Tach on 71 2.2 dies

[DByers]

While driving my 71T targa with 2.2 and stock ignition my RPM gauge (tach) suddenly stopped. I have pulled the unit and checked all connections both at the gauge cluster and in the engine compartment and everything is tight and clean. No blown fuses either. So I think I am left with either the RPM sensor or RPM transducer failing. Any one know what reading I should have from an OHM meter with these two units? Part# 54 and 72 in the following diagram.
Anything else I should check? I dont think the Tach is bad as it was at NHS about 2 years ago for R&R.

[Early_S_Man]

Dan,

You can test the tach out of the car on a bench using my tach calibrator below ...

Does the engine run OK? Since the RPM transducer, CDI, and tach share the same trigger signal from the points ... I doubt that the RPM transducer [ AKA Speed Switch and Speed Relay] could fail in such a way that the tach signal would be degraded and still have the CDI operating normally.

It is my belief that your car doesn't have a Ballast Unit or RPM Sensor #72 in the diagram. If it does, then the tach uses a different signal than the CDI and RPM Transducer.

My suspicion is that a connection is loose or corroded. Check the Black/Violet wire at the forward 14-pin connector for presence of a signal with a 12-Volt test lamp [or CMOS logic probe] when the engine is running, and then at the back of the tach.

Cleaning the Faston connectors is a fairly simple, but tedious procedure:

1. Clean the six female Faston connectors inside the 3-pin wiring harness connector for the CDI unit [and fourth wire at ground lug terminal 'D'] with Q-tips, pipe cleaners, and 90% isopropyl alcohol. Crimp both sides of each connector SLIGHTLY with needle-nose pliers -- don't close the gap on each side completely, just tighten it so the connector has to be pushed in firmly to mate with the CDI unit! See annotated diagram from AMP Faston document.

2. If corrosion is present -- whitish or greenish surface buildup -- after cleaning Faston with alcohol, then I recommend cleaning with vinegar [9% or 10% pickling vinegar recommended] and Q-tips, then rinse with distilled water after deposits have dissolved.

3. After all connections verified operational ... apply Dow Corning 4, 111, or 112 Silicone Grease to prevent future corrosion.

[Vereeken]

Well I am no expert on tachs nor on the 2.2 but my tach on my SC went belly up all of a sudden. I was already pulling the plastic to get the expensive parts, when I realized that there is a spade connector for the tach wiring in the box behind the frontseats under the carpet where the shift coupler is... if you also have that on a 2.2 then you might want to check. Often this gets stretched when they pull (lower) the engine and vibration does the rest from there.

This check is free of charge.

Just my 2 cents.

Good luck.

Michel

[DByers]

Thanks for the responses guys.
Warren the engine runs fantastic. No mis-fires, stumbling or erratic idle. The tach just went dead while driving. It was during some spirited driving where the rev limit was found several times. I will recheck the connections and also the 14 pin connector in front. The 3 pin at the CDI box I will pull and clean. I have something where #72 is shown so I will pull that and see what part# is on it.
Michel, good suggestion and I will pull the cover to see whats there. When I did my shift coulpler last year I didnt notice any wires in there though.

[304065]

We STILL can't figure out exactly what #72 is, or does. I just bought another one, had to get the whole electrical panel, just so I can cut it open and find out.

Warren is right on the money with his advice, as always. Oxidation is the likely culprit and wires rarely fail in the middle, usually at the ends.

Here's what we know about the #72 can. . .

electrical stumper question

Tach - oh - woe (pre '74)

[DByers]

John, Thanks for those links. Something tells me they will come in very handy. For now its back to grilled fish tacos and margs in the SoCal sun.