Ammeter not working

[Patrick H.]

Hello all.

On my 1971 350/270 AC coupe, the ammeter has never worked in the almost 5 years that I have owned the car. Now, I know that these do not fail often, and can also appear not to work because of other wiring problems leading to the gauge itself.

Before I take apart half the interior to fix and/or replace the gauge, does anyone have suggestions as to 1) How to easily test the gauge to see if it is the source of my problems, and 2) What other wiring problems can cause an ammeter not to work?

Electrical problems are the "low point" of my Corvette knowledge, so any suggestions would be appreciated. Thanks in advance.

Patrick

[Gary Schisler]

Re: Ammeter not working

I had a very similar problem with the ammeter on my 66. No indication of charge or discharge. First, the back of your ammeter should have a "V" shaped plug that connects to the two prongs on the ammeter itself. The ammeter is just a sensing device that tells you which way the voltage is flowing. Put a voltmeter lead on one terminal of the ammeter and the other to ground. You should read normal system voltage. Do the same to the other side. Next make sure that your alternator is properly charging. That should put out around 14.6 volts. The final area to check is the firewall connections. This is where my car had a problem. When I pulled the two male connections off of the engine side of the firewall, there was a lot of corrosion on the various pins. I finally used a cheap glue brush and brushed naval jelly on the pins, left it there for about 30 seconds then hosed it off. I just experimented with the time to find the one that cleaned the connectors and didn't harm anything else. When I was done, the pins looked like new and my ammeter started working. When I put it back together, I used another glue brush and put di-electric grease on the plugs to help lessen the corrosion.

[Jack H.]

Re: Ammeter not working

A stone cold dead ammeter is either simple or hard. Most of the time the unit itself saw over-voltage and went into open circuit/self-destruct mode. You check this by looking for continuity between the terminals on the back (should read relatively low ohm from the internal coil windings), and while there make sure you've got no loose connections at the guage itself.

WARNING, WARNING

To get at the ammeter in this era Shark, you have to pull the center console. Read the Chassis Service Manual and follow their instructions to the letter step-by-step. The center console bezel is NOT being reproduced with 100% accuracy and it'll cost you +200 for a replacement if you break it during removal.

Next, GM designers intended this part to break easily! It has designed in weak spots just above the guage cluster intended to snap in a collision (CSM documents) and you've got to 'pop' the cluster out from under the dash pad at the top. If you don't do this correctly and gingerly, you'll break the cluster at it's designed tear points and probably become unhappy with yourself.... ---------------------------------------------------------------------

Getting the cluster free from the center console is also an exercise in 'toolsmanship'. You have to remove the forward side covers on your center console, then snake a deep dish socket onto the nuts holding the bottom of the guage cluster against the console. This requires some manual dexterity as well as a good set of sockets and length matching extentions with articulating joints. Essentially, you're forming a C-curve and working backwards/upsidedown.

Getting 'em loose in one hardship. Putting these nuts/washer back on and torquing 'em down is slightly worse..... ----------------------------------------------------------------------

If your Shark has A/C, the job's a slightly bigger challenge because there's virtually NO clearance behind the guage cluster and the air make-up box. Here' a time when you can smile for not having that particularly desirable factory option. ----------------------------------------------------------------------

Last, the ammeter does NOT sit in series with your battery. If it did, 100% of the car's total branch circuit current would flow through it and you'd need a brick s*&^house of an ammeter as well as wiring guage to support cold cranking conditions. No, if you study the wiring diagram, you'll find the ammeter sits in parallel to what appears to be 'nothing' and scratch your head forever trying to figure this one out.

Essentially, the ammeter has controlled resistance in its coil windings. The 'nothing' wiring path it sits in parallel to is actually another low resistance (controlled) run so a 'shunt' circuit is established. If you look more closely at your wiring diagram, you'll find places where, for no apparent reason, Chevy designers decide to change wiring guage and color code along a simple point-to-point run for no apparent reason.

There IS a reason. Splicing different guage wires together in series and controlling the length of individual runs is one way of building a controlled resistance into a wiring run. It's this technology that sets the main path resistance your ammeter's shunt is set across.

Now, prior owners and previous repairs/modifications often involve cutting into wiring harness and 'doing things'. I will tell you that not all the would be fix-it/improve-it folks really know what they're doing. If wiring has been changed, it's possible for your ammeter to check good and still not work despite proper connections because some genius before you changed the designed-in IR drop dimensions of a critical point-to-point wiring run and re-taped the harness....

If so, GOOD LUCK! You'll be a long time trying to figure this one out (obviously the guy before you was stumped). You'll could wind up 'shot gunning' by wholesale replacement of engine wiring harness assemblies. ---------------------------------------------------------------------

Last, the advice given to check the ohmic contact integrity of your firewall connector plugs is EXCELLENT. Add to this, the inspection of your engine compartment B+ bus bar integrity (everything that's clamped to the metal side of your horn relay) being careful to fully disconnect the battery when you 'play' here since these are areas that are ALWAYS HOT. And, finally, check the integrity of connections at your starter motor because the circuit points your ammeter is essentially 'looking across' lies between the run position of the starter and the horn relay B+ engine bus bar. ----------------------------------------------------------------------

Good luck. Have fun and let us know how your situation works out.

