tach problems
 

I just installed stainless marine exhaust system to a set of 502MPI's with captains call. I sent my computers to Arizona speed to be re-mapped. I also added 1.8 roller rockers as well. Other then these 3 upgrades the motors are stock. Now my tachs work properly up to 3000 rpm and then they start bouncing around the dials. The engines are running fine and all other guages are working normal. My mechanic is showing it as some sort of RF(radio frequency) problem. We've checked all the grounds, tried different spark plug wires, and added noise diodes to the lead tach wires nothing worked. Anybody have any ideas ??? help

stock ignition or are they msd's.

and how bout the distributors

I had this problem on my stock 500 . Was told by Lavorsi to run a ground wire from the dist to the engine block. Was told the intake piant causes some problems with grounding. I did this an have had no problem for three years. Unitll this spring after I had some work done. But I see my guy didn't put the jumper wire back on. So I will do this.

Just FYI, you problem may be diff

they are stock ignitions/distributors. We did put new wires,plugs, cap and rotor (basic tune up)

they are stock ignition and diistributors. We did change the wires,plugs,cap and rotor (basic tune up).........

The only new electrical component is the Captains Call, and they use a large capacitor and a constant duty solinoid, so the potenial for EMI/RFI is there. Maybe try disconnecting them and retest.

does this occur when they are NOT engaged.

The boat already had thru hull exhaust with captains call from the factory and the tachs worked fine. All I did was replace the cast iron exhaust with the stainless marine exhaust .

not engaged, meaning the captains call ? If so yes

yes.

yes it still happens when there not engaged.

How about the grounding on the box with the ECU and other electrinics mounted to the riser?
Aluminum Manifolds vs. Cast Iron?
Maybe run a ground from that box to the block...
I had a similar problem and ran a ground wire for the tach to the block.

we tried grounding both the distributer and the computer to the block and that didn't work either. We ran a test light to every ground on the motors and they all checked out. We also tried adding msd tach filters and they were not compatible with my vdo guages......... We did find that my tachs have dip switches on the back does anyone know what there for ???

I think the switches are for 4, 6 or 8 cylinder engines. You change the setting of the switches based on the number of cylinders.
Dave

Having the same issues with Gaffrig gauges. Just changed over to efi. Jumps at higher rpms @4000.
Need to find some more grounds also I guess.

I'm having a similar problem. Medallion Tach, MSD 6M2 box and MSD distributor. Tach works fine to about 3400 RPM, then as you give it more throttle the needle starts to jump or fluctuate until finally at WOT it reads zero. So far I have run a separate ground from the tach directly to the battery, and a separate trigger wire from the box to the tach. Still works the same. Next I'm going to try a separate power wire from the battery to the tach. If that does not work maybe I'll try the ground from the distributor to the block.

Dave

each tach had an 8 position dip switch panel. I spoke to a formula tech and he suggested contacting VDO and giving them the dip switch configuratuion and see if they can make anything from it. Both MSD and Gaffrig make tach filters that might solve your problems. It's worth a shot. I may be heading for a whole new set of guages if we can't figure this out soon. Formula is now using Livorsi instead of VDO on all there boats now.

update?

I was having the same problem. I checked all my grounds and ran additional grounds just to be sure.
What fixed the problem was a resistor from Livorsi. If you call them and explain the problem they will know the part. On my tachs what you do is splice the resistor onto the signal wire to the tach, and it takes care of the problem.
They also had a newer style resistor that just bolted onto the back of the tach, but it did not fit onto my older tachs, so I used the splice. It worked great, and fixed the problem.
Good luck.