Gauge problems

[Chevy Dave]

I just replaces the printed circuit on my 70 SS w/gauges. Everything works good accept for the temp gauge and fuel gauge. These worked OK before, but now both needles  are pegged with the ignition off or on. Running the engine does not change anything. Has anyone else had this problem ? Any idea what is wrong?

[cbolt]

I replaced a printed circuit for a member about 2 years ago and the new circuit was defective, causing one of the gauges to not function at all. (Dont remember which one) I had to go back in and discovered the new printed circuit had a slice in it across several electrical runs that was difficult to detect. 

 

If I recall correctly a pegged meter is indicative of an open circuit, so the way to determine the issue is to take it back apart and visually inspect each electrical run, tracing them to see what is up. 

[DragCat]

From my blue oval experience, especially fuel gauge, a pegged gauge is a ground issue. with the ignition on. 

[Dtret]

Are you using the original idiot light harness? If yes you need to repin the connector. Two or three wires get switched around. 

[MCfan]

It sounds like you are simply replacing the printed circuit on an original full gauge dash instead of doing a conversion from a non-gauge dash? 

Do your dash lights function properly?  If so, the issue has to be a connection or continuity problem as addressed above if some gauges are also working since they all use a common ground. I agree with the suggestion to check the main plug followed by the individual circuits and their connections at the gauge studs.

Of all the many repairs and upgrades I've done on my Montes, working on the instrument cluster has been the most frustrating - I hate it!  Good luck!

[Chevy Dave]

My Monte is a factory gauge car. I disconnected the battery and the gauges are still pegged all  the way to the right. Do you think that I some how destroyed the gauges or could  the printed circuit defective? The only changes that I have done is to replace the printed circuit. The ammeter and all the lights work.

[MC1of80]

The ammeter is directly wired into the horn relay so the circuit board is not used. Sounds like a ground issue to the cluster/pod. 

[Chevy Dave]

Does the temp and fuel gauge pod have a separate ground?

[MC1of80]

I believe there is a 5/16ish hex head screw that goes through the circuit board into the metal housing. 

[MCfan]

Dave,

I will attempt to help troubleshoot and fix your printed circuit replacement problem but the only way I know to approach this is to ask some basic questions, take a close look at the factory full gauge printed circuit, and do some continuity testing.

First, what was the reason you replaced the printed circuit? What was and wasn’t working properly?

Second, did the new printed circuit fix the original problem(s) and/or create new problems? In post-replacement testing, what functioned properly and what didn’t (I know you have already said the Temp and Fuel gauges aren’t working)?  If things that were originally failing were not fixed with the new printed circuit, their root cause may well be upstream in the instrument cluster plug, wiring harness, fuse panel or other connections.

Answers to the above questions are extremely important to pinpointing the possible problem(s).  For example, if your dash lights, turn signals and tach are all working, you do not have a grounding issue as they all require a good ground through the printed circuit.

The printed circuit has fourteen serpentine, copper ribbon conducting paths (as a careful examination will show).  Twelve of them begin at the instrument cluster plug socket and two of them are grounded directly to the metal instrument cluster housing with three attachment screws (be sure they are snug). Those fourteen paths provide 12V DC power or modulated electric signals to the low-wattage lights and analog gauges in the instrument cluster. 

NOTE: The CLOCK is completely independent of the printed circuit as it has its own power wire and needs no signal wire. Also, the factory AMMETER IS powered through the printed circuit (positions #3 and #4 of the instrument cluster plug) as it simply measures the current flowing through the 12V system.

When you look at the factory printed circuit from the front (facing rearward), the instrument cluster plug socket is numbered as follows: Upper right tab is #1 and then consecutively down to #6 at the lower right. Switching to the left bank, the lower tab is #7 and then consecutively up to #12 at the upper left.

NOTE: For now, I am assuming that your replacement printed circuit is not damaged and that all circuit paths are functional.  However, it may be necessary to confirm the integrity of individual power/signal connections/paths with a DC continuity tester or multimeter.

