68 Barracuda........ Temp and Gas Gauge Issue.

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Al T

Al T

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I recently purchased a '68 Barracuda convertible and am trying to chase down a non working temperature gauge and gas gauge.
Step one. Take a driving car with a couple of faulty gauges to this.....



I've got continuity from the temp sending unit to the round plug on the back of the gauges and I've checked the resistance at the sending unit and had 193.5 ohms with it cold and 22.8 ohms with the engine at 160F (checked with an infrared gun) so I believe it's OK. I've measured the resistance across the gauge and have around 15 ohms.
For the gas gauge I've got continuity from the sending unit connection to the round plug at the gauges and have checked continuity on the circuit board. I've got a good ground between the sending unit in the tank and the car.
I've got power at the black wire terminal (K position) on the round connector which according to the schematic is the feed for the temp and gas circuits. The previous owner had installed an aftermarket 4 volt regulator that when I pulled the dash out wasn't connected.... and the wiring on the round plug was as it should be for a stock system so it should have not had any input to the issues I'm having. It's all been removed and as far as I can tell, the wiring is as it should be.


The more I read about it, is it possible the regulator in the gas gauge is the issue? Is there a way to check it with caveman electrical testing equipment? The other thing I don't understand is on the printed circuit board, how does the black wire feed power to the temperature gauge? I don't see any physical connection between the two. Should there be a jumper wire?
I've got the gauges set up on a spare circuit board and still have no movement on either gauge.



Any advice/insight would be appreciated.

Allan


 
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Yes the VR in the gas gauge is probably the culprit. Search for threads about replacing it with a solid state component.
 
Your temp sensor readings make sense for the resistances and temps you recorded.

To make a crude check of the IVR in the gauge, pull the wire off of the temp sensor, turn the key to RUN, and look at the voltage coming out of the temp sensor wire. It should be pulsing from 0 to some + voltage. If this voltage is always 0 or 12, then it is bad. 'splains why the 5v regulator was put in at one time.

IIRC, the connection to the temp gauge from the IVR is on the circuit board.
 
I should have mentioned the oil pressure gauge does work.
Does it not use the same IVR in the gas gauge?

Allan
 
Yes it does. Unless some snuck another one in there somewhere..... Does the oil pressure seem reasonably accurate?

So have you checked at the end of the temp gauge lead for the pulsing voltage? You should see the same at the end of the fuel sender wire. If you have the pulsing voltage, since the temp sensor is in spec, then likely the temp gauge is not good. If no voltage, then the traces/wiring within the cluster have some problems.

Also, read the resistance into the fuel sender with what you think is in the tank.
 
I can't respond to "black wire" unless we're speaking of park brake indicator lamp wiring. I can say, remove and discard previous owners BS attempt at repair. It didn't work anyway.
A dark blue w/ white tracer, at 10 oclock position of round connector, feeds 12 volts to limiter inside fuel gauge at switch on position. The limited/reduced gauge voltage is supplied to the fuel gauge inside that gauge and also comes back out to printed circuit trace to connect the other 2 thermal gauges.
Dark blue wire at 12 oclock position of round connector comes from the fuel sender. The gray wire is from oil sender and violet wire is from temp sender.
Take a glance at the pot metal inst' housing where this circuit board was removed. You'll notice cut aways where the temp and oil gauges are isoluated from chassis ground. Not so for this 3 post fuel gauge. Not only is the casting solid there but the inside will have a unpainted spot about the diameter of a quarter to further insure this gauge housing is grounded. No a fuel gauge doesn't need chassis ground. A limiter or regulator does though.
So pull the fuel gauge off the board and look at its backside. See a thin piece of metal there. That is a critical component in the mechanical limiters operation. Sometimes a weak or broken ground path there is the only fault. With circuit board floating as seen in your pic, aint chit gonna work.
 
Did a little more checking this morning. I've got 25 ohms at the sending unit with this much fuel in the tank.... lol



I checked voltage at the temp sending unit and gas tank and am reading mV. I was wondering about the ground on the gas gauge. Can I just run a jumper ground to check whether I have voltage at the sending unit leads with the gauges "floating"?
 
OK. Back in and properly grounded.




None of the three "thermal" gauges work. If the IVR was working I should be able to get a voltage reading on this circuit?



Thanks again for all the advice!

Allan
 
Look for the pulsing on off 12 volts DC, as others advise. If you put straight 12 volts to the hot wire thermal gauges, they will probably be wrecked.
No cracks in the copper foil traces?
 
There are bunches of old threads on gauges, clusters.

I would "rig" the cluster out of the car so you can test it. Be sure the studs/ nuts at the gauges are making good contact.

Use alligator clip leads from your fuel sender, or get some other resistors to use as sender substitutes.

Rig battery power to the proper harness connection and verify that it is transferred to the proper post on the fuel gauge

(Be aware a popular problem is broken / loose connector pins, IE not making contact with the traces)

Here's how the fuel gauge works. It has THREE terminals, unlike the oil / temp

1.....One terminal is 12V INto the gauge from the harness connector pin. This feeds power to the IVR unit

2.....Second terminal is common with the fuel gauge itself as well as the regulated power coming out of the IVR. This pin feeds the "5 v" to the other two gauges

3....Last terminal of course is the sender

There are numerous forum pages and sites on this

And this one is excellent

mopar rallye gauge voltage limiter - Google Search

RTE Limiter Faq - rte

A big discussion about the Chrysler gauge tester which is a simple box with three resistors

Hey Del, Look what I found!!! An original Miller Gauge tester!!!

