Tachometer issue-very strange

[crause]

Have you done any work on the tach. in your C3? Mine has been doing some strange things and not sure where the problem is. I took the 75 out for a ride the other day, after sitting for several weeks, and noticed that the tachometer was not working. The needle was pointing between and 15 and 20 and did not fluctuate with engine speed. No other problem and figured I would check the connection at the distributor later. I needed to change the oil so I took it out for a run this morning and when I got home I pulled it up on the ramps and waited for it to cool down to change the oil. A little later I went out and changed the oil and filter and lubed the frontend with no complications. Before I started it up I checked the wire plugs going into the distributor and made sure the tach wire and the power wire were tight and not cut or scuffed. Everything looked ok so I went to start the engine. It would turn over fine but would not start. Never had any trouble starting it before and was very confused. I checked the power wire to the dist. and it was getting power when I turned the ignition switch on. I unplugged the tach lead going into the dist. Turned the key and it started right up. What the?:confused I turned it off and plugged the tach lead back in and tried to start it again; would not start. Unplugged it and it started right up. Any suggestions? :W

 

[moonshadow]

Same problem twice, mustang and roadrunner, tach wire was grounded via small cut in insulation on tach wire. hard to spot on rr but easy on my friends mustang

 

[Dragon]

Had the same problem on an 81. If you have a tach filter, check the connections or replace it.

 

[TimAT]

Follow the wire from the TACH terminal- it runs down to a filter- (looks like a condenser with a wire out each end). It's shorted internally and is grounding your ignition. Replace that- your tach will work and the car will run.

 

[crause]

getting close

I did some experimenting today and discovered that the capacitor is defiantly causing the engine not to start. I bypassed the capacitor and plugged the connector directly from the line coming out the dash into the distributor and the engine started. If the capacitor is plugged inline, it won't start. Now, with the wire coming out of the firewall (which was plugged into the capacitor) going straight into the distributor the tach is still not reading. It does move around a little but basically stays between 15 and 20 on the gauge. The question before I start changing parts is, does the wire coming from the dist. have to go through a capacitor to make the tach work? I have bypassed the shorted capacitor and the tach is still not working. This capacitor is not cheap and I hate to just throw money at the problem. What do ya'll think?

 

[Dragon]

Tach Issue

I still think your tach problem is a bad filter. Please do some price checking but I seem to remember that there is a non OEM style one for about $30 & a near OEM style for about $40 (maybe at Ecklers?)

Of course if you are a gambler, you can try a salvage yard. Just snip off theleads on either side & plice it in to yours. Good luck getting one that works!

If you have a BFF with a working tach, maybe you can convince him to snip his out, try it on yours & then resplice his back in. Good luck finding that BFF

Your call, but nothing on these cars is cheap.

 

[BigJimZ16]

I did some experimenting today and discovered that the capacitor is defiantly causing the engine not to start. I bypassed the capacitor and plugged the connector directly from the line coming out the dash into the distributor and the engine started. If the capacitor is plugged inline, it won't start. Now, with the wire coming out of the firewall (which was plugged into the capacitor) going straight into the distributor the tach is still not reading. It does move around a little but basically stays between 15 and 20 on the gauge. The question before I start changing parts is, does the wire coming from the dist. have to go through a capacitor to make the tach work? I have bypassed the shorted capacitor and the tach is still not working. This capacitor is not cheap and I hate to just throw money at the problem. What do ya'll think?

Electrically speaking, yes the filter is required for the tach to function properly. The capacitor charges to the RMS value of the pulses coming from the distributor creating a varying DC signal that the tach meter reacts to. Feeding the pulsating DC signal raw from the distributor will cause pretty much what you are seeing.