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Temp and Fuel Gauge don't work

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amcenthusiast View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amcenthusiast Quote  Post ReplyReply Direct Link To This Post Topic: Temp and Fuel Gauge don't work
    Posted: Oct/30/2013 at 6:41am
All the time I see AMCs for sale on EBay & it'll say the fuel gauge doesn't work and it must be one of the most embarrassing things in the world to be seen walking with a gas can 'cause you ran outta gas! The temp and fuel gauges weren't working on my rebuilt-from-a-junkyard Gremlin. I'd been checking the connections and testing the sending units with my cheap voltmeter set on ohms for continuity and all seemed ok, but still no gauges. I kept re-reading my AMC technical service manual to educate myself on the gauge circuits to get them operational. The TSM says 'the constant voltage regulator is located inside the temp gauge and supplies a regulated 5 volts to the temp and fuel gauges... if the CVR is bad, neither the temp or the fuel gauge will work & the temp gauge must be replaced...' WELL, humpf! This morning I decided I'd get up real early and go for it -to take the temp gauge apart & see what the heck they're talking about. Sure enough after carefully removing the crimped on temp gauge facia there's a bi-metallic spring inside with a set of contact points. I cleaned the contact points with a knife (super careful), reassembled for testing and -now- I've got my gauges back working again! This fix applies to Gremlin/Hornet and probably Spirit/Concord. Jav/AMX, Rebel/Matador/Ambos have external removable pronged CVRs, probably having a similar spring & contact set inside... Of course, always test the circuits first for voltage and ground continuity. Hope this helps somebody else get their gauges working again.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rocklandrambler Quote  Post ReplyReply Direct Link To This Post Posted: Oct/30/2013 at 8:12am
Good Job I think most would just go and buy a new temp gauge. I have a few of the external VR's in the garage. Think I'll try to open one up and see what I can see.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 401MATCOUPE Quote  Post ReplyReply Direct Link To This Post Posted: Oct/30/2013 at 9:20am
I am not aware of all models, but all 2 hole gauge set ups (Javelin/AMX/Rebel Ambo, etc) have external CVR.  Great troubleshooting, fuel gauge systems are not that hard to troubleshoot.  I should also be rembered on 1970 and 1971 cars with low fuel warning system the CVR is re-routed to the Low Fuel Warning relay...very common to find dead CVR internal to relay.
Ross K. Peterson
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: Oct/31/2013 at 8:19am
I did not know CVRs go bad often as they are a sealed unit... I have an extra one myself as a backup for later on down the road. Will test both, and hopefully one will be bad. If so, I can modify the thing to be a connector for a 5v regulated power to my instrument panel. Hate to do that to a good CVR, but this thread got me to thinking.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Oct/31/2013 at 12:07pm
They can and do go bad or get flaky - They are an electro-mechanical device with CONTACTS or points and a heating element. As such, the points can either stick (pegging the gauges) or get dirty (low or no readings - no gauge deflection), and the winding that heats the bi-metal strip can fail just like the winding or heating element in any device, be it a heating pad or hot plate.
They don't strictly actually "regulate" anything, they pulse the supply so that it averages to the voltage required for the gauges and keep it constant, otherwise the gauges would read high when the car was on a long highway trip or the voltage regulator under the hood was calling for 14 volts instead of 13, etc.
They take the variables out of the equation. More voltage in the car's system, the faster that coil heats the bi-metal strip breaking contact inside, lower voltage, like a bad charging system, the coil takes longer to heat the strip upping the average a bit.
The OP found the contacts "dirty" so nothing was going through - not even the heating coil, so the thing never passed any current at all, and never heated at all (or not enough)
So in a sense it's a whole lot like the old mechanical voltage regulators used with generators or early alternators on most cars but they control the voltage by controlling the input to the field, so the output really IS the regulated voltage, they do more of a direct control, not regulating so much.
They go bad quite a bit, more than you'd think - however, bad doesn't always mean open circuit. The metals can age causing the thing to cycle too fast or too slow leading to gauge readings that are off, like with my car, and so on.
I've experienced the same thing on Ford as well as AMC, so it's not a unique to AMC thing.
