[ponchjoe]

Hi guys,

I am preparing to install rally gauges and tach in my 70 LeMans. I know I have to change 2 wires in the connector, and do something at the ignition switch with green wires, but what? Also, do I need to purchase an adapter for the temp sender? I know I need to buy or make a coil connection for the tach too.

Can you guys share your experiences and any instructions you may have please?

Thanks,
Joe

[Peter Serio]

This answer will come in 2 posts.

The first post is written for '68 rally gauges but should be enough for you to figure out your 1970.


A non-gauge dash harness (1968-71) can be field converted from a standard gauge car to rally gauges as long as the wiring including the fuse block is still good.

IMPORTANT: you need to disable the twin green wire which runs out of the plug-in and over to the back of the ignition switch and then on out thru the firewall into the engine compartment to the coolant temperature sender.

This is something most people forget to do. A true rally gauge car dash harness has just a single green wire which connects the back of the gauge plug-in out to the sender via the firewall plug.

A car w/o gauges branches the dark green wire over to the back of the ignition switch & then on to the firewall plug. This is because when you crank the starter the temperature overheat lamp comes on bright red just to let you know that it is indeed working.

I have seen the dark green wire branched at the back of the key switch (1968) or the switch on top of the steering column (69-71) on all non-gauge cars. In some cases this wire could be forked at the dash gauge plug-in tab instead of the ignition switch plug.

All you need to do is remove the brass terminal from the black plastic plug-in behind the ignition switch and tape that off. DO NOT CUT THE GREEN WIRES!!!

Pete

[Peter Serio]

The big black plastic plug-in on the back of the main cluster is arranged so that each cavity is numbered one thru 12.


1 thru 6 across the top and 7 thru 12 across the bottom. NOTE: The plug-in is also designed that it will only fit into the gauge housing one way. (The 2 locking tabs on the short sides are not the exact the same in width.)


Here is a "map" of how the plug needs to be arranged for a car with factory rally gauges (1968-71 only).


#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 (possible 2 wires joined as one (Main 12 volt battery + power input to gauge cluster from Ignition switch-key on only).



#12 Tan wire with a black stripe (when grounded this wire turns on the bright-red "BRAKE" warning lamp in the speedometer circle).





Silver metal gauge housing, = car body ground.



The plug has no ground wire in it so MAKE SURE the metal housing of the gauge cluster is grounded to the body of the car!! This is a source of many problems during an install (a missing ground).



Pete

[ponchjoe]

Pete, I really appreciate your help and expertise. Couple of questions, do you see an advantage in making a ground from the housing to the chassis over and above the straps? Do I need a harness extension that plugs into the engine harness for the temp sender?
And if memory serves from reading threads, you swap #5 and #7?

Thanks!!

[Peter Serio]

I would add a supplemental ground via a black wire. On 12 volts DC It never hurts to have an extra ground. Rolls Royce cars have redundant grounds on everything and those cars are designed to last for 75 years or more if properly maintained and stored in a garage when not driven.

I don't know for sure about a 1970 engine harness, that big 3 tab switch is strictly for emissions control, but the green wire runs to it and then beyond to get to the single post top sender which gives the dash gauge it's temperature reading.

I would say call somebody or a company who specializes in new GM engine wiring.

[69gtocv]

Originally Posted by Peter Serio

This answer will come in 2 posts.

The first post is written for '68 rally gauges but should be enough for you to figure out your 1970.


A non-gauge dash harness (1968-71) can be field converted from a standard gauge car to rally gauges as long as the wiring including the fuse block is still good.

IMPORTANT: you need to disable the twin green wire which runs out of the plug-in and over to the back of the ignition switch and then on out thru the firewall into the engine compartment to the coolant temperature sender.

This is something most people forget to do. A true rally gauge car dash harness has just a single green wire which connects the back of the gauge plug-in out to the sender via the firewall plug.

A car w/o gauges branches the dark green wire over to the back of the ignition switch & then on to the firewall plug. This is because when you crank the starter the temperature overheat lamp comes on bright red just to let you know that it is indeed working.

I have seen the dark green wire branched at the back of the key switch (1968) or the switch on top of the steering column (69-71) on all non-gauge cars. In some cases this wire could be forked at the dash gauge plug-in tab instead of the ignition switch plug.