[Jack H.]

Re: Ammeter not working

A stone cold dead ammeter is either simple or hard. Most of the time the unit itself saw over-voltage and went into open circuit/self-destruct mode. You check this by looking for continuity between the terminals on the back (should read relatively low ohm from the internal coil windings), and while there make sure you've got no loose connections at the guage itself.

WARNING, WARNING

To get at the ammeter in this era Shark, you have to pull the center console. Read the Chassis Service Manual and follow their instructions to the letter step-by-step. The center console bezel is NOT being reproduced with 100% accuracy and it'll cost you +200 for a replacement if you break it during removal.

Next, GM designers intended this part to break easily! It has designed in weak spots just above the guage cluster intended to snap in a collision (CSM documents) and you've got to 'pop' the cluster out from under the dash pad at the top. If you don't do this correctly and gingerly, you'll break the cluster at it's designed tear points and probably become unhappy with yourself.... ---------------------------------------------------------------------

Getting the cluster free from the center console is also an exercise in 'toolsmanship'. You have to remove the forward side covers on your center console, then snake a deep dish socket onto the nuts holding the bottom of the guage cluster against the console. This requires some manual dexterity as well as a good set of sockets and length matching extentions with articulating joints. Essentially, you're forming a C-curve and working backwards/upsidedown.

Getting 'em loose in one hardship. Putting these nuts/washer back on and torquing 'em down is slightly worse..... ----------------------------------------------------------------------

If your Shark has A/C, the job's a slightly bigger challenge because there's virtually NO clearance behind the guage cluster and the air make-up box. Here' a time when you can smile for not having that particularly desirable factory option. ----------------------------------------------------------------------

Last, the ammeter does NOT sit in series with your battery. If it did, 100% of the car's total branch circuit current would flow through it and you'd need a brick s*&^house of an ammeter as well as wiring guage to support cold cranking conditions. No, if you study the wiring diagram, you'll find the ammeter sits in parallel to what appears to be 'nothing' and scratch your head forever trying to figure this one out.

Essentially, the ammeter has controlled resistance in its coil windings. The 'nothing' wiring path it sits in parallel to is actually another low resistance (controlled) run so a 'shunt' circuit is established. If you look more closely at your wiring diagram, you'll find places where, for no apparent reason, Chevy designers decide to change wiring guage and color code along a simple point-to-point run for no apparent reason.

There IS a reason. Splicing different guage wires together in series and controlling the length of individual runs is one way of building a controlled resistance into a wiring run. It's this technology that sets the main path resistance your ammeter's shunt is set across.

Now, prior owners and previous repairs/modifications often involve cutting into wiring harness and 'doing things'. I will tell you that not all the would be fix-it/improve-it folks really know what they're doing. If wiring has been changed, it's possible for your ammeter to check good and still not work despite proper connections because some genius before you changed the designed-in IR drop dimensions of a critical point-to-point wiring run and re-taped the harness....

If so, GOOD LUCK! You'll be a long time trying to figure this one out (obviously the guy before you was stumped). You'll could wind up 'shot gunning' by wholesale replacement of engine wiring harness assemblies. ---------------------------------------------------------------------

Last, the advice given to check the ohmic contact integrity of your firewall connector plugs is EXCELLENT. Add to this, the inspection of your engine compartment B+ bus bar integrity (everything that's clamped to the metal side of your horn relay) being careful to fully disconnect the battery when you 'play' here since these are areas that are ALWAYS HOT. And, finally, check the integrity of connections at your starter motor because the circuit points your ammeter is essentially 'looking across' lies between the run position of the starter and the horn relay B+ engine bus bar. ----------------------------------------------------------------------

Good luck. Have fun and let us know how your situation works out.

[Dave Koch]

Re: Ammeter not working

Patrick, I just got the non-functioning Ammeter on my 66 working a few weeks ago. After removing the connector I placed an ohmmeter across the terminals and found an open circuit. I had to fix my tach also, so out came the instrument cluster (that was a fun thing to do..). Anyways, I removed the cover to expose the meter movement and found that one end of the movement's coil wire had broken at the point where it was connected to one of the solder lugs. It looked like it was stretched a little too much during assembly. I used my low power soldering iron (20 watt) and was able to splice in a piece of #22 wire, checked out the meter on the bench and it functioned ok. Put it back in the cluster then put the cluster back in the car (that was almost as much fun as removing it..) and it is working just fine. Not saying that this is your problem, but something to check.

Dave

[Dave Koch]

Re: Ammeter not working

Patrick, I just got the non-functioning Ammeter on my 66 working a few weeks ago. After removing the connector I placed an ohmmeter across the terminals and found an open circuit. I had to fix my tach also, so out came the instrument cluster (that was a fun thing to do..). Anyways, I removed the cover to expose the meter movement and found that one end of the movement's coil wire had broken at the point where it was connected to one of the solder lugs. It looked like it was stretched a little too much during assembly. I used my low power soldering iron (20 watt) and was able to splice in a piece of #22 wire, checked out the meter on the bench and it functioned ok. Put it back in the cluster then put the cluster back in the car (that was almost as much fun as removing it..) and it is working just fine. Not saying that this is your problem, but something to check.

Dave