Engine Tach. gone crazy

[bayhoss]

The tach on my e28 seems to have lost it's mind. It will jump to 3200, then down to 900, then back up to higher readings. It will do this for several minutes, then after a while it seems to settle down and read correctly. All while sitting at an idle, or at constant higher rpms. The Admiral says its suffering from mood swings. I say one more swing and it takes a trip to the dumpster. But, before I do it is there a simple fix that I may be missing. I've checked all connections. Anyone out there had a similar problem?

Best always,
Frank

 

[mherrcat]

Does the engine RPM actually fluctuate as well? If the engine is an Atomic 4 with a distributor you might want to check that the distributor is not somehow coming loose from it's mounting to the engine block.

I had a 1967 MGB-GT whose RPM would start to fluctuate at speeds around 65 MPH. Turned out the distributor was mounted to the engine with a clamping ring and the small lip that the ring clamped onto had broken. The distributor was loose and would start to move around at that speed, causing the engine to surge and the tach to go nuts.

 

[Loren Beach]

Clicking on Frank's name and then on his Profile: "about me", the boat is an '86, and therefore should have a diesel engine.
If this were happening on my boat, I would first check Every electrical connection from the alternator to the panel and to the tach.
Next I'd call my mechanic....

LB

 

[exoduse35]

What engine do you have? Gas or diesel, as the tach's get their signal differently. Edd

 

[bayhoss]

Thanks to all that replied! I should have included that the engine is a Universal M 12 diesel. The engine rpm's remain constant - it will do it at an idle or while running along. I've checked, cleaned, and tightened all connections - maybe I should go back and check and re clean and re tighten. It happens at random, and the really strange thing that I forgot to mention is sometimes when I come to the marina to go sailing the tach will be reading 3200 or any other number - with the engine off and the key removed.

Best always,
Frank

 

[Loren Beach]

Thanks to all that replied! I should have included that the engine is a Universal M 12 diesel. The engine rpm's remain constant - it will do it at an idle or while running along. I've checked, cleaned, and tightened all connections - maybe I should go back and check and re clean and re tighten. It happens at random, and the really strange thing that I forgot to mention is sometimes when I come to the marina to go sailing the tach will be reading 3200 or any other number - with the engine off and the key removed.

Best always,
Frank

Probably little help, but perhaps worth mentioning is that my tach needle stays at the last rpm reading when the key switch is turned off. i.e. when it's no longer energized it just freezes at whatever rpm it's on.

I wonder if there is an intermittent open circuit in the sensing wire from the alternator?
Long shot, probably, but I once fixed a low-voltage problem with my Universal diesel electric lift pump by bypassing the engine harness with a new wire to the engine panel. By checking the voltage and wiggling parts of that wire bundle I isolated the fuel pump problem to the plug with all the old black tape wrapped around it!
LB

 

[bayhoss]

Thanks Loren, I think that I'll just have to wiggle one wire at a time until I find something that is behind this one.

Best always,
Frank

 

[exoduse35]

also when it is acting up tap the gauge, It sounds like the needle may be sticky. there is a very fine spring inside that pulls the needle toward zero and the electric input moves it upward. If the needle shaft has developer rust or the lube has hardened it will want to reed were ever pulling to overcome the sticky spot then sticking too high and pulling back to zero until the friction frees everything up. Tapping it should help free it back up and if the needle moves with the shock the problem is in the gauge. if so a good speedometer repair shop can fix it. Edd

 

[bayhoss]

Thank you Edd, I'm going to re check all connections first. We have a good instrumentation shop in town, it looks like that may well be the next step.

Best always,
Frank

 

[u079721]

Thanks Loren, I think that I'll just have to wiggle one wire at a time until I find something that is behind this one.

Best always,
Frank

I fought this problem once too, and the wiggling didn't seem to help. In desperation I just replaced every single spade and ring terminal in the circuit and that did the trick.

 

[Tom Metzger]

Try putting a load on the system, such as the blower, bilge pump, or other large load. When the battery is fully charged the alternator has enough capacitance that the tach doesn't always have enough signal. Adding a load helps.

The other suggestions are also good.

 

[Rocinante33]

Ground circuit?

In that situation I suspect the ground circuit. With a bad ground, the 'trons will seek another path to ground. That may pass through the circuit of another instrument or any of a myriad of pathways. Hence, it acts crazy, as we boaters are prone to from time to time. Double check those ground connections.

 

[bayhoss]

Hi Keith, I won't even attempt to deny crazy. The thing that stumps me is that the conditions are never the same. I've checked all connections and everything seems to be as it should be. I wondering if the short or malfunction lies inside the tach itself. I tried knocking on it and that ended the malfunction. The tach was reading 2600 while at an idle, I knocked on it with my knuckles and it retreated to 900. I'm thinking its the tach.

Best always,
Frank

 

[exoduse35]

IT IS THE TACH! That is just as a sticky one should act. The instrument shop can make quick work of it, they will most likely just clean and lube it up and have you set. I have been inside a couple just to see what was inside and know enough to send it to someone who does it. Luckily they are usually not too unreasonable.

 

[bayhoss]

Many thanks to all for the help! I'm going to wait until the boat is winterized (sadly soon) remove the tach, and to the shop it goes.

Best always,
Frank

 

[Spirit Moon]

Same problems

I was having and am still having the same problems though my tach doesn't go beyond the RPM I'm running at. It will show what I'm doing and then drops off. After several attempts to fix it a marine mechanic found the issue was as Tom Metzger describes in his post. If I put some load on it pops right back into business.

 

[Stu Jackson]

Many folks write in about their tach operation. In addition to the "check the wires it's usually a connection" answer, I always ask: "Are you plugged into shorepower or just recently disconnected?" So many people ask this question when their batteries are fully charged.