All you need to do is remove the brass terminal from the black plastic plug-in behind the ignition switch and tape that off. DO NOT CUT THE GREEN WIRES!!!

Pete

What happens when you don't disable that green wire???

[Peter Serio]

The green wire on a red warning lamp car is grounded by the ignition switch during cranking. This gives you a quick bulb test to let you know that the overheat lamp is working. On a 1968-72 Pontiac A body gauge car the range span that the coolant temperature sender will give you is between 600-800 ohms (cold) and 51 ohms (max-overheat). When you ground the green wire on a car with gauges and the key is on this creates a condition at the gauge which is something the gauge could never seen during any normal operation; including a huge loss of coolant and resulting overheat enough to blow a head gasket (250+ degrees).

There are 2 metal tabs that stick up on the front of each gauge dial, they are there as mechanical "limit" stops.

The tiny tabs located underneath & just outboard of the curved view-window for each gauge. Just in case of an unforeseen electrical failure or some other extreme condition. They are there to prevent the pointer from going outside the field of view as you drive the car.

If you allow the green wire to go to ground (zero or 1 ohm) you are jamming the pointer up against it's high limit stop, and HARD! The pointer is made from a very thin piece of aluminum, painted a day-glow orange/red color. Grounding your green wire on a gauge conversion will damage the new gauge in a very short time. Repeated jamming leads directly to gauge failure. The pointer hub will at first loosen it's grip on it's shaft (skewing the gauge out of calibration) if you continue with a green wire to ground every time to crank your starter the pointer will eventually fall off the shaft.

The other failure mode is when the shaft loosens in the magnet. Same deal here; the gauge now reads incorrectly. Once the shaft gets loose in the magnet (that is a press fit) when the gauge is made. Your gauge is now ruined.

I rebuild these old GM gauges almost every day around here, I also match gauges to senders to make sure that they read true.

In the photos below I hope that you can see the low and high limit stops. I have outlined the tabs in a blue circle in one of the photos.

[69gtocv]

Originally Posted by Peter Serio

The green wire on a red warning lamp car is grounded by the ignition switch during cranking. This gives you a quick bulb test to let you know that the overheat lamp is working. On a 1968-72 Pontiac A body gauge car the range span that the coolant temperature sender will give you is between 600-800 ohms (cold) and 51 ohms (max-overheat). When you ground the green wire on a car with gauges and the key is on this creates a condition at the gauge which is something the gauge could never seen during any normal operation; including a huge loss of coolant and resulting overheat enough to blow a head gasket (250+ degrees).

There are 2 metal tabs that stick up on the front of each gauge dial, they are there as mechanical "limit" stops.

The tiny tabs located underneath & just outboard of the curved view-window for each gauge. Just in case of an unforeseen electrical failure or some other extreme condition. They are there to prevent the pointer from going outside the field of view as you drive the car.

If you allow the green wire to go to ground (zero or 1 ohm) you are jamming the pointer up against it's high limit stop, and HARD! The pointer is made from a very thin piece of aluminum, painted a day-glow orange/red color. Grounding your green wire on a gauge conversion will damage the new gauge in a very short time. Repeated jamming leads directly to gauge failure. The pointer hub will at first loosen it's grip on it's shaft (skewing the gauge out of calibration) if you continue with a green wire to ground every time to crank your starter the pointer will eventually fall off the shaft.

The other failure mode is when the shaft loosens in the magnet. Same deal here; the gauge now reads incorrectly. Once the shaft gets loose in the magnet (that is a press fit) when the gauge is made. Your gauge is now ruined.

I rebuild these old GM gauges almost every day around here, I also match gauges to senders to make sure that they read true.

In the photos below I hope that you can see the low and high limit stops. I have outlined the tabs in a blue circle in one of the photos.

Thank you very much for that explanation. I'm not sure if I disabled that wire when I converted mine. I went off of someone's how-to on this forum as to how to convert the wires, but that green wire doesn't ring a bell.

[nytrainer]

Great info Pete. Thanks for sharing. Would you shed more light on the swapped wires (that was mentioned when converting standard to rally gauges) and also if the oil and temp sending units need to be changed.

