Meter Does Not Turn Off (Problem 1); Meter Enters Wrong Function (Problem 2) - Test Equipment Depot 88 Service Manual

Automotive meter

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Service Manual

1. The Meter will not turn off when the rotary switch is in the OFF position.

2. The Meter does not enter the function indicated by the rotary switch setting.

Both of these problems can be intermittent and can be caused by different malfunctions.

3-31. Meter Does Not Turn Off (Problem 1)

If the Meter does not turn off when the rotary switch is in the OFF position, the wiper

contacts are probably not making contact with the PTF sections. This may not allow the

Power Down Signal (PDS) line to be pulled low by VBT-. Check the wiper contacts for

good spring tension against the PTF surface. Make sure that the PTF surface and spring

contacts are clean. If they are not, clean them with isopropyl alcohol. To get to the wiper

contacts, perform the procedures under "Disassembling the Case" and "Removing and

Reinstalling the Circuit Assembly" previously described in Chapter 3.

This problem could be unrelated to the PTF, in which case a power supply component,

such as Q8, may be at fault. Refer to TROUBLESHOOTING THE POWER SUPPLY

earlier in Chapter 3.

If the Meter does not enter the function indicated by the rotary switch selection, the most

probable cause is an out-of-tolerance PTF resistor section. This can be checked by

performing the procedure under "Function Selection (Rotary Switch) Test" given earlier

in Chapter 3. If the display reading is outside the tolerances given, the PTF resistor

section is bad, and the pca must be replaced. (This assumes that the PTF wiper contacts

are good. If the wiper contacts are not making proper contact with the PTF sections, the

Meter can also select the wrong function. See "Meter Does Not Turn Off", above.

However, poor contact is more likely to cause an intermittent rather than a total failure.)

If the Meter powers up in ac mV and will not change functions, the problem is caused by

the inability of the a/d converter to measure, NOT BY THE PTF. The a/d converter must

do a ratio measurement of the PTF resistor in order to change functions. U4 or one of its

peripherals can cause this problem.

3-24

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