Weird Tach Issue

[68goldrunner]

Im having a strange problem with my vintage late 60s early 70s 8000 RPM half sweep Stewart Warner Greenline tachometer in my 1970 F250. I apologize I know it's not Mopar related but I figure the cumulative knowledge in this forum may be able to help solve my issue.

The tach has its original sending unit mounted behind the tach head. The sending unit is simply wired with three wires. A signal from the negative post of the coil, a clean ground straight to the block, and a lamp wire straight to the instrument cluster lighting. I found an original SW wiring diagram for the tach online and wired as stated. My truck is running a stand alone MSD electronic ignition distributor with a MSD coil. I have researched my tach and from the information I found it was designed to work with electronic ignitions.

My problem is when I turn on the headlights the tach jumps exactly 2000 RPM. This problem is consistent every single time the switch is activated. The tach does not jump around and is not sporatic. The tach works perfectly while driving and goes up and down with engine RPM as expected however it's still 2000 RPM higher with the headlights. Turn the headlight switch off and the tach works and reads no problem.

I have checked all the grounds. I have run the tach with and without a resistor in line with the signal wire however the problem persists. I have searched online for some fix to my problem but have not found one. I've been thinking about possibly sending my tach in to be updated or rebuilt however I'm not totally convinced the problem is within the tach or sending unit because it looks brand new.

All suggestions or opinions appreciated. I'm at a dead end and hoping someone as had this problem before.

Thanks,
Brad

 

[Durandal25]

....I'm having a similar issue....cant wait to hear the feedback....

***Update***

I'm pretty sure this is a low voltage issue with the alternator I got from NAPA a while back, but from from several posts I've seen, certain idle conditions can make it worse....or the VR can need a looking into as well...as for me, I'm going to check the grounds again......

 

[68goldrunner]

Durandal,

This issue is incredibly frustrating. Hate to say it but Im glad somebody else has the same problem because I was starting to feel like it was just me! Id like to keep this tach because it matches the truck so well it looks stock. I can only work on trying to diagnose this issue for so long before I have to take a break. Somebody has to have an answer?

 

[Beekeeper]

the first thing would be to check that the ground is a good low risitance connection
and then you could check and note the voltage at the tach with the lights off. Then check the voltage at the tach with lights on. Also another thing to try would be to get the 12v supply from another source other than the dash lights and see if it still jumps 2000 rpm.

 

[Durandal25]

the first thing would be to check that the ground is a good low risitance connection
and then you could check and note the voltage at the tach with the lights off. Then check the voltage at the tach with lights on. Also another thing to try would be to get the 12v supply from another source other than the dash lights and see if it still jumps 2000 rpm.

.......I'm going to do this tonight. After that, I'm drinking two bud lites, then go back at it.....

- - - Updated - - -

Durandal,

This issue is incredibly frustrating. Hate to say it but Im glad somebody else has the same problem because I was starting to feel like it was just me! Id like to keep this tach because it matches the truck so well it looks stock. I can only work on trying to diagnose this issue for so long before I have to take a break. Somebody has to have an answer?

We'll figure it out sir. This forum is chock full of salty mopar pirates.

 

[68goldrunner]

Beekeeper,

I will double check the grounds and then test voltage. I disconnected the lamp wire from the dash lights and I still have same issue. Only thing I can think of is somehow the extra load on the alternator is affecting the signal from the coil? Trust me Durandal this problem as driven me to drink on a few occasions!

Brad

 

[mopar 3 B]

Just got to ask, is the MDS wired correctly? The old style tach reads an electronic paulse if you had access to an oscilloscope it would show the status of the paulse with the lights on and off. Back before ignition shielding it could be done with a TV set. Anyone out there old enough to remember the fuzzy TV every time someone drove by?

 

[Durandal25]

Just got to ask, is the MDS wired correctly? The old style tach reads an electronic paulse if you had access to an oscilloscope it would show the status of the paulse with the lights on and off. Back before ignition shielding it could be done with a TV set. Anyone out there old enough to remember the fuzzy TV every time someone drove by?

star

......only happened to me when I was watching Battle Star Galactica.

 

[68goldrunner]

Mopar 3 B,

I actually have an old Sun tune up station with an oscilloscope but it doesn't currently work. It's on my list of projects...somewhere after my Road Runner and F250. Even though the truck has been running for about 10 years with the MSD distributor I did double checked the wiring diagram. Come to find out the pro billet MSD distributor I installed should have a grey wire for the tach signal...but it doesn't! I installed the tach to the negative terminal of the coil but since the MSD has a different signal wave or frequency it can't read the old analog signal. My MSD distributor is over ten years old and only within the last year did I install the tach. I doubt MSD will take the distributor back even though it doesn't have the grey wire for tach installation. On top of all that one of the two machine screws that held the cap in place seized and snapped off in the aluminum distributor body. I drilled and tapped the threads but it's not the best. So now I'm in search of a stand alone electronic ignition distributor which will work with my old Stewart Warner tach. Of course I hoping this is the only problem but I won't know until I install the new distributor. I was searching for simple but as normal it was a little more complicated.

Brad

 

[Durandal25]

star

......only happened to me when I was watching Battle Star Galactica.

........figured my issue out. Tach wire got caught under a linkage and was grounding out.....lol. Hilarious......been bugging me for years....