Rally Gauge Swap Debugging
 

Had a set of rally gauges restored by Pete Serio a few months back and they look fantastic. After some work refreshing the dash, I finally swapped my idiot lights out for the new gauges. I did the 5/7 wire swap and unplugged the dark green wire from the ignition switch - the temp and oil pressure appear to be working great, but I think I've got a couple other issues to resolve.

1) The lights for the rally gauges are really bright. They're so bright, I'm not sure if the rest of the lights in the pod are working. Specifically, if I take out a bulb for the speedo, it doesn't seem to make a difference. The clock lights are clearly working as it's a contained unit.

2) My gas gauge doesn't appear to be working. It's on E, but it's not buried. It's been a while since I've driven the car, but I'm pretty sure it's got at least a 1/4 if not a 1/2 tank. I don't think any of the wiring changed for the gas gauge so I'm not sure what's causing this.

Usually these things end up being caused by a bad ground somewhere, but I'm not sure what I might be missing. Any one who has done the swap or has rally gauges have any input?

Maybe I should start by putting more gas in to see if the gauge is really broken :D

http://forums.maxperformanceinc.com/...1&d=1577413484

http://forums.maxperformanceinc.com/...1&d=1577413679

Some pics of Pete's awesome work for reference:

http://forums.maxperformanceinc.com/...1&d=1577413097

http://forums.maxperformanceinc.com/...1&d=1577413105

As far as the fuel gauge, you could detach the signal wire in the trunk that goes to the sending unit. The gauge should go full if the wiring is correct between there and the gauge, and the gauge is working. I think it's tan...if I recall?

Put 5 gallons in it, the needle buried itself past the E. Disconnected the sending unit wire and the needle didn’t move. There must be a bad ground somewhere.

That's a strange one. For it to move in the opposite direction as the float comes up. I'll have to try to wrap my head around that one.
You'd think if it was an open or a short that it wouldn't care where the float was.
at any rate, at this point I would probably disconnect the sending unit and check for voltage with power on and then check for a resistance or short between the plus wire and ground with power off... Just to see what's going on.

Took a look up under the dash and I didn't see anything missing or out of place. I did confirm only 1/2 of the lights in the pod seem to be working for whatever reason. The ones on the speedo side aren't lighting up.

I unplugged the main harness and reseated it, but it didn't fix anything. The only thing I can think of is there's a good layer of paint between the ground straps and where they attach to the metal dash. I was like that with the old setup and it worked fine, but when I get a minute I'll try to clean those tabs off so it's a metal to metal contact see if that helps anything. Hoping I didn't ruin the gas gauge by bottoming it out. I need to pick up a new multimeter to get rid of some of the guess work.

I know the simple circuit from the 'plus' side of the gauge to the 'negative' side of the sending unit pretty well but nothing about the gauge itself...could it be wired backwards?
That's the only thing I can think of that might make it move towards 'empty' as the sender moves up. I really don't know if it looks at current direction or not though. Mr. Serio would know that I'm sure.
Resistance should increase as the float moves up and that should cause the gauge to move towards 'F'. Yours seems to be moving in the opposite direction.
Yes, a digital volt/ohm meter is needed.

Alright, so I'm confused here.

I disconnected the sending unit in the trunk and measured the resistance. I put the multimeter between the connection on the harness and the connection to the sending unit and got reading of 115. Seems high considering the 90 is supposed to be full, but I'm pretty sure this thing is filled pretty close to the brim right now. I also cleaned the sending unit ground to the body. It was already pretty clean but better safe than sorry.

Next I measured the resistance between the poles on the gauge with the key off. It's 44.5 which seems perfect considering a 0-90 scale.

However, when I turn the key on the resistance at the gauge goes negative and keeps building. It went to 150 before I shut it off which explains why the gauge is burying itself below E. Note that it did this without the sending unit wire attached.

Electrical debugging has never been my strong suit. Did I measure things correctly? What does it mean that the resistance for the gauge builds like that? Is there a short somewhere? Why isn't it affecting anything else?

Everything else on the gauges works as they're supposed to - turn signals, gen light, high beam indicator, etc. Pete also said the lights look great so I'm not sure what's up with the gas gauge.

I also added an extra ground wire from a screw attaching the gauge pod to the dash housing to the metal dash to ensure I had a good ground.

Fuel gauge testing
 

This is a book from the dealership around 1969-70.

The Ser con tester is basically an upscale multi-meter. The cost to each dealership back in 1969 was $187.50 so if you translate that into modern $ it was a pretty expensive piece of test equipment.

It also had built a built-in rotary switch with various resistance settings in ohms.

That is how you test a fuel gauge. Never ever try to measure resistance using the ohms scale setting with a multi-meter on a "live" 12 volt + circuit!!

Thanks Pete, I appreciate your wealth of knowledge. I'm always amazed by how simple these machines are but how much more there is to learn.

Pete, is it possible the lights are being used as a path to ground ? Is the cluster only ground in the harness ?

