PS Magazine - TB 43-PS-428
PS, The Preventative Maintenance Monthly
ISSUE 428
JULY 1988
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Alot of gages are tossed OuT when there's no±hing wrong wi² them. For instance,
µ¶,
'fÀ
¶
' ·
a ³el gage That won'T woRk or one with a needle that ´Ips beTween ³lL and empty
¸
¹º
fµr no apparenT reason Is Not necessarily a bum gage.
So, what's wrong?
Some mechanics neveR leaMed-r Have fµrgotte¶·abouT one of the basic ¸cts
in DC (dIrecT currenT) e¹ectrIcal systems:
ºhe chassis o» your equipmenT is the
"second wire¼ in the elecTrIcal system.
ºhe battery's negatIve Termi½
r¸¹º»:¼½¾";¿ÀÁÂÃ
ÄÅ
nal is connected To the chassis¾
usuall y
the
engine
or
¿
the
Àame·by a wireÁ
["ÁÂÃÄÅÆÇÈÉÊËÌ
ºhis means all of tÂose elecÃ
trIcal components wi² chassis
ground are connected To ²e
baTtery's
negatIve
Terminal
through the chassis.
ºhe Äel gage, År example, is
inteÆally connecTed To The
ÍÎÏÐÑÒÓ
ground
by
The
mounTiÇg
bracKeT.
ÈT musT maKe solid
conTact with clean, bare metal.
AnyThing between ²e meTal
chassis and Éhe ground connecÊ
r;
/
:±²
³´µ
¶·
!
¸¹
º»
¼
Tion, lIKe rUsT or corrosion, stops
The ´ow of electricity To ²e
gages. ºhe gages won'T worK.
The bottom mounting stud is connEcted
diRectly to the electRical systeM inSiDe
the gage foR gÆ\unding
8
JUL88
Ëemove tÂe instrUment panel Àom
the dashboard.
»
Set your multImeter to the lowest
ohms scale and adjusT.
Ìeasure resistance beTween The
gage's mountiÍg bracKet and the panel.
ºouch one probe To The bracKet and the
o²er To ²e panel where they meeT. An
ÔOÕ
reading means tÂe bracKeT and tÂe
chassis have a good conÎectIon.
Èf you get any other reading, remove The bracKeT. Scrape oÏ any paint, coÐosion
or dirT ²aT's buIlT up on the panel. PUT ÉÑe bracKeT bacK on and try tÂe TesT agaIn.
Ìultimeter sTIll giving bum reading? Do this:
ºouch one probe to the bracKet and the o²er to ²e bracKet's attachIng stud. An
ÔOÕ
readIng means you have a good co¶necTion. WIth any oTher readIng, cleÒ¶ The
sÉUd, washer and bracKeT and Try ²e test again. STill a no½go? ºry this:
ºuÓ The ignition swITch
oNÖ
SeT your meter to voltage and seT ²e meter volTage range selecTor swiTch To the
righT range.
PulÔ ²e hoT wire(+) oÕ its TerminalÁ ºouch Éhe meTer's posItive probe To ²e
hoT wire and ground the negative probe to metal.
I
The needle regIsters volTage, tesT
The gage År resistance.
Ìove Éhe meter's switch to ohms. DisconnecT the sending uniT wire Àom its
Termi¶al o¶ the gage.
Do a conTInuiTy Test by touching one probe To the posITive TeÖinal and ²e oÉher
probe To ²e sendi¶g uniT termInal on the gage.
I
The needle moves To
ÔO,Õ
²e gage
is
oK×
×e problem's ²e sendIng uniT circuiT.
Èf ²e needle stays still or ´IcKers, ²e gage's deØcTive. Ëeplace the gage.
JUL88
9
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