuals. Additionally, Sections 0A of the "D/E/K" Service Information Manual and "C" Service Information Manual currently contain information on "Vibration Complaints and Corrections." This portion of Section 0A provides a systematic procedure for classifying a vibration condition in one or more of the following categories: (1) car speed sensitive, (2) engine speed sensitive, (3) torque (throttle) sensitive, or (4) jounce sensitive. Each of these categories subsequently identifies the probable areas for investigation and lists procedures to repair the condition. This information also will be included in the 1986 Cimarron Service Information Manual.
To diagnose a vibration condition using these charts, two inputs are required:
1. Vehicle speed or engine RPM - Vehicle speed can be read off the speedometer when the vehicle is in 4th gear (3rd gear on Cimarrons) with TCC/VCC engaged. Vehicle speed can be verified using the onboard diagnostic parameters (except Cimarron), refer to Section 6E of the Service Information Manual. Engine RPM can also be read from the diagnostic parameters, refer to Section 6E of the Service Information Manual. Engine RPM can be read directly from the Tachometer on Cimarrons.
2. The Frequency of the Vibration - The frequency of the vibration can be read from the scale on the Reed Tachometer; frequency is read in Hertz (Hz).
FIGURE 1 - Reed Tachometer:
FIGURE 2 - ROTATIONAL ORDER CHART1985 "C"-CAR WITH 4.1L V-8 ENGINE AND 440-T4 MODEL AY TRANSAXLE IN 4TH GEAR WITH VCC ENGAGED:
Example: A 1985 C-car with a 440-T4 transaxle is travelling at 55 MPH, and the 13 Hz reeds are vibrating (Figure 1). The Rotational Order chart for 1985 C-cars (Figure 2) indicates, by following across the chart on the 13 Hz line to where it intersects the 55 MPH vertical line, that there is a 1st order tire vibration.
FIGURE 2 - ROTATIONAL ORDER CHART1985 "C"-CAR WITH 4.1L V-8 ENGINE AND 440-T4 MODEL AY TRANSAXLE IN 4TH GEAR WITH VCC ENGAGED:
FIGURE 3 - FIRST ORDER VIBRATION - ONE DISTURBANCE EACH ROTATION:
FIGURE 4 - SECOND ORDER VIBRATION - TWO DISTURBANCES EACH ROTATION:
As noted on the chart, some components may induce more than one vibration at a given speed (i.e., tires, identified by the * in Figure 2). These multiple vibrations are referred to as the order of a vibration. The order of a vibration refers to the number of disturbances created by one rotation of a component. For example, a tire with one heavy spot will produce one disturbance each rotation--a 1st order vibration (Figure 3). An oval-shaped tire will produce two disturbances each rotation--a 2nd order vibration (Figure 4).
In general, the suspect components should be isolated to verify the cause of a vibration before parts are replaced. For a 1st order tire vibration, the responsible component can be anything which rotates at wheel and tire speed (i.e., wheels, tires, brake rotors, brake drums). These can be serviced or replaced with known good parts to identify the component causing the vibration. Once verified, the component can be balanced or replaced as necessary.
Using a Resonant Reed Tachometer in a vehicle will take some practice. One of the more important things to be aware of when using the Reed Tachometer for the first time is that the Reeds are very sensitive and will pick up many low amplitude vibrations. These will appear as slight movements of many Reeds, and do not correspond to any particular component. Reed movement that corresponds to a vibrating component will be greater in amplitude, traveling the full range of the viewing area. The Reed vibrating will also change as engine RPM or vehicle speed changes, and will follow the lines on the Rotational Order Charts.
FIGURE 2 - ROTATIONAL ORDER CHART1985 "C"-CAR WITH 4.1L V-8 ENGINE AND 440-T4 MODEL AY TRANSAXLE IN 4TH GEAR WITH VCC ENGAGED:
FIGURE 2 - ROTATIONAL ORDER CHART
1985 "C"-CAR WITH 4.1L V-8 ENGINE AND 440-T4 MODEL AY TRANSAXLE IN 4TH GEAR WITH VCC ENGAGED
FIGURE 5 - ROTATIONAL ORDER CHART1983 "C" AND 1984 AND 1985 "D"-CAR WITH 4.1L V-8 ENGINE AND 3:42:1 RATIO REAR AXLE IN 4TH GEAR WITH TCC ENGAGED:
FIGURE 5 - ROTATIONAL ORDER CHART
1983 "C" AND 1984 AND 1985 "D"-CAR WITH 4.1L V-8 ENGINE AND 3:42:1 RATIO REAR AXLE IN 4TH GEAR WITH TCC ENGAGED
FIGURE 6 - ROTATIONAL ORDER CHART1984 AND 1985 "J"-CAR WITH 2.0 L-4 ENGINE AND 125C MODEL CA OR CB TRANSAXLE IN 3RD GEAR WITH TCC ENGAGED:
FIGURE 6 - ROTATIONAL ORDER CHART
1984 AND 1985 "J"-CAR WITH 2.0 L-4 ENGINE AND 125C MODEL CA OR CB TRANSAXLE
IN 3RD GEAR WITH TCC ENGAGED
FIGURE 7 - ROTATIONAL ORDER CHART1985 "J"-CAR WITH 2.8L V-6 ENGINE AND AUTOMATIC TRANSAXLE IN 3RD GEAR WITH TCC ENGAGED:
ROTATIONAL ORDER CHART
1985 "J"-CAR WITH 2.8L V-6 ENGINE AND AUTOMATIC TRANSAXLE 30mIN 3RD GEAR WITH TCC ENGAGED
FIGURE 8 - ROTATIONAL ORDER CHART1984 AND 1985 "E/K"-CAR WITH 4.1L V-8 ENGINE AND 3:15:1 RATIO FINAL DRIVE IN 4TH GEAR WITH TCC ENGAGED:
ROTATIONAL ORDER CHART
1984 AND 1985 "E/K"-CAR WITH 4.1L V-8 ENGINE AND 3:15:1 RATIO FINAL DRIVE 30mIN 4TH GEAR WITH TCC ENGAGED
FIGURE 9 - ROTATIONAL ORDER CHART1985 "C"-CAR WITH 4.1L V-8 ENGINE AND 440-T4 MODEL AY TRANSAXLE IN 4TH GEAR WITH VCC ENGAGED:
ROTATIONAL ORDER CHART
1985 "C"-CAR WITH 4.1L V-8 ENGINE AND 440-T4 MODEL AY TRANSAXLE 30mIN 4TH GEAR WITH VCC ENGAGED