Yes it is possible. Electricity always seeks the path of least resistance and the gauges as well as the night time driving lights in the dash all share a common ground which is the steel housing they are mounted into. (There is no ground wire in the 12 cavity plug-in.)

Please note that the ground for the night time dash lights has to go thru several copper pathways in the printed circuit first to get to the housing. The 3 gauges are wired directly to ground at the housing with the 12 volt + power supply as well as the seeker wires to each sender going thru the PC.

The 12 volt + (key on) power to the electro-magnetic coils of all 3 gauges are wired in parallel so high resistance in one gauge or one gauge's seeker wire could result in a back-feed into one or more of the other coils, that or one of the light bulb or bulbs; the filament(s) for the gauge cluster's night time driving lights.

I'm not sure if this is any kind of clue, but with the e-brake on and the key turned to ACC, the BRAKE light in the cluster is really dim and the GEN light is really bright. When you turn the key to ON, the GEN light goes out (I think that's expected) and the BRAKE light gets really bright. Seems to me either light should be the same brightness no matter of the key position.

I'm thinking I should just pull the cluster out again, there must be something I'm missing. Either I did the harness wire swap wrong or I don't have a good ground somewhere that I can't see.

I would pull the cluster & go thru the plug-in one more time, making sure that the right color wire is in the right cavity. Since this is a car that did not come with rally gauges the wiring needs to be re-positioned in the plug for the gauges to work.

GM has a color theme to their car & truck wiring so once you understand the wire insulation colors the wire itself tells you what circuit it pertains to.

Here is the list for a '68 to 71 Lemans or GTO with rally gauges (option). Note that one cavity in the plug is not used. So you have 11 wires total; in the plug-in.

The black plastic plug-in on the back of the main cluster is arranged so that each cavity is numbered. (1 thru 6 on the top & 7 thru 12 on the bottom.)

The plug-in is molded on purpose so that it will only fit into the gauge housing one way. (The two locking tabs on the sides on the plug-in are not the exact same width.)

#1 lime green (high-beam headlamps on indicator).

#2 Dark Blue with a white stripe (Right turn signal: green arrow).
#3 empty.

#4 Gray wire (dash lights, for night-time driving).

#5 Dark Brown ( Alternator charge "gen" light).

#6 Dark Green (Coolant temperature gauge).

#7 Tan (fuel gauge sender wire)

#8 Gray wire (dash lights, for night-time driving).

#9 Light Blue (Left turn signal: green arrow).

#10 Dark Blue (oil pressure gauge).

#11 Pink = This is your 12 volt + battery power feed to the gauge cluster from the Ignition switch- (key on).

#12 Tan wire with a black stripe (When grounded this turns on the "BRAKE" warning light in the speedometer circle).

As I turns out, the lights are all working fine it's just the fuel gauge. I made a bonehead mistake a confused the turn signal lamps for gauge lights.

I pulled the panel out tonight and went through everything. As expected, the fuel gauge was pegged way beyond E.

http://forums.maxperformanceinc.com/...1&d=1578806916

Double checked all my wires, they match Pete's list above. #7 is the fuel gauge wire.

http://forums.maxperformanceinc.com/...1&d=1578807291

http://forums.maxperformanceinc.com/...1&d=1578807307

I took off the PCB to make sure it was making a good connection around the fuel gauge and reattached it. Didn't see anything out of the ordinary. Looks like all terminals on the harness are making a connection to the PCB too.

http://forums.maxperformanceinc.com/...1&d=1578807463

I cleaned off all the ground points on the body and on the instrument panel and reinstalled the cluster. Now I've got the opposite problem - the gauge went all the way to full regardless of whether the sender is plugged in or not. So I pulled the cluster again.

http://forums.maxperformanceinc.com/...1&d=1578807497

Tomorrow I think I'm going to start from square one, reverse the wire swap on the harness, and reinstall the non-rally cluster to confirm the fuel sender works (it did before).

Just as a precaution take an eraser and clean all of the copper "tabs" in the plug. They make special erasers for cleaning metal. Hobby shops sell them to clean the metal rails of model train tracks. Around 15 volts of current. The current runs thru the rails on the track to get to the electric motor inside of the engine.


In some cases you may need to very carefully "re-arch" the copper curve of each tab to make 100% sure that you have a good electrical contact from the plug-in to the printed circuit.

I swapped back the 5/7 wires and put the green wire back in the ignition switch and the Fuel gauge on the original non-rally gauges worked just fine (and shows almost full).

Put the rally cluster back in and the fuel gauge pegged past full again.

Are there any other wiring harness differences with an automatic car vs. a 4 speed?

Success! After a few tests, I started to suspect the gauge itself. Sent it back to Pete, he diagnosed a bad wire inside the gauge and fixed it up in short order. Got them installed and working :thumbup:

:clap2:Good deal!

Those guages look fantastic! I like the brightness, I can barely read mine at night

Good thread Ken. Thanks for all the detailed pictures and input. Good to hear

you have this problem solved.