Erratic Tach

**Veek** · 09-24-07, 07:51 PM

Has anyone had the problem of your tach reading going silly when you turn on additional accessories? Turn on the lights, A/C and wipers at the same time and it shows 10,000 RPM.

Jonathan S · 09-24-07, 08:19 PM

Try putting your tach on a separate ground.

Phil · 09-24-07, 10:45 PM

Veek
Try a good ground as Johnathans mentioned, and also look into a tach filter. This was used on some of the early GM cars with fuel injection to filter out stray pulses that were affecting the tach.
I would bet that your ground is bad or there is a poor connection causing the tach to find ground through one of the other accessories. Another thought is maybe the tach isn't getting enough voltage because of a poor connection, and this will cause crazy readings. I recently had an alternator go bad and just before the car stalled because of the lower voltage the tach and speedo went full scale.
Hope some of this helps
Cheers
Phil

David Briggs · 09-24-07, 11:19 PM

Does this only happen at idle or low RPM? probably alternator is not up to the load requirements. You can change the ratio of the alternator pulley but you have to careful to avoid overspeeding the alternator at high revs. You should have at least a 100 amp alternator with most of that available at idle(on a CAV) with the AC and all fans, fuel pump, lights etc running.
Dave

Tom_Hughes · 09-25-07, 12:42 AM

Veek, if it's still a problem, please call me. You may still have my number.

Thanks, Tom Dallas

Cliff Beer · 09-25-07, 12:53 AM

Definitely check the ground as suggested above. Make sure the ground strap for the engine is clean and sufficiently sized and also ensure the grounds for the guages aren't corroded. I'd guess you'll solve the problem starting with the grounds before going to anything more exotic.

**Veek** · 09-26-07, 06:14 PM

Thanks for the help. I have already backed up the ground by adding another wire and checked the connection. I also have added another ground strap to the engine. So far, not much luck.
Tom, I'll give you a call.

Thanks and any other suggestions are most welcome.

David Briggs · 09-26-07, 10:32 PM

The VDO tach in the CAV get erratic when voltage drops below about 11 volts.
Veek, have you added and relays or changed your ignition switch?
The early CAV's route all the power for the car thru the ignition switch and that switch is only rated at 15 amps, it gets very hot and creates a lot of resistance.
Dave

**Veek** · 09-27-07, 07:12 PM

Thanks again for the suggestions. I modified the ignition switch with the relay modification some time ago so I don't think it's the ignition switch. The suggestion of a filter makes sense though. Any suggestions as to where I can get what for the VDO and what type? Would I need one for the power, distributor input and the ground?

Again thanks to all.

David Briggs · 09-28-07, 12:00 PM

You could run a seperate wire for the tach input, not in the harness. It is possible that the wire in the harness is picking up pulses from the rest of the system.
When this is happening what voltage reading do you have on the in car volt meter?

johann hamm · 10-04-07, 04:50 PM

Have a problem with my Smiths Tach. When I push rpm to past 6000 the needle bounces between 4500 and 6000. Any ideas?

Canuk40 · 10-05-07, 04:24 AM

Hi Veek,

All good sugestions, IMHO it's the alternator charging capacity at idle. Check your volt gauge when the tachs' bouncing and you'll see 10>11 volts. Anything under 13.4 starts upsetting electronics including ignition boxes and EFI computers. What we've been doing is fitting the smallest v-belt pulley possible on the alternator. Most cars are running aluminum pulley sets, the crank pulley is smaller than OEM Ford and the Alternator pulley is bigger. This just slows down the alternator so much it cannot keep up at idle. Increase the idle RPM or the speed of the alternator via pulley dia. and you problem will go away. That should do the trick.

Cheers

PS Pic is of latest install 347 Ford Racing with Webers, short accessory drives, low and forward engine mounting. The distributor is under the bar below the bulkhead window! No problem seeing out the back either...

Auto Futura-CAV Factory · 10-05-07, 05:47 AM

Quote:

Originally Posted by jhamm

Have a problem with my Smiths Tach. When I push rpm to past 6000 the needle bounces between 4500 and 6000. Any ideas?

Hi Johann
Please phone me. I have answer.
Jean

**Veek** · 10-05-07, 07:14 AM

Thanks for the note Ian,
Do you have a suggestion on the parts number or where I could source a smaller pulley?

Canuk40 · 10-09-07, 11:28 AM

Hi Veek,

The pulley is a Ford OEM peice we had fitted as the new alternators come with sepentine pulleys stock. A good alternator rebuilder should have something in stock and pop it on fot you.

Cheers

**Veek** · 10-10-07, 07:27 AM

Thanks for the help Ian.

Tom_Hughes · 11-13-07, 10:04 PM

Veek-did any of these fix it for you? had a chance to try any yet?

**Veek** · 11-19-07, 07:53 PM

I've been doing some experimenting and I think Ian was on to something. I went for a long (70 mile) ride where the car was at 4,000RPM for over an hour. Then I went full on with all the electrics and the tach was quite steady. Surprisingly it remained so even after I slowed down to participate in the Roar N Soar show in Lakeland, FL. I guess the state of battery charge was so high that the volts didn't fluctuate. So I'm looking for a tach filter and a smaller alternator pulley. The one I have is a stock Ford item. My concern is not to overcook the alternator when I am doing spirited cruising.

On a unrelated note, at the show, the subject of the picture below did a low level full speed flyby and the sight and sound was magnicificent. I really need to get to Goodwood...

Canuk40 · 12-07-07, 07:41 AM

Hi Veek,

Thanks for posting the Mustang pic, Wow. One of my techs also works on warbirds for a private collector near here. He has a Messerschmidt Me109 with Daimler V12 (only one flying I beilieve), Spitfire and a Hurricane. Fantastic stuff.

About the alternator, getting it up to speed for good charging at idle won't fry it at the occasional times you run at high RPM. With the smaller crank pulley on aftermarket mills you're probably turning the alternator no faster than the stock engine with a larger crank pully at a lower redline.

Cheers