Tachometer Grommet Location on Firewall for '73 rr

[73RedRunner]

My '73 road runner didn't come with a factory tach in the rally gauge cluster. I am replacing my rally cluster with a rally gauge cluster that has a tach.

My question: Just where does the wire that goes from the tach to the coil pass through the firewall? Pics would be great, if ya got them.

 

[Nacho-RT74]

If your car didn't get AC, you could use one empty cavity should left on engine harness... I meant if you don't care about use the original location for it

 

[73RedRunner]

I want to use the location Ma MOPAR would have used if the car came with the tach as an original option.

 

[rt-man]

I'm very curious for the answer/pics. Doing the same thing to my '73 SSP this winter.

 

[manic_mopar]

I do not know if this is the original location for the grommet, but this has been in my car since we purchased it, and where i have my tach wire coming in. It's right next to the speedo cable hole. The grommet definitely feels like original 44 yo rubber, and the hole inside it is barely big enough for a tach lead so it seems appropriate.

 

[pjm8047]

From a 74 Plymouth chassis manual. Pin 4 would be for the tachometer.

 

[Nacho-RT74]

Thats for 74 JUST if AC since AC runs the harness by its own grommet on firewall. And just in 74 for same reason. 74s got more cavities used due the interlock system.

72/73 were different. Since no interlock system on them, they got one empty cavity on engine harness, used for compressor clutch feed, and the 4th pin was used for the blower ( with or without AC ). This made the tach run on its own wire through the firewall.

 

[73RedRunner]

Here is more background on this question. Below pic is what the 1973 factory manual shows, a grommet that passes through the "dash" which must be the firewall. I am not looking for the power to the tach, that is provided through a yellow aux connector.

Also below is a pic of the factory tach harness that I have. Unfortunately the wire going through the grommet to the engine was cut at some point, this is the gray wire.

So, where does that gray wire with the grommet pass through the firewall?

View attachment 496677 View attachment 496678

 

[73RedRunner]

Ready to route this tach wire........no one knows where it is supposed to pass through the firewall??

 

[Nacho-RT74]

floating this up...

71 diagram shows on bulkhead LOL

( there is a longer thread about this topic, talking about a 71 RR I think but couldn't find it )

 

[pjm8047]

on a 71 rr on the bulkhead pin 20 would be used for the tach. 72/73 through a grommet and 74 back to the bulkhead pin 4. In 72/73 they do not use pin 20 at all. weird

 

[Nacho-RT74]

on a 71 rr on the bulkhead pin 20 would be used for the tach. 72/73 through a grommet and 74 back to the bulkhead pin 4. In 72/73 they do not use pin 20 at all. weird

agreed.

on 72/73 was added the NSS signal to the cab for the seatbelts relay warning system on same harness, while just two cavities left for the Blower ( NSS bulkhead plug ) and A/C compressor ( when equipped, on engine harnes ). 71s allways got the 20th cavity empty when no tach, and NSS BH plug allways got 3 and 4 for Blower and compressor

on 74 not just the NSS signal but the interlocked starting system, just left one cavity at 4th cavity, what it was used for the tach. A/C harness did run through the firewall with a grommet.

 

[72RoadrunnerGTX]

Since its getting specific, the ’72 seatbelt warning system is not wired to the neutral safety switch circuit. It is grounded to the parking brake switch circuit, not requiring the use of a bulkhead connector cavity.

 

[Nacho-RT74]

mmmm dunno, but you are a 72 owner, so... LOL

 

[NoCar2023]

Hey RedRunner,
Did you find info for the tach grommet? seems weird no one has pics of a 72/73.
I think it is close to the center of firewall, doing mine this week.
Maybe just find a dimple close to center and drill it. maybe near bulkhead connector?

 

[73RedRunner]

Hello 1972GY8SE,

Nope. I never did get an answer that addresses the question. Does seem odd no one has a car with a factory tach that can share where this wire was routed for 72 or 73? I did notice there were a couple of pre-punched circular areas on the OE firewall insulation pad, above and just to the passenger side of the trans tunnel that "might" be where the wire was routed when a tach was installed as a factory option. As you suggested, it might be the location more toward the center of the firewall. This would make some sense since the destination of the wire is on the passenger side of the engine bay. But dunno, just guessing.

Not being sure, I didn't want to drill a hole in my firewall. So my solution was to route the gray tach wire through the hole that is punched beside the brake master cylinder/booster (driver side of it). I used a speedometer grommet to run the wire through. That hole is the same diameter as the hole for the speedometer cable, so a speedometer cable grommet fit perfectly. My rr is a non-a/c car, so that hole beside the brake booster on the firewall was not being used. On a car with a/c, that hole has a grommet that is used to route a vacuum line and also a control cable for the water cut-off valve. There is a hole beside the speedometer cable (to left/driver side of it) on my car and that hole is used to route the wire for the manual trans clutch switch. My car is a factory 3-speed (changing to a 4 speed) car so that hole is occupied, otherwise it would have been a candidate. On cars born with an auto, this may be where the tach wire was routed....again, I dunno, just guessing.

If ya need pics of anything, just let me know.

And a Merry Christmas to ya!

73RedRunner

 

[Nacho-RT74]

Shouldn't the 73 ( or 72 ) FSM say it that just like the 74 FSM does somewhere ?

http://www.mymopar.com/index.php?pid=109

I'm not downloading it because I have the 74 one ( original book )

 

[idrivemopar]

I have a suggestion, since I restored my car, I had the same dilemma when I did my wiring and I am using after market tach, but wanted a nice place for the wire go through the firewall, so, I chose to use one of the insulation "nails", the rubber deals that hold the insulation onto the inside of the firewall. Here is a picture from the engine compartment of the one I used. It was easy, just cut off the very end of it, white greased and ran the wire through. As you can see, the white wire, its the one near the accelerator cable and to the right of the bulkhead connector. This way it fits tight and no need to make another hole.

 

[Nacho-RT74]

well, on my 74 I run the ammeter parallel wires using same grommet factory used for the AC harness ( it's posted on one of the pics some replies ago ). I just enlarged a bit the grommet hole to get the wires throught allong with the AC ones.

This harness grommet it is however in a diff place of the firewall the diagram shows which is actually between bulkhead and brake booster as far I recall!

 

[73RedRunner]

Nacho, I have a FSM for '73 and it doesn't show the location on the firewall. It has the diagram I posted earlier, which is all I have been able to find. See post #8.

That's a good suggestion, idrivemopar. If I hadn't already come up with a work-around, I would be using your method of putting the wire through one of the firewall insulation fasteners.

Merry Christmas!

73RedRunner