Bouncing Tach

[Ranger16]

The tach started bouncing around..........again. Here is the history to this puzzle that I hope someone can help with.

First time was last spring, shortly after I bought it and drove it home (from Detroit). That time it turned out to be the orange & blue ECU going out from being over charged by a bad factory voltage regulator. Replaced the ECU and the VR with a Wells VR706 Voltage runs a consistent 14-14.5v. Problem solved. Reference (long thread): If anyone is interested. http://www.69roadrunner.net/mopar/sh...Gone-Wild-HELP!

Months later (last summer) it started again. No misfire involved so I suspected the Tach. Pulled and cleaned the tach ground wire and it held an accurate and steady RPM. Problem solved.

A few weeks ago at the Farmington Mopars At The Park show in Minn. it started again, but this time it had an associated misfire. That turned out to be a badly oxidized 47 yr old connector from the battery to the starter relay. Cleaned it up and it ran fine all the way home (375 miles). Problem solved.

Last night it started bouncing around again. Cleaned and moved the tach ground to no avail. Thought maybe the tach was bad. Put a tach/dwell meter on the coil and it acted the same way, bouncing erratically.

HELP! I'm running out of ideas.

I have cleaned and checked all grounds. The car starts, idles and runs fine.........so far. It seems to be an indication problem. Now electronics is not my strong point so someone correct me if I'm wrong. My theory is that it is the ECU..........again. The tach gets it's signal from the coil, but if it where the coil, the problem would have persisted from last year and I should have a misfire, correct? The coil gets it's signal from the ECU. I don't know why it would fail after only a year, but that's my suspicion. New ECU is ordered so I'll know soon, but would appreciate any comments, thought or suggestions.

 

[NJRR]

You mentioned a badly oxidized connector from the battery to the starter relay. Did you change the whole battery cable or just that lead? What about the neg battery cable? I had the same bouncing tach problem on my 70 RR. Also, had a problem starting. Had to jump it. Would not jump with neg connected to the battery post, only to a ground (the block). Changed the neg battery cable, both problems solved.

 

[Darthomas]

Check bonding of engine to ground.
Should be connection from neg post to engine, but also a ground strap from engine to firewall on passenger side.
These often get thrown away as it seems useless to most people.
It is there that the tach finds its return for the pulses from the ignition, and also very important in eliminating ignition noise in radio reception. If it's missing, you are relying on "incidental" grounding through exhaust, drivetrain, etc.
Even if it's there, take it off, clean up the terminals and the mounting spots, and reinstall it to be sure poor grounding is not the issue.

 

[GetX'd]

Just went through a lot of this. FBBO was a great help. Turned out to be the battery cables which looked perfect. Also didn't have a ground strap so I installed one. So far problem solved.

 

[bee71]

Mine did that and it turned out to be a bad coil.

 

[Ranger16]

New I'd get more help on THIS site.
Let me answer one by one.
NJRR, I didn't change any battery cable. I cleaned both negative & positive cables & posts and ALL the connectors on the starter relay. It has no problems starting.

Darthomas & GetX'd, grounds from battery to engine and engine to firewall where cleaned when I had this problem last year. Hard to imagine them rusting or oxidizing so bad in one year that they loose connectivity. Likewise with the tach ground, ECU ground & coil connections.

Bee71, the coil is the one thing I have not yet tried, but as I mentioned, I never expected it could act this way. If the ECU doesn't do the trick, I'll keep it as a spare (I need one anyway) and go for the coil (it's about the only thing not yet tried), but the fact that you had the same problem and it WAS the coil is VERY interesting.

 

[bee71]

New I'd get more help on THIS site.
Let me answer one by one.
NJRR, I didn't change any battery cable. I cleaned both negative & positive cables & posts and ALL the connectors on the starter relay. It has no problems starting.

Darthomas & GetX'd, grounds from battery to engine and engine to firewall where cleaned when I had this problem last year. Hard to imagine them rusting or oxidizing so bad in one year that they loose connectivity. Likewise with the tach ground, ECU ground & coil connections.

Bee71, the coil is the one thing I have not yet tried, but as I mentioned, I never expected it could act this way. If the ECU doesn't do the trick, I'll keep it as a spare (I need one anyway) and go for the coil (it's about the only thing not yet tried), but the fact that you had the same problem and it WAS the coil is VERY interesting.