[Peter Serio]

To go from non-rally to rally: yes you take out 2 wires and exchange places with them. It has been posted here before; I think you are trading places with a tan and a brown wire in the big plug-in.

However. I would go by the diagram I posted just to make sure each wire is where it belongs.

Yes, when you upgrade from non-gauges to gauges the senders for both oil pressure and coolant temperature need to be changed. The senders for non-gauge cars are are basically an on-off switch where as the senders for gauge cars: the oil pressure sender is a variable resistor. The coolant temperature sender is a thermistor.

[Eric71T37]

Peter,

Thank you for this information. My ‘71 was originally a non-gauge car. It has twin green wires at the 12-terminal connector at the back of the (now rally) gauges. I need to disable the single green wire at my column-mounted ignition switch, correct?

[Peter Serio]

Hi Eric71T37, I can't see your car from here but Would say that you need to remove the single green wire out of the ignition switch steering column plug. The one that runs from the plug on top of your steering column over to the back of your twin-green at your gauge cluster plug-in.

1969-71 GM A and G body cars (it depends on the year) can be "twined" at the ignition switch wire plug-in while others are twined (2 green wires) at the copper tab which goes to the back of your gauge cluster.

The photo below is part of a larger document (it's a PDF file so I can't post the whole thing here). If anybody is converting a '68-71 Pontiac A or G body non-gauge car to rally gauges and would like to see the whole document all you need to do is PM me with your EM address.

[Eric71T37]

Thank you, I will do that as soon as possible.

[Mister Pontiac]

Originally Posted by Peter Serio

The photo below is part of a larger document (it's a PDF file so I can't post the whole thing here). If anybody is converting a '68-71 Pontiac A or G body non-gauge car to rally gauges and would like to see the whole document all you need to do is PM me with your EM address.

pm sent, Peter. Thanks!

[Mister Pontiac]

Originally Posted by Peter Serio

To go from non-rally to rally: yes you take out 2 wires and exchange places with them. It has been posted here before; I think you are trading places with a tan and a brown wire in the big plug-in.

Thanks for all the intel here, Pete. Great point to bring up, and you are correct about 72. It is different than 68-71, but not by much. The process is the same, but the wire locations in the connector appear to be different for 72.

Nevertheless, I took a look @ my 72 today, including the wiring diagram from the factory manual, and confirmed the swap. Just like 68-71, in 72 you have to trade places with the Brown (Gen Light) and Tan (fuel gauge) wires in the connector. It does appear they are in a different location than the 68-71 plug, but the physical wires are the same and the process is the same. (I did not have my 71 harness available to confirm if the location of the wires in the plug were different, but fortunately, swapping the colors is exactly the same, nonetheless.)

Also, the green wire to the steering column plug is the same for 72, and easily removed @ the top of the steering column. Same as 68-71.

I sure don't mean to hijack the thread, but I hope this info helps others with a 72 doing the same Rally Gauge swap (like me).

[Eric71T37]

Yesterday I removed the green wire terminal from the black terminal housing at my ignition switch and taped it off. Now the temp gauge needle doesn’t slam against the lower (hot) limit on cranking, which is great. It actually hits the upper cold limit when cold (50 degree ambient temp yesterday here). However, I am thinking years of pegging the hot limit as it tried to test the idiot light damaged the gauge.

Where do these gauges typically read when a car is fully warmed up? I would think about halfway like modern cars, but maybe not? There are only two numbers, 100 and 250, so I assume the line halfway between is 175? Mine doesn’t get anywhere near that even fully warmed up. It does peg to 250 if I ground the wire at the temp sending unit in the (Edelbrock aluminum) intake manifold. Do people experience inaccurate temp gauge readings with aluminum intakes? Could be a stuck open thermostat...I need to get a temp reading with my laser gun.

When the key is off, my temp gauge sits in about the middle. The oil pressure and fuel gauges work normally.

[Two_70_Converts]

I had different readings with several senders none of which were satisfactory. I finally did a temperature reading with an infrared thermometer and then used a variable resistor between the sender and the gauge wire to dial in the needle to where I wanted to see it at normal operating temperature. For me I set it at halfway. I then measured the resistance and found a corresponding resistor to place in line. This will only work if your gauge is reading too high.

