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74 C20 amp guage stopped working

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74 C20 amp guage stopped working

1robert1 1robert1
New User | Posts: 2 | Joined: 10/13
Posted: 10/22/13
08:34 PM

I looked for similar threads but found nothing: may not have looked far enough; anyway here goes.
The amp guage in my 74 C20 stopped working. I checked the fuse with a circuit tester:its OK. Going to have to take the dash apart. I am worried about the instrument cluster > I know I am going to break an end : or worse yet the carriage where the gear selector is: It's 40 year old plastic and breaks it you look at it funny.
My question is this: Is there any way to test the guage while it is still in the cluster: in the dash? is there a way to test the circuit ribbon on the cluster? For some reason, can't say why really, I have a feeling it might be the ribbon. Is the only way to test the guage and circuit to take out the cluster? That is what the service manual says to do. Any tips would be helpfful.
Thank you

wayne712222 wayne712222
New User | Posts: 25 | Joined: 10/13
Posted: 10/23/13
01:33 AM

OH... chevy truck AMP meters... a bunch of things could be wrong..

gm had a VERY creative guy who designed this circuit...

on the starter motor..

top post...

battery cable ...

fusible link to the 10 or 12 gauge wire that goes to to the terminal block on the firewall.. where it will go into the cab and supply current to a lot of things.. including one side of the AMP meter circuit...

smaller fusible link that goes to a PAIR of wires..  one that is routed into the cab of the truck.. usually to the other side of the amp meter... and the other wire on this.. goes up to the #2 terminal that plugs into the side of the alternator..

so.. before you do anything...

lets test a few things.. perhaps clean a few..

with a test light or a volt meter...  engine off.. do you have power to the big red wire on the back of the alternator.. and also to the RED wire that plugs into the side of the alternator..   you should have power on both.. this indicates that both fusible links on the starter solenoid are actually working...

so test there first...

second.. lets run a voltage drop test with your digital volt meter...

engine running.. headlights on...  volt meter set to 20 volts DC

1.  Battery Negative post or bolt.. to the Positive battery post or bolt. 14.1 to 14.8 volts DC...

2.  Negative battery post or bolt to the engine block... 0.04 volts DC MAX..

3.  Negative battery post or bolt. to the body/firewall.. 0.02 volts DC MAX..

4.  Engine block to the firewall.. 0.02 volts DC max..

i would like you to post the results to the 4 part test above..





lets continue... engine running, headlights on...

5. positive battery post or bolt to the alternator output stud on Back of alternator..  usually under 0.07 volts..

6. Positive battery post to the RED wire plugged into the side of the alternator.. 0.04 would be acceptable.. but you might only get 0.02 or 0.1.. but above ZERO..

7. alternator output terminal to the red wire on the side on alternator terminal #2..  again.. this should be below... 0.1 volts.. and above 0.00... 0.1 is 1/10 of a volt..

how the amp meter works is it looks at the difference in voltage between the battery positive and the alternator positive output..  the back terminal and the side...  if one is higher than the other.. then the meter swings in one direction.. if one is lower than the other.. the meter swings in the other direction.

warning.. if your truck is an automatic.. please prepare.. note if your truck has a straight steering column or a tilt column.. buy a new ignition switch..    so when you drop the column to remove the cluster.. you can swap out the old ignition switch..

sorry.. i am falling asleep right now..

lets look at a wiring diagram i have for a later model ...

77 80 Chevytruck Fusible Links

do you see the 2 wires that lead into the cab on the lower right of the image..

the wire into connector cavity 2J goes to one side of the amp meter...

the other wire into connector cavity 2E..  that seems to feed a lot of stuff and is the main power into the cab..  i could be wrong.. and this is a 77 to 80 diagram.. not a 74...

and why change the ignition switch.. because they are under 20 bucks.. you will have the column down... and they tend to burn out and cause weird issues like you are talking about..

please also.. before you drop your column.. get around the passenger side down below the shifter lever. and release the string/wire from the side that goes up to the P R N D 2 l in the cluster.

if you drop the column and don't notice.. you will break a pulley off in your cluster and won't be happy as i don't think there is any way to fix it.. other than replacing the whole cluster..

wayne712222 wayne712222
New User | Posts: 25 | Joined: 10/13
Posted: 10/23/13
01:38 AM

here.. i shrank a copy so you could see this under hood section...

762Cd242 B126 4D25 A5df 20535C76cc96 Zps7237bddb  

wayne712222 wayne712222
New User | Posts: 25 | Joined: 10/13
Posted: 10/23/13
01:40 AM

A5aec22a Bfc3 4108 A318 15A4af353625 Zps3cabf5f5  

1robert1 1robert1
New User | Posts: 2 | Joined: 10/13
Posted: 10/24/13
07:13 PM

Thanks for this tip > and the diagrams; Going to have to study up on this one I think. going to have to have a free weekend too. was worried about pulling the cluster so guess, maybe, wont have to.
One question: although I doubt this is the cause, some time ago I replaced the points distributor with an HEI. You might remember your advise and diagrams helped on that project : much appriciated. Anyway, the amp guage was working fine up until a week or so ago > so I doubt that distributor swap would have anything to do with this : what would you think?
my ignition switch is not very old either, not that you indicated that might be the problem: but you do recommend replacement. not sure I will do that. I will post test results as you asked for when I get to this project > probasbly around Thanksgiving. Looks like that is when I will have a full weekend I can devote to this. I will let you know how it goes.

wayne712222 wayne712222
New User | Posts: 25 | Joined: 10/13
Posted: 10/25/13
12:28 AM

if you pull the cluster...

with a test light.. you should have power on either side of the amp meter circuit..

with a digital volt meter...   if you set it to 20 volts DC scale..  and hook up on either side of the amp meter circuit..

engine off.. probably have ZERO volts.. across the two wires... and battery voltage on each when checked to ground..

if you have the engine running and you hook the digital volt meter across both wires to the volt meter..  turn on the headlights.. turn on the defroster..  when you power up various circuits.. including when the engine is off... but the key on..  there should be slight measurements of voltage..  as there will be slight differerences in voltage on either side of the wire..

i found some of the 74 diagrams.. and i have some in print... but i need to study them to see how the amp meter is wired.. its not clear as it spreads across several pages.. and i have a new to me lap top so none of my old programs work.. leaving me stranded..   since you are not in a rush.. i will work on it.. so do check back..

as for the HEI... wiring... i have done them in several different ways.

i have grabbed some positive connection from the terminal block on the firewall. and used that to pin 87A on a relay.. wired pin 30 to the HEI BAT terminal...

grounded pin 86 and ran the old ignition wiring including the resistor bypass wire from the starter to relay pin 85..  this turns on the relay and sends a full battery voltage to the relay.. really makes a HUGE difference in how the HEI works..

i also on many hot rodded GM cars and trucks from the mid 60s thru the late 70s.. used a 70 dodge dart starter relay on the firewall.. to BOOST the solenoid current to the starter solenoid..  this also makes it start so much better...

Starter Load Dropping Relay

you would not believe.. but i have measured the current needed on some GM MT10 starters.. and its over 50 amps to pull the plunger in to mash the pin to close the contacts and make the starter spin.. this load dropping relay. reduces the load on the wiring from the battery to the ignition switch and then to the starter to about 1 amp..