I had checked or replaced everything ignition or electrical wise and the coil was the last thing I tried. One way to check it for sure is to pull the coil wire and have someone crank the engine. That is when I saw an intermittent and weak spark jumping out from the coil.

 

[Ranger16]

I just found a very badly corroded alternator wire. While trying to pull it, I broke the capacitor stud. Going to replace the alternator and see if that helps. Best I can tell from the service manual is that it needs a 34.5 (+ or - 3 ) amp alternator. O'reilly has 50, 55 or 60 amp. What would happen if I use a 50 amp?

 

[Darthomas]

That's argument territory.
I suspect you'll have less output at idle.
There is no free lunch, and just like economy vs power, you can't have both.
I love 35 amp alts, because I don't have any non factory accessories to hog power, but do like to see the strong charging at idle if needed.

 

[Ranger16]

My biggest concern is doing damage or burning something up. Can I use the O'Reilly 50 amp without problems?

 

[Darthomas]

Yes, it's gonna be OK to use, you will burn nothing up as a result of a 50 amp alternator.

 

[Shorthorse]

Yes, it's gonna be OK to use, you will burn nothing up as a result of a 50 amp alternator.

2X - An amp is a measurement of consumption, not power output. This would be a great opportunity to address some common issues with Mopars. Inspect & clean the associated connections at the bulkhead terminal and at your amp gauge. Don't forget to use a small amount of dielectric grease on the cleaned terminals.
Bulkhead terminal #2 pins P & J (Dark Blue & Black in the engine compartment, Red and Black in the interior).

 

[Ranger16]

Well, I chickened out and went with the 35 amp. Figured I can't go wrong in staying with what the factory first designed. Napa will have it in the morning.

I have checkeded the bulkhead connectors. They where clean, but I did what I could anyway and did use dielectric grease on those and every other connector I cleaned, including the alternator, boy that one was really nasty. The more I think about it, that might be my problem. No additional accessories, just factory lights and at idle, with lights and brakes on, volts will drop to 10, amp gauge will no negative and the turn signal will slow drastically. Don't think it was designed that way. Probably unable to accept the voltage that the alternator was putting out. Also found the rubber over the alternator connector crimp was a bit distorted. Probably due to overheating due to excessive resistance. That too was cleaned, recrimped and soldered. Will find out tomorrow.

 

[rrTor-Red]

Just went through a lot of this. FBBO was a great help. Turned out to be the battery cables which looked perfect. Also didn't have a ground strap so I installed one. So far problem solved.

Where did you install the ground strap? I know it should be to the block, usually on the passenger side, but where to on the firewall? I've seen a few places people put these from the block: fan motor stud, VR bolt, etc. I could be wrong, if my memory is good but just seem that's where I remember. On my car, one end went to the block. The other was broken off but by the way it was laid out, looked like it went to the stud for the fan motor.

 

[Darthomas]

Fan motor stud is perfect, make the metal shiny before locking it down.

 

[rrTor-Red]

Fan motor stud is perfect, make the metal shiny before locking it down.

Fan motor is what I was thinking. No where in the FSM do I or have seen anything regarding a ground strap

 

[Ranger16]

Problem solved.
Once I saw that alternator connector I highly suspected it was the problem. Installed a new Napa 35 amp alternator this morning with the new shinny clean connector. Tach sits steady at idle and best of all, I now have 14v at idle, with all 4 headlamps, flashers and brake lights on. Confidence level is high once again and I'm a happy camper. Survivors are nice, but they come with hidden gremlins and need TLC.

P.S.
The ground wire is on the RR manifold bolt and is bolted to the firewall. It shares that spot with nothing else.

 

[Darthomas]

I have a major problem, my son is threatening suicide on FB and we can't find him, please pray he will not take action

 

[rrTor-Red]

Problem solved.
Once I saw that alternator connector I highly suspected it was the problem. Installed a new Napa 35 amp alternator this morning with the new shinney clean connctor. Tach sits steady at idle and best of all, I now have 14v at idle, with all 4 headlamps, flashers and brake lights on. Confidence level is high once again and I'm a happy camper. Survivors are nice, but they come with hidden gremlins and need TLC.

P.S.
The ground wire is on the RR manifold bolt and is bolted to the firewall. It shares that spot with nothing else.

Glad you got the gremlins figured out.

As far as the ground wire. Don't know because there's no empty hole for this.

 

[Darthomas]

I have a major problem, my son is threatening suicide on FB and we can't find him, please pray he will not take action

My son is found and is OK for the moment.