Water temp and gas gauge not accurate while engine running.

[SlimPikins]

I got my my engine back in my car and running. Before the install I had a problem with the gas gauge being accurate with the key on, but as soon as I started the engine, the gauge would read a lot higher. Now both the gas gauge and the temp gauge do it. The temp gauge makes me nervous since while driving I wont know how hot it is.

A little background.
The original dash has aftermarket gauges in it. It had the original firewall forward harness in it, now it has a slightly used year one harness, and MSD Digital 6 Plus ignition. The year one harness is not for a 73, it's a big block harness not sure of the year. Everything seems to work fine except for those two problems.

Any insight would be greatly appreciated.

Thanks,
Slim.

 

[67Dart273]

This could be a few things. In fact, on my 67 it WAS a few things.

First, run the car at a good fast idle with the battery "up and normal," at an RPM to simulate low to medium cruise. Accurately measure the battery voltage right at the battery posts. You'd like to see 13.8--14.2, not below 13.5, not above 14.5.

Next, you need to determine if the ignition circuit is not suffering from voltage drop. The basic path here is..............

Battery -- fuse linke -- bulkhead connector -- ammeter circuit -- in harness splice -- ignition switch connector -- through the switch -- back OUT the switch connector on the "dark blue" "ignition run" (IGN1) circuit.

THAT branches off back through the bulkhead into the engine bay, and the other branch feeds power to the cluster and warning lights.

So if there is a bad connection in there, the switch connector, the switch itself, your ignition switched power may be cheesy to begin with.

"You are going to have to pull the cluster"

Unhook the battery ground, drop the column, and git 'er done.

Get the column "out" where you can gingerly get to it, maybe with a towel layed under it, etc

Now make sure everything is "safe" hook the battery back up, and turn the ignition switch to "RUN" Carefully measure the cluster switched power wherever it feeds the gauges.

While you are at this, you might as well check for voltage drop out through the bulkhead into the engine bay. Identify switched ignition power out in the bay. This should feed your red trigger wire of your MSD, and your regulator "I" terminal. To measure this drop more handily, stick one probe on the blue "igniton run" and the other probe on the battery POS post. You are hoping, here for a very low reading, NOT OVER .2 -- .3V (three tenths of one volt)

ALSO some gauge circuits are what is known as a 'bridge circuit' and those type guages have a ground post which MUST have a good ground, such as to the steering column support behind the cluster.

 

[SlimPikins]

While you are at this, you might as well check for voltage drop out through the bulkhead into the engine bay. Identify switched ignition power out in the bay. This should feed your red trigger wire of your MSD, and your regulator "I" terminal.

ALSO some gauge circuits are what is known as a 'bridge circuit' and those type guages have a ground post which MUST have a good ground, such as to the steering column support behind the cluster.

All of those seem to be great places to look, thanks.
The two I quoted stand out for these reasons:

The switched ignition power that I run to the msd is the brown wire that went to the ballast resistor. I've been told that just that wire works, and I've also read that you need to use both wires that run to that side of the ballast. Well with just the brown wire we only got power during cranking. We had to use both wires together to get power during cranking and the "ON" position.

Also, checking the grounds on the back of the cluster sounds plausible since I replaced the auto column with a manual one also during this adventure, and had the cluster out.

 

[67Dart273]

Yes, the reason you need to tie the "IGN1" (ignition run, dark blue), and the IGN2 (bypass, brown) is that most mopars the "run" circuit GOES DEAD in start.

What you are checking in with key in "run" / engine off is for bad connections through that circuit path, although the MSD "trigger wire" relieves a lot of the load. You still have the field and regulator, and on some cars, some smog junk, and such as sixpacks and a few others, distributor retard solenoid, electric choke if used, and carburetor idle solenoid.