In other words you can test all three gauges with three resistors, for empty, half, and full

A photo of part of the chart on one Mopar tester. The yellow resistor values were added by someone else. Those are what you need

c-3826-jpg.1714848381
 
If your meter was reading just mV (millivolts) at the end of the temp sender or fuel sender wire, then the IVR is kaput or your connections are bad somewhere. Testing at the end of the tmep sensor wire is the quick way to check; your meter should be pulsing from 0 to some voltage up to 12 volts about once per second (but this will vary a lot from meter to meter).

If the 'this' circuit' that you are referring to is from the gas gauge to the temp gauge on the circuit board, then yes, you should see the pulsing voltage there.
 
If your AM radio is working, tune to a spot where the band is silent, and you should be able to hear the ticking about once a second, sounds almost exactly like when you are listening to a weak signal and driving on a road with an electric fence alongside.
It's even visible on some cars' ammeters.
In my '71, I put in a 5 volt solid state regulator, to get rid of the AM radio ticking, which capacitor bypassing did not seem to help. Unfortunately it creates more battery drain than the factory regulator.
Not much, but for sure.
My '72 Dart seems to have a "smoothed out" ammeter; the pulsing is only visible if you look VERY closely, and I'm looking right now with ign turned on.
 
nm9stheham said:
If your meter was reading just mV (millivolts) at the end of the temp sender or fuel sender wire, then the IVR is kaput or your connections are bad somewhere. Testing at the end of the tmep sensor wire is the quick way to check; your meter should be pulsing from 0 to some voltage up to 12 volts about once per second (but this will vary a lot from meter to meter).

If the 'this' circuit' that you are referring to is from the gas gauge to the temp gauge on the circuit board, then yes, you should see the pulsing voltage there.
Click to expand...
I've already confirmed continuity on both circuits so it's obvious the IVR is bad.
My apologies to everyone for beating a dead horse. I didn't realize this is a pretty common problem. I'll do a little research on existing threads and hopefully get this resolved.

Thanks to everyone for the help.

Allan
 
If 'twere me, I'd get one of the RTE IVR's (limiters) and work it into the circuit. Check the link in post #10.
 
Al T said:
I've already confirmed continuity on both circuits so it's obvious the IVR is bad.
My apologies to everyone for beating a dead horse. I didn't realize this is a pretty common problem. I'll do a little research on existing threads and hopefully get this resolved.

Thanks to everyone for the help.

Allan
Click to expand...


Certainly no dead horse. All I was meaning is that there's a lot of threads you might find useful. I agree with the suggestion to the RTE solid state limiter. That was one of the links I posted RE: reworking the fuel gauge. You really cannot tell too much with resistance tests except maybe point to point in the cluster traces, and senders to ground. You need to do as suggested earlier.........rig it up so you can test it with sender resistors.
 
One might think some of these members own stock in RTE. WHAT BRAND NAME solid state limiter DOES NOT MATTER ! Even the homemade add on like your previous owners attempt can provide 5 volts for these gauges just fine,, if done correctly. There are legitimate reasons to open these very old thermal gauges. "bend the little thingy" is not one of them. Disabling the original limiter is as simple as removing that previously mentioned little thin piece of metal from the back of the gauge.
So why would anyone suggest opening this gauge ? To see if it's cooked,,, and it very likely is.
The seeing and knowing leads to sending the gauge out for repair. Odds are you will send it to whoever you purchased a solid state regulator from. There, 10 years of smoke and mirrors were blown away this Memorial Day.
 
I have made a few gauge testers and it is usually the IVR that is toast if more than one gauge is kaput.
 
halifaxhops said:
I have made a few gauge testers and it is usually the IVR that is toast if more than one gauge is kaput.
Click to expand...

In the case of 13 ohm gauges common to standard panels, yeah they can look good as new on the inside and work just fine. This is where something like the RTE, other, aftermarket solid state regulator has much success. Just change it out like a light bulb and happy motoring again.
3 X 16 ohm gauges in a rallye panel is not the same animal. To open this fuel gauge will reveal black crunchy carbon build up on the ni-chrome winding and that bi-metal beam bowed at room temperature. It aint gonna respond correctly. That's why they say open it.
Then none of the fuel gauges will be correct with aftermarket senders anyway.
I apologize to the owner/thread starter for any chit storm I may have created. Good luck with it.
 
No problem at all......... it's enjoyable reading. lol.

So I need the IVR3...... not IVR4?

I just want the gauges to work.

Allan
 
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I recommend the RTE because it emulates the pulsed output voltage of the original limiter, and thus keep the heat load minimized on the IVR. The big issue with using a standard regulator is that when the oil pressure is high, the tank is full, the temp is high, and the alternator is putting out 14 volts, then the heat load on the regulator is pushing into the 5 watt range. With no heat sink paste and a small heat sink like shown above, the junction temp is going to soar immediately well beyond 125*C and that is moving into eventual failure territory. The PO's 5v regulator obviously did not last based on the pix in post #1.
 
OK, before you proceed,,, I clicked one of your pics for a better look. Both paint and primer missing, bare metal showing at the fuel gauge needle near its base tells me someone has lifted that needle at least once before. Lifted as in flag pole raising. Please do not do that again. This material is a lot harder than the kitchen aluminum foil. It will break ! 3 up down cycles at max. Most often broke in 2 cycles. Separate the crimp at can perimeter and move screen and cover as a whole due north from under the needle. Beyond that , good luck
 
see it be the issue more times than i can remember , volt limiter dam you ! so thats the first check on the list for that pair of gauges are going way- wire , no pun intended.
 
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