1971-1974 Rally restoration tip

JosephGiannini · Post by **JosephGiannini** » Thu Oct 04, 2007 12:07 am

I just recently found out when rebuilding a rally dash without the
tachometer. If you cannot find a usable 1971-1974 instrumentation
voltage limiter. You can use a Instrument voltage limiter, 1966-1970 B-body models from year one and it works fine. Some fabrication is needed to mount the device to the back of the cluster. However it works wonderfully. Also how the voltage limiter works internally is it is a copper piece of metal that is wound acting as a relay.

mopar71 · Post by **mopar71** » Thu Oct 04, 2007 1:24 am

Thanks for the tip!

rr6pak · Post by **rr6pak** » Thu Oct 04, 2007 4:31 am

I see Harms has tachs for $185. What kind of "mods" are needed to it?

rr6pak · Post by **rr6pak** » Thu Oct 04, 2007 4:34 am

OOps...I guess the mods is without tach.

72RoadRunnerGTX · Post by **72RoadRunnerGTX** » Thu Oct 04, 2007 2:10 pm

It happens that I too am currently restoring a rallye cluster for my 72â€™ RR project. Itâ€™s my understanding that a mechanical voltage limiter was used on almost every automobile built since the fifties when the change over to 12volt electrical systems occurred. As with any mechanical device, it will wear out eventually and at its best functions completely inaccurately in regards to regulating a 5 volt power supply for the analog gauges. Someone here on this site posted a link ( http://www.demonivr.com ) not to long ago about a solid-state replacement for the limiter. I couldnâ€™t locate the posting, not sure who to credit. If you can handle a soldering iron, an internet search on â€œ5v voltage regulatorâ€, you will find instructions on how to build your own with about $4 worth of parts from your local Radio Shack. Donâ€™t forget to use a heatsink.
With or without a tach, shouldnâ€™t mater as the tach is a 12volt device having nothing to do with the limiter. Speaking of tachs, I just ordered mine from Charger Specialties, currently 10% off.

mopar71 · Post by **mopar71** » Thu Oct 04, 2007 4:58 pm

I think it was Dave! or rr6pak.

rr6pak= http://www.demonivr.com/index.html

Dave= http://rt-eng.com/mediawiki/index.php/R ... 4_Tach_kit

patrick · Post by **patrick** » Mon Oct 08, 2007 11:27 pm

I was thinking of either building my own or ordering the DemonIVR, but my question is, since I have the rallye dash, how difficult is it to remove the regulator bits from the fuel gauge?

And is it the same process for both rallye gauge options, whether it's the larger TEMP/GAS gauge or the smaller OIL/FUEL gauge (such as if you have a tach or clock)?

72RoadRunnerGTX · Post by **72RoadRunnerGTX** » Tue Oct 09, 2007 11:57 am

Both versions of the rallye cluster have the limiter secured to rear of the casting with one screw. With the non-rallye dash it plugs into the circuit board on the rear of the cluster. It is not located in the fuel gauge on these cars. BTW, the condenser is removed as well, its function is to control the electrical noise created by the contacts in the limiter. The fuel gauge mounted limiter described on the demonivr.com site I believe was an older a-body of some kind.

patrick · Post by **patrick** » Wed Oct 10, 2007 10:05 am

Cool thanks, I guess the confusion was because on the gauge cluster I bought, it already had the IVR and noise suppressor removed and I had not yet looked under the dash at my own. Thankfully it's not in the gas gauge!

That being said, I have secured most of the parts just from work and scouring my dad's garage to build the solid-state 5v+ regulator. I'm so happy! Just one quick trip to Radio Shack and I should be golden! And at work we have heat shrink and a heat gun and solder, so, yeah!

Boss: What are you working on?
Me: It's technical...
Boss: Okay. Lunch?

patrick · Post by **patrick** » Tue Oct 16, 2007 11:35 pm

Okay, one last question, how important is it that the 7805 silicon chip be 1.5 amps? The one I got from Radio Shack is only rated at 1.0 amp. Is this sufficient? I have everything else, but most instructions I see online use 1.5 amps.

rr6pak · Post by **rr6pak** » Wed Oct 17, 2007 4:01 am

O.K now I'm confused..SORRY! I have my insturment cluster out of my car..What now?

72RoadRunnerGTX · Post by **72RoadRunnerGTX** » Wed Oct 17, 2007 12:16 pm

I have learned to preface my comments here with â€œitâ€™s my understanding thatâ€, here goes. The numbering system used for semi-conductor components are setup as a standard for manufacturing. In other words, a 7805 is a 7805 no-mater where it was manufactured; it should have been built to the same electrical/physical specifications. Quality? Who knows. The one Iâ€™m using was sourced from a Radio Shack store here and the specs. on the packaging is the same as what you have, the component itself may have or may not have been manufactured at the same plant. Radio Shack tends to be rather conservative with their printed specs. I suspect to account for the variations in quality. With that said, I have measured the current draw for the gauges I have at roughly 200ma per gauge for a total of 600ma or .6 amps total for all 3 at their max. Any voltage regulator rated at 1amp should work without any problems. BTW, this is where the heat-sink comes in; if you can keep it cool it will handle considerably more that rated current without failure.

If we have lost anyone here, I would advise that maybe you should look at the demonivr.com site and buy their piece pre-assembled. The site has instructions on how the install goes.

patrick · Post by **patrick** » Wed Oct 17, 2007 10:31 pm

Thanks for the great info about the 7805 IC. I'm kind of over-building my solid-state IVR with a little-bigger than normal copper heat sink and arctic silver, just to be cute. Hmm, maybe I'll even hook a PC fan up to it, running off 12V from the coil. Just kidding...