Would the real V8 Cobra II Tach stand up please? Adaptor or no adaptor?

[fastmerc]

Greetings!

I have a factory in dash tach that I pulled out of a 1978 Mustang Cobra II. I only have L-Merc parts book so cannot look up numbers for a V8 Tach in an "F" style body (Mustang) in 1978 since Merc Cougar went HUGE and Mustangs had already gone small....!

Tach is marked D7ZF-17360- B C With what appears to be a 7 L 20 date ( 1977 Nov 20)...1978 car line...but the B C marking is not clear as to if this is a -B or a -C or a -BC!

Parts book would hopefully clear this up.

So searched Google and got everything but what I needed...... There was a mention of a tach adapter needed for V8 cars to slow the tach down... However, the screw hole in the rear of my tach shows no evidence of having ever had a screw mounted in it......

My question is was the tach adapter REQUIRED by 1978 or did separate tachs have the calibration built in then? (V6 or V8 specific tachs)

Possibilities:
1) A tach adapter was used on the one I found, but not screwed down
2) tach adapter was there but I left it hanging in the car
3) no tach adapter was used by 1978????? ...in the V8 cars.

I know it was a V8 Car as I removed all of the smog and top end components, motor mounts etc and recently sold it all as a kit. I just recently came across the tach packed in another box for safety and promptly lost for at least a decade on my shelves! Ugh!

Anyone have access to a 1974-79 Ford Master Parts Catalog??

Thanks a heap!
Steve

 

[LILCBRA]

Hello and welcome to the site and group! I don't have a parts book but I know that the tachs require a tach adapter, so you probably should have grabbed it. And I'm about 90% sure that they're matched to the year of the tach, so early tachs and adapters only work with each other and later tachs and adapters only work together, you can't mix/match them. I'm laid up now, but there are 2 different part numbers for the adapters, one I'm almost positive has a 75 date code, the other is a later code although I don't remember when - I'm gonna throw a wild guess and say it's a 77 code? In any event, all factory tachs are only wired for 4 or 6 cylinder cars which is what necessitated the adapter. What I'm not sure of, and probably isn't a concern here, is if they were specific to 4 and 6 cylinder cars, but from what I recall the adapter basically cuts the signal in half for the tach to read properly, so the tach that it's attached to would definitely work for a 4 cylinder. I believe I posted about tach adapters here somewhere, I'll have to see if I can hunt that info down....

 

[fastmerc]

Missed posting these pic's in original post!

I took this out SOOOO long ago.....can't recall if there was anything in the dash like the adapter! Who knows.....may have been driving around at "half speed!"

 

[LILCBRA]

Here's one post that I shared a year or so ago. It shows the tach adapter's part numbers and date codes on the pieces I have. I'm pretty sure there's an earlier version though I don't know what it's part number is. Regardless though, if you know yours was out of a 78 AND the current date code matches, one of these are the adapters you'd need.

I guess I lied about the date codes. The tach in my cluster is indeed a D4, but it's the speedometers that are D5. The "new" tach is D7 as well as both adapters. So I think I can probably get the D7 tach working with one of the adapters as long as I can get the needle fixed and presentable.

 

[fastmerc]

Lilcbra, good stuff!

Now I'm wondering if that wiring harness in the dash would have plugged right into the tach (no adapter)???

Because of the "no threads" in the adapter mounting hole condition. I'm just thinking that had I seen the adapter plugged to the rear of the tach, I would have grabbed it "just because"..... OR it came loose when I pulled the cluster forward....... and I didn't think much of it....at the time! Where's the key to my way back time machine......!

Thanks for the pic's!
Steve

 

[extra_stout]

Tach can be modded:

Electrical - tachometer conversion from 6 to 8 cylinder (will also work for 4 to 6 or 4 to 8 but with different resistor size)

I bought an complete instrument cluster, because it had a KPH speedometer. So there was a spare tachometer to work with, without having fear to destroy it. I removed it from the instrument cluster. The complete tachometer consists of three main parts: - circuit board (which the 4 wire plug from...

stangnet.com

The electronics inside the tach produce a voltage depending on the frequency of the ignition coil (which depends on the cylinder count). My mod is to manipulate the voltage of this electronics to get the right RPM. the original tach adpater, should manipulate the frequency of the ignition coil in some way.... at least that is my theory.