1969 Dash Grounding

[69rcode 10]

I'm doing a ground up build of a '69 Mustang. The entire car has been apart, repainted, and I'm in the process of re-assembly. I have validated all electrical function including interior lights, dash, etc. I am now installing aftermarket parts (Vintage Air) under the dash that require a good ground and I assumed the dash/frame would be grounded. However, after checking it with my gauge, I found that was not the case. I do have one item mounted the interior side of the firewall and it has a good ground to the firewall. Since I knew the firewall was grounded, I assumed the dash frame would also be grounded since it was bolted to the body.

Should the dash frame be grounded and if so, how is it usually grounded?

[det0326 179]

It's been a long time but I seem to remember a ground coming from the under dash harness  on the drivers side that connected to the dash panel.

 

[Midlife 814]

Dash frame should be electrically tied to the firewall via welds.  If no continuity, you'll need to re-weld the dash structure to the frame and/or eliminate paint at the joints.

[det0326 179]

after thinking about the ground wire  from under dash harness connecting to the dash panel maybe ground to instrument cluster

[RPM 1,190]

3 hours ago, Midlife said:

Dash frame should be electrically tied to the firewall via welds.  If no continuity, you'll need to re-weld the dash structure to the frame and/or eliminate paint at the joints.

Is not the lower dash bolted to the hinge pillars, pedal hanger and  V shaped rod which bolts to the firewall? Mine is attached with bolts.

[RPM 1,190]

My belief is if you paint the attachment areas that in my case were left bare metal from the assembly plant, continuity might be an issue.

[RogerC 136]

11 minutes ago, RPM said:

My belief is if you paint the attachment areas that in my case were left bare metal from the assembly plant, continuity might be an issue.

 

Yup, if everything is painted you're not getting good continuity between the lower dash and the body, need some bare metal at those contact points. There's at least 3 places that I can recall where the dash harness grounds to the lower dash

[RPM 1,190]

17 hours ago, 69rcode said:

Should the dash frame be grounded and if so, how is it usually grounded?

I'm with det0326 in thinking the under dash harness has a ground which attaches to the lower dash, which connected to the body. And the hip bone is connected...

[det0326 179]

1 hour ago, RPM said:

I'm with det0326 in thinking the under dash harness has a ground which attaches to the lower dash, which connected to the body. And the hip bone is connected...

I was thinking that too Bob to start with but instead I think that wire coming from the harness being a ground for the dash I think it is actually the other way, it picks up a ground from the dash and goes to the plug at the instrument cluster and grounds it.

[69rcode 10]

Thanks for the responses. I did some more poking. I can confirm the car was COMPLETELY disassembled (and cut apart in some cases) and welded/painted/re-assembled. The paint is real purdy...on all the bolt holes and I'm betting that is causing me continuity issues :-(. The block is not yet in the car, so I have the negative cable temporarily grounded to the rad support. I do have a good ground throughout the engine bay and at a bolt coming through the firewall (ground for my epas system). In all cases, those points show as about 3 ohms to the negative post. Under the dash, however, is a different story For example, I thought the steering column support shown in the picture above would have decent continuity given that it's bolted in. If I test the continuity on a bare spot on this bracket, it isn't open but is 85-95 ohms to the negative post. Maybe that's good enough? That measurement seems to be consistent across other portions of the dash, when I get my probe onto bare metal.  The interesting (or maybe not) part is that I function tested the instrument cluster, clock, all lights, turn signals etc and all tested fine before I tore it back apart to add the Vintage Air.

The instrument/clock panels and stereo is not in yet though I'm still not very motivated to pull the dash frame back out again to get metal-metal connections at the connection point, assuming that's the problem. I think my options are:

1. Leave it, assuming 85-95 ohms is just fine.
2. Pull the kick panels out and get the hinge pillar connection points and the dash support brackets into clean metal.
3. Run a good ground strap from the frame to the dash.

Right now I was leaning to #3. I'm not sure if #1 is sufficient since I don't want to have grounding problems down the road. I don't recall a dash ground in the underdash harness as discussed above but maybe I missed something putting in the new harness. Any thoughts on the options above?

[det0326 179]

1 is not sufficient .  Not sure what the assembled state of your interior is but if it's not in yet then you can do as I did. I ran a #8 wire from neg battery post connection at the block all way to the rear of car grounding things along the way.  I ran it in a way  that it was hide and could not seen. 

[aslanefe 333]

+1 for option 3.

[RPM 1,190]

Ya, I'd just do #3. I try to use the KISS process. Even with no interior pulling the lower dash is a pain. 

[Midlife 814]

You should be seeing 1 ohm or less between chassis ground and the battery ground.  Anything more than that, and you'll begin to have problems.  Ask me how I know...

OK...I'll tell you.  My wife is trying to put me 6 feet underground so I can test continuity to ground...I'm trying to convince her that a copper or steel red/pipe stuck in the ground is just as good but she's not having any of that...