Since both the Temperature and Fuel gauges are 12V analog instruments that convert a modulated electrical signal to a mechanical indicator needle, they both require connection to a 12V power circuit and an individual signal circuit. They both get their power from the #7 path which is fused in the fuse panel on the inside firewall (the fuse should be good if other gauges/lights are working).  The Temp gauge gets its signal from the #10 path and the Fuel gauge gets its signal from the #11 path.

Note that the #7 path also provides fused power to other devices (i.e. tach, brake warning light, oil pressure light) so, if any of those are currently functioning correctly, you possibly do not have either a power supply or a grounding problem with your Temp and Fuel gauges.  HOWEVER, the right-hand stud of those gauges might not firmly attached and in contact with the printed circuit.   Be sure to check both connection and path continuity between the right-hand stud and the #7 tab in the cluster plug socket for each of those gauges.

If both gauges are getting power, the gauge failure must be lack of signal. Since you already told us that those gauges were functioning properly before you did the replacement, the missing signal problem must be in the printed circuit or its connection at the instrument cluster plug.  I suggest you carefully examine and thoroughly clean the #10 and #11 contacts in both the plug and the respective tabs on the printed circuit.  Be sure the springy contacts in the plug are not damaged or dirty and are making good contact with their respective printed circuit tabs when the plug is in place.

The last thing I know to check are the connections at the left-hand stud of each gauge to be sure they are snuggly contacting the printed circuit.  Also check the connection and path continuity for each gauge between the left-hand stud and the #10 or #11 tab, respectively.

Obviously, if you find any continuity issues in the replacement printed circuit, it will have to be repaired (not easy to do) or replaced.

Sorry this got so long – someone may have a quicker, easier method to troubleshoot your problems.  Good luck!

 

[Dtret]

Very descriptive. Nice response. Well said. 

[Chevy Dave]

Thank you Dennis for some great information. The reason that I changed the printed circuit was because some of the lamps were not illuminating and I wanted to replace all of the lamps with LED bulbs to make the display brighter. During bench testing I short circuited the new printed circuit. I replaced it, but I may have blown the two gauges at that time. With the second new printed circuit  everything works accept the temp and fuel gauge. What is the best way to test the gauges to see if they are good? I have added an extra ground wire just in case I might get a bad connection at some time. 

 

[MCfan]

Dave, I would still do all of the steps I listed originally. If you confirm at all of your connections for power and signal are good and that each of the power and signal paths have continuity, then I would reasonably conclude the gauges have been damaged.  The fact that everything else that uses the new printed circuit is working also points to the possibility of gauge failure.  I don't know any better way to test the gauges than to put them into a system that has everything esle working, especially if you have confirmed that everything the gauges need to function properly is operational.

If you need new gauges, I suggest you contact sonnie24 on eBay as he provides those exact gauges in his full gauge dash conversion kits.  Be sure to specify '70 gauges to get the green tinted numerals and gauge markings.

[Dtret]

I ordered a volt gauge from him yesterday. Also goes by hi-tech classic on eBay. 

[Searsman]

Had simular problems with my 71 after replacing pcb for gauges. Problem

ended up being the plug pins not making connections to the pcb… I would expect gauges to go back down when connections are good. Just my 2 cents.. 

[Paul D Valle]

On 11/24/2023 at 12:18 AM, Chevy Dave said:

I just replaces the printed circuit on my 70 SS w/gauges. Everything works good accept for the temp gauge and fuel gauge. These worked OK before, but now both needles  are pegged with the ignition off or on. Running the engine does not change anything. Has anyone else had this problem ? Any idea what is wrong?

The grounding connections are critical on the dash clusters. 

I have found that the new printed circuit mylar can partially cover the #10 screw grounding connection to the cluster can.

From your old circuit determine the location of the grounding connection for the guages that are not working and ground it with a jumper to anypoint on the dash Steel frame. 

 

[Dtret]

On 11/25/2023 at 11:53 PM, Dtret said:

I ordered a volt gauge from him yesterday. Also goes by hi-tech classic on eBay. 

Forgot to post. This was a last week. 

[Grant MacPherson]

Test your volt gauge before you close up your dash, mine is a few volts too high according do multimeter.  Not sure if it is adjustable,  thought maybe could just pull the needle and reinstall.  Never called supplier,  didn't want to go back in.

