Instrument Cluster Voltage Limiter Replacement

[martincom]

There have been at least two other threads on the forum in this regard. My creating a third is by no means stating their methods are wrong. I just took a different course, but did research the pitfalls they mentioned.

One poster utilized two solid state, IC chip, voltage regulators in a stepped series arrangement along with an external heatsink. This was to overcome an overheating issue. I measured the current draw of all the gauges combined, including the optional oil pressure gauge, and compared this to the rated capacity of the IC chip regulator I had chosen (it was in my inventory). The IC chip regulator had more than ample current capacity for the current load of the gauges.

Another poster made reference to the potential of "frying" the gauges in the event the regulator ground connection was inadvertently lost. I agree this could be an issue, depending on the internal design of the regulator utilized. I didn't research this issue on the regulator I had deployed, as my installation actually preceded that thread by months. However, on the regulator I deployed, the integral mounting tab is also internally connected to the regulator's internal ground circuitry. As my installation has the regulator secured to the instrument cluster housing with a machine screw, it has a redundant ground connection. Further, it would be highly unlikely the screw type mechanical mounting would become open.

I opted not to solder connecting wires to the regulator chip's leads. It would create additional bulk/mass that could impact the internal connection of the lead with temperature expansion/contraction, road vibration and time. More, significantly, it has an unprofessional appearance. Rather, I chose to utilize a connector to make the connection. This also provides for easy replacement of the regulator chip in the unlikely event of it's failure.

I mounted the IC chip regulator to the top of the instrument cluster, directly above the speedometer neck. Be sure to mount it off center, so the machine screw nut does not interfere with the speedometer illumination lamp. Also, be sure to mount it far enough back on the cluster so the regulator does not contact the top of the dash opening metal. The speedometer housing serves as a heatsink for the regulator. The white paste seen around the regulator mounting is Wakefield Thermal Compound. It is thinly applied to the regulator chip mounting flange to maximize heat transfer. It has a property in common with anti-seize-----you wear it when using it. I had it on-hand, so I utilized. You could substitute silicon dielectric grease or mount it dry. The regulator did not not generate much heat.

I utilized 20 gauge wires to connect the regulator. I did solder the connector contacts to the wires as I could not obtain a strong mechanical connection by crimping alone. I followed the vehicle wirings' color codes with the regulator chip connecting wires:

Red = 12 volt supply
White = ground
Black = 5 volt instrument supply

The brass alloy bus bar between the fuel gauge supply post and the male slip-on gang strip, affixed to the back of the amp meter, must be trimmed to open the path.

The 12 volt supply voltage lead is re-located from the fuel gauge to the gang strip. The regulator chip wiring connection points are obvious within the photo:

Below is a photo of the instrument cluster ready to mount:

The circuit breakers mounted to the backside of the instrument cluster housing are for the trailer brake controller and the auxiliary 12 volt feed to the 7-way RV trailer connector. The additional wiring, attached to the amp meter studs, is to reinforce/increase the bulkhead connector current capacity. More on this in an upcoming thread.

Bill of Materials

22-01-3037 Molex connector housing https://www.mouser.com/ProductDetail/538-22-01-3037

08-51-0108 Molex contact https://www.mouser.com/ProductDetail/538-08-51-0108-LP

LM340T-5 5-volt regulator https://www.mouser.com/Search/Refine?Keyword=LM340T-5

[swptln]

I'd be very careful with that regulator. If the middle pin (ground) is lost the output voltage will reach the unregulated input. If the ground is reconnected with power going to the input, it will damage the regulator. This regulator needs a solid ground on that middle pin before powering it up.
Whether the heat sink is connected to the middle pin, maybe questionable as the data sheet....mentions nothing about it. It does show ground in the photo but, I'd do some testing on that regulator to see what happens. If it is connected to the sink, you'll probably be ok as long it's grounded and don't remove it from the cluster.
I'd also make sure the cluster ground wire in your harness is in good condition so as your not relying on the dash to cluster housing for a ground. Removing the cluster from the dash for testing, could cause a failure, if the ground wire in the harness isn't grounded as it should be.

This is a major problem with line voltage regulators and they need have protection built into the circuits.

[martincom]

Thanks for the heads up, Mark. I did confirm the mounting tab and the center, ground, pin were common with an ohm meter. I have a few more of these regulators on-hand. The next time I'm at my electronics bench--and I remember--I'll pull one out and run some tests to see if the input voltage is coupled through to the output, with a similar load in place as the gauges, if the ground connection is opened. None the less, I'm more than comfortable with the tab mounted ground connection.

Another somewhat unrelated question: Is the ground connection point of the white instrument cluster ground wire at the headlight foot dimmer switch? Is there anything else that connects to this ground wire?

[WarEagle]

I'm interested in replacing the voltage limiter housed in the stock fuel gauge as well on my 1967 D100. Scoured the internet, this forum and other Mopar forums on this topic and ran across another approach:

https://www.youtube.com/watch?v=UrjEQYBoLM8