There are 3 connections required - ground, "12v" supply and output to gauges. Current goes in, through the heating coil or winding and in parallel to the gauges, then to ground. The part that goes through the winding heats the strip causing the bi-metal to bend and open the contacts. This breaks the circuit so the coil loses power, and the gauges get nothing either. The strip cools, the contacts make contact, and it starts over.
An analog volt meter will show the "average" output - it's not a true voltage reading!
You can't say that they "put out 5 volts" or "they put out 10 volts" as that simply is not true. When the contacts are closed full voltage gets trough (less the normal voltage drop across connections, etc.) The voltage out is a DIRECT connect to the voltage in when the strip is cool and relaxed.
Because the gauges themselves work on heating a coil to move the needle, they do not respond quickly to high or low voltage - but give more of an average of the duty cycle. The cycle happens so fast the gauges can't respond so you don't see the needle quiver. It's a "dampened effect". Because of this, the circuit doesn't even need any filters or capacitors to even out and smooth the output.
Anyway it's such a bloody simple circuit, albeit very delicate inside, yet one of the lesser understood areas.
By the way, it's a great reason to disconnect the battery when working on the dash and being careful to not subject the dash or gauges to bumps and sudden movements.
"amcenthusiast" has seen why first-hand. Pretty small stuff in there, right?  Wink

Guys you made me feel really old again as I'm not supposed to be able to remember all that old-skool stuff and the details. Those went away many years ago and as such it's been a very long time since I've had them open or done anything with 'em.
Gauges after those years worked more like a "regular volt meter", measuring the "voltage" using a coil inside a magnetic field - more voltage, more field from the coil, more needle deflection.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote W1ngzThinBlueLine Quote  Post ReplyReply Direct Link To This Post Posted: Nov/05/2013 at 4:24pm
Do you mind posting a picture? I'm trying to diagnose mine and I'm electrically illiterate. Thanks.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RebelYell Quote  Post ReplyReply Direct Link To This Post Posted: Nov/07/2013 at 6:54pm
I am having this problem on the Barcelona I just purchased. This car also has a factory clock that doesn't work so I was wondering if it might be supplied from the CVR?  I spent time today replacing a completely melted away ignition control module, and cleaning up all the mess it made. I ran the car after it got dark and when I turned on the lights I had a little bit of a fluctuation in the intensity of the lights, especially the interior lights. I noticed it looked like voltage regulator on the fender well is brand new. The alternator is supposed to be relatively new even though it doesn't look like it. Could the voltage fluctuation be caused by a bad CVR?  
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Post Options Post Options   Thanks (2) Thanks(2)   Quote rocklandrambler Quote  Post ReplyReply Direct Link To This Post Posted: Nov/08/2013 at 2:10am
Offhand I would say NO. The CVR is used only for the temp/fuel gauges. If you look in your TSM and trace out along the printed circuit board it should only affect those two items. Clocks are clocks, none of them work after a few years.
Past AMC's
1974 Hornet X (new)
1975 Gremlin X (new)
1964 Classic 660 Cross Country
1965 American 440-H
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Post Options Post Options   Thanks (1) Thanks(1)   Quote 401MATCOUPE Quote  Post ReplyReply Direct Link To This Post Posted: Nov/08/2013 at 4:11am
Rebel Yell,  Your Mat Coupe clock uses straight 12 volts, the CVR (Constant Voltage Regulator), function is to keep the fuel and temp gauges at a known constant while the "system" voltage fluctuates from a low right after start  (less then 12 volts) to a high that could be over 14 volts.  Improves accuracy of indicators.  I have only seen one Mat Coupe clock that worked, I sent the one from the "Prototype"Matador to Instrument Services, they do excellent work, they can convert to Quartz too.
Ross K. Peterson
68X,GoPac,343,AT,52A(1stCar)
68X,GoPac,390,4sp,52A
69X,GoPac,390,4sp,64A
70X,GoPac,390,4sp,87A,8
70X,GoPac,390,4sp,BBO,8
70 Jav SST,390,AT,BSO
74MatX,401,AT,Prototype
74MatX,401,AT
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Post Options Post Options   Thanks (1) Thanks(1)   Quote sidewinder Quote  Post ReplyReply Direct Link To This Post Posted: Nov/08/2013 at 8:43am
Chuck Page ,AKA Sidewinder
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