[smokinquack]

I got the sonnie24 kit to switch from idiot lights to gauges, new printed circuit, gauge and sending unit for temp. My gauge  goes from zero to pegged then goes back to zero, its all over the place. If I hook the sending unit wire directly to ground it pegs. Is the sending unit bad out of the box? Can I test continuity or ohms or something with it on the car? 

[MCfan]

Greg, there's obviously something wrong with that circuit but it might or might not be the sending unit. The new gauge seems to be getting both power and a signal so it is logical to suspect the sending unit.  There was one important change in the wiring of that circuit and that was to move the dark green wire from position 9 to position 10 in the dash harness plug that goes to the PC board.  I assume you did that but you may want to check.

The signal strength should only vary gradually with the water temperature.  Since the gauge is jumping around, I suspect it is a connection/continuity issue. The instructions call for replacing the factory sending unit on the passenger side with the new sensor, so I assume that is what you did.  I actually did not follow that instruction and mounted the new sensor/sending unit on the driver's side of the block just above the exhaust port (see first photo).  I left the original sending unit in place and just disconnected it.

I don't remember if I sent you my enhanced instructions for installing that sonnie24 kit, but I wll paste in here the section about installing the new temperature sensor/sending unit:

Replace the engine temperature sending unit:

1.      The new temperature gauge required a different sending unit which is included in the kit.

2.      Locate the current temperature sending unit in the driver’s side cylinder head just above the #3 spark plug.

3.      Disconnect the dark green (temperature) wire from the sending unit and leave it lay for now.

4.      Use a 24 mm deep socket to unscrew the old temperature sending unit from the cylinder head.  CAUTION: There is coolant present in the cylinder head that will pour out when you remove the sending unit so have the new one ready to insert immediately (Teflon tape already on the threads) and a rag to catch some spilled coolant.

5.      Immediately insert the new sending unit and tighten it securely to prevent leaking.

6.      Clip the old connector off the end of the dark green (temperature) wire, strip back a bit of insulation and crimp on the new style connector that is supplied in the kit. (see second and third photos)

7.      Connect the dark green (temperature) wire to the new temperature sending unit.

8.      All wiring for this project should now be complete.

Also, I assume you changed the connector on the end of the dark green wire as stated in #6 above.  If so, be sure you have properly crimped the new connector onto the dark green wire.  The only other thing I can think to check is to be sure the new connector is firmly seated on the stud of the new sending unit (see last photo).  Regarding checking to see if the new sending unit is working, I suspect it is putting out a very low voltage analog signal.  When the engine is cold, I would not expect to see any millivolts between the connection stud and a good ground.  With the engine warm/hot, I would expect to see a positive millivolt reading.  If you don't get a reading with a warm engine, I would suspect the new sending unit is bad.  But, again, the fact that your new gauge swings wildly suggests a circuit problem so I would definitely check all connections and the end-to-end continuity on the darkgreen wire at their respective connectors.  Good luck.

[smokinquack]

Thanks for the reply, I feel like I followed the directions pretty well but I cant see the #9 & #10 connector position without taking it all apart. The directions actually say to replace the drivers side sending unit. The passenger side looks like a broken sending unit that went to a TCS that has been deleted???  I was checking the gauge with a multimeter but just ended up confusing myself. I'm taking it to my mechanic once the snow melts.

[Blazer1247]

I also recently replaced the PCB on my '71 Monte.  All the lights work, turn signals and such. As others have mentioned the fuel gauge and temp gauges do not work.  The temp gauge pins out and the fuel gauge does not move. At first, if I banged on the dash, they connected and worked.  This was an intermittent fix and is no longer effective.  That made me think I had a loose connection.  I took the nuts and washers off both and reconnected them thinking they were loose and still they do not work.  I am guessing the ground is good or the lights and such would not work.  I replaced the PCB because over time working on the dash it was physically damaged at the voltage meter and that light point.  By the way I installed all new LED bulbs, and it looks great at night.

As anyone knows that has worked on this issue, it is a royal pain taking the dash and gauge cluster apart.  Also, the PCB is fairly fragile, and I don't want to damage it.  In the vernacular, "it ain't cheap".

I appreciate the long explanation MCFAN and will try each step and let you know if I am successful getting both gauges working.