Guage Dead End

ECJ amigos,

Patient: 1975 CJ5, 4.2L 258 I6

Symptoms: Both Fuel and Temp guages went out.

History/Testing/Repairs: I began by reading every post on this blog, and other Jeep blogs, that address fuel and temp guage problems, testing, etc. Watched a dozen youtube videos too. I am not electrically inclined and the words "automotive electrical" make me cringe... Tested the guages with a multimeter for voltage and resistence. All readings were "normal" except the temp guage was receiving 12V. Many of the posts indicated the voltage regulator is notortious for going bad and if the gauges get 12V (vice 5V) they get fried. Triple checked wiring from fuel sending unit and temp sensor, including grounds, and all appear in proper condition. Purchased a new Omix instrument cluster (l know their bad rep, kept my OEM cluster just in case) and installed it. Speedo working fine, both guages still no life.

My thought is it is a grounding issue. I've learned CJ's have infamous grounding issues. There is of course no ground wires/terminals on either guage and all indications are the gauges are "grounded" to the intstrument cluster body, which I assume is then grounded to the dash through the mounting screws. I checked the contact between the new cluster and the dash and it's good. Verified I've got good 12V from the ignition switch going to the "I" post of the fuel guage. Fuel sending unit and temp guage wires correctly attached. Any guidance is greatly appreciated.

Good news is, it's not a show stopper. I write the mileage on the dash with a dry erase marker and fill her up every 100 miles or so. She only goes maybe 30 miles per week. It'd be nice to have a temp guage and the fuel gauge is a nice convenience. If I can't figure it out, next step is stand alone guages and abandon trying to get the cluster guages to work.

Thanks y'all! Great CJ community here, I've found so much helpful info in my never-ending Jeep CJ tinkering, repair, and improvement hobby/frustrations.

Brian

 

The voltage regulator (VR) needs a 12V input, the 5v-ish output to the gauges (internal for one of them and a strap to the other), and a ground. The VR is pulse width modulated, ie it's mechanical and turns on and off to make an average of 5V. You may be able to see this with your multimeter, especially if you have an expensive digital meter that updates quickly or an analog meter with a needle instead of a digital display. A cheap meter may get confused and tell you nothing.

This may be asking too much, but replacing the mechanical VR with a 7805 chip will be a huge win, and is not difficult. The 7805 is an integrated circuit in a package that makes a constant 5V. If the 7805 loses ground, it just goes open - no danger to your gauges. If the factory mechanical VR loses ground, it stops on-off cycling, which sends a constant 12V to your gauges. A few minutes of this will let the magic smoke out.

I need to work now, but I can pick on this later. Look for articles describing this conversion. There is probably a thread or two here about it, and I know the Wagoneer forums have several threads. The parts are cheap too.

You can also test your gauges on the bench easily with a 5V source; A 6V lantern battery is close enough, or three 1.5V D cells in series will work too. The senders are variable resistors, which you can replace with fixed resistors. Not difficult.

 

Timgr - good stuff, thank you. I will start looking into the 7805 conversion as a possible solution long-term. I'm still hoping to find the problem with the current set up and get these new guages working. Thanks again, I always appreciate your insight and learn something new.

 

Try these links
CJ Gauge Troubleshooting
Jeep CJ Gauge & Sender Diagnostics

 

Came across this thread...googled that 7805 circuit you referenced. Good Timing, good info. I'm redoing the dash right now and my Cluster is getting restored. To avoid a burned out future fuel/temp due to loss of ground seems this 7805 is a good option.

So Tim, for the non-electrical engineers...out there like me, and to keep super simple layman terms. Can I run two 7805s w/ heat sinks and capacitors, one to each gauge Fuel and Temp, outputs for the 5v power run to the gauges? Do I then remove the factory 'link' between the gauges? And both power leads from the 7805s go to the 'A' terminals of each gauge? Just want to make sure source 12v power is coming through the 7805 before hitting the gauge.

 

Ok, you have at least one assumption here that is wrong.

The fuel gauge includes the "CVR" constant voltage regulator. You can abandon this CVR in place. If you use the 7805, this replaces the CVR which normally supplies both the gauges, temp and fuel. You do not need two 7805s. One 7805 can supply both gauges. You would be abandoning the "CVR" and replacing it with the 7805.

Remove the 12V wire to the fuel gauge and send it to the 7805. Then send 5V from the 7805 to the link between the fuel and temp gauges.

 

Ok... I think I'm following with 50% retention did I mention I am not an engineer. I probably mixed up the wire path in my mind. Agree to abandon the fuel CVR in place and leverage the 7805. Just not sure how that wiring then goes to bypass and power both gauges. Any chance you could sketch it out on a napkin and post?

 

No, I'm not going to go through it again. It's not complicated. There is plenty of explanation available if you search.

site:earlycj5.com 7805 - Bing

 

I usually keep my mouth shut, But.......
If you carefully read the links that Timgr referenced, the info is there, but maybe this will help

Remove the 12v feed from the fuel gauge. and run to the input of the 7805. Leave the 12V terminal on the fuel gauge open
Remove the VR strip from both terminals
from the output of the 7805 connect to both terminals that the VR strip was connected.

Here is a simple circuit of how the 7805 s/b wired



Hope this helps and doesn't make things worse.
(which is what usually happens when I put in my .02)

 

Yet another thread about this topic: Instrument cluster voltage regulator upgrade - Full Size Jeep Network

The Wagoneer has the same sort of CVR, except they move it from the fuel gauge to the temperature gauge.

 

Thank you Desert Runner... you nailed it. Wasn't sure what to do with the 12v input to gauge and how to correctly bypass the VR, which I 'assumed' was inside the Fuel gauge because I cracked the gauge open. Found info on another site for how to bypass. Just afraid I'd hook the 7805 up wrong and fry my restored gauge (pic)...that's how a 45-yr old gauge should look

Shouldn't feel need to keep quiet. Forums are for sharing! The more information flow the better. I'm on P-4x4, and that site can be brutal. I've answered the same question a hundred times on the hard core forum for people asking. Never once flamed someone. Not butt hurt because folks have sand in their cl*T for answering something or pointing to it. I'm always happy to share my jeep knowledge, admittedly my knowledge is more suitable to rock crawlers, D60s, 5.3/6.2FI, 4-link setup, swapping 1-ton drivelines, and fabrication. Probably not as extensive as a 5-paragraph Op order in Triplicate on a Relay, but still rather knowledgeable. After this project...add '74 renegades to my list, without the condescension.

Sorry OP for a big highjack...I'm tapping out.