[Mister Pontiac]

Eric, mine sits about the same spot, so I don’t think your needle is out of whack. Mine hovers at the first tick mark on a cold ambient day (when fully warmed up), so that is roughly 175-180 degrees to me. On warmer ambient days, the gauge moves into the middle of the range, which I assume to be 200 degrees or so, and it will go 3/4 scale when the outside temp is in the 110 degree range (probably 220 coolant temp).

I still need to put an infrared gun on it to confirm all of those readings, but it seems accurate. That’s where the car typically ran back when I had aftermarket gauges on it, and nothing has changed.

I’d say keep driving. You’re fine. Hope that helps.

[Eric71T37]

Thanks! I actually drove it yesterday (first time in months we haven’t had a ton of salt on the roads) and it stays right at the second line (there are five total) when warmed up. I haven’t had a chance to shoot my infrared gun at it yet. Ambient temps were upper 40s. Will update after I use the infrared and also when ambient temps are warmer.

[Peter Serio]

I believe that GM purposely left some owner interpretation to the temperature gauge.

The coolant temperature gauge is non-liner where as the Fuel gauge and the oil pressure gauge are linear.

On cars that I know are all original and in verifying this against the GM service manual (I have a ser-con electrical troubleshooting manual printed for Pontiac dealerships in 1968) this is what I see on 68-72 Pontiac A body coolant temperature gauges.

The top line is cold, from 69 to 72 it is labeled 100 degrees but this needs to be taken with a grain of salt. All the top line really means is the coolant is stone cold and no heat can be generated from your heater core in case you are trying to de-ice a frozen windshield in the wintertime. (There was a time when these cars were all used as daily transportation.)

The 1/4 scale line is 180 degrees, this is where your thermostat opens. The 1/2 way line is 210 degrees. In actual use these gauges are reasonably accurate (and were designed to be reliable and last the normal life-span of the car, the gauges will not last forever). They all loose their buffering fluid over time.

That said rally gauges are certainly much better than a red warning lamp. In my opinion, at best, these factory gauges are within approximately 3 to 5% of true when working correctly and matched with the right sender. Most modern replacement and aftermarket senders sold for cars 50 years old can't possibly be trusted to be fully tested on a real car by their manufacturer. Often replacing the sender with a "new" sender can cause one of these older GM car gauges to read, in error, 25 to 30 degrees higher on the dial than actual.

To compare to a non-gauge car the threshold temperature for the red warning lamp cars to turn on is at least 240 degrees and taking into consideration production tolerances, probably somewhere around 240 to 245 degrees coolant temperature before your overheat dash lamp comes on bright red.
By the time that light turns on serious damage could befall your engine block and/or head gasket(s).

On a gauge car the 3/4 line is 230 degrees and the maximum overheat line is labeled at 250 degrees.

In normal operation with your cooling system in good condition you want to start up top at or just above the 100 degree line. You want to monitor the car (under all outside air temperature and weather conditions) to warm up and at least reach the 1/4 scale line at which point your thermostat will open.

After your thermostat opens your "normal" coolant temperatures are anywhere in-between the 1/4 scale line and the top edge of the 1/2 way line. Ideally right in-between those two lines.

What you need to watch for is if you ever cross over the 1/2 way line you need to keep a close eye on your gauge. The gauge is non-linear so that once the pointer goes over the 1/2 way mark any further increase in coolant temperature of just 5 or 10 more degrees, that(or a sudden coolant loss, such as a blown radiator hose) the gauge is designed so that the pointer accelerates rapidly past the 1/2 line and beyond alerting you to this condition.

The 1/2 line is the intersection between the magnetic fields from the slow-moving top 1/2 and the rapid moving bottom half of the dial. From 100 degrees to 209 degrees the pointer covers the dial very slowly, from the 1/2 line and beyond (your danger warning) the pointer really takes off!!!




A common problem is loss of silicone buffering fluid (inside of the gauge) and/or mating an older GM car (with rally gauges) to a modern aftermarket or replacement sender which is made incorrectly, but sold by the car parts store as fitting said car. Running an aftermarket sender (made wrong) puts the gauge at or past 1/2 scale under normal coolant temperatures which is bad. You want the pointer somewhere up above the 1/2 scale line under all normal coolant temperatures (engine fully warmed up).