THE PORTAL HUB - Pinzgauers, Haflingers, Volvo 303s and more!

Okay, weird thing happened today.

I took the pinz out to set up a water catchment tank about 100 miles from my house. Going everything was very normal, did my work, took a few hours, the old girl did very good.

I filled up with some gas (fresh) and headed home. Went about 50 miles and stopped to top off from my last jerrycan (again fresh gas). Killed the engine, filled up and when I went to start the truck, it took an abnormally long time to start and did not come to life but sort of limped along until I gave it a good bit of gas. A little more smoke than usual when it started. Figured, maybe was hot, fuel in the line vaporized, who knows.

Got on the highway and noticed my tach was bouncing around like crazy, 3k-4k, sometimes up to 5k... Performance was also lacking, seemed like it wanted to go 50 mph and had not get up and go, almost like it was governed. I normally try and keep it under 60 @ 3500-3800 rpm's or so.

I drove home, not stopping, then at my exit when I had to slow down, the problem seemed to fix itself, tach was acting normal and the power seemed restored. Although speed limit is 40 in my neighborhood, it felt normal.

I know my tach is powered by my 24-12v converter, the 4500 light never came on, and this is not the first time it has happened but by far the worse one. Last time was similar but only lasted a few seconds. I am having a hard time figuring out why a 12v tach would affect performance or why the lack of power would manifest in a bouncing tach.

Does anybody have any thoughts about this?

Thanks

Since both the tach and engine performance were acting up, I suspect the 4500RPM box is failing - it will both cut ignition power (often gradually) which will start to confuse the tach. The weak ignition will result in weak engine power. The RPM box is a cigarette pack sized box under the dash with a six terminal connector plugged into it (two rows of three 1/4" terminals). Unplug the connector and create a jumper with mating terminals on each end of a short piece of wire and use it to jumper from the center terminal of one row with the center terminal of the other row. Now the RPM box is out of the circuit and the truck will run without it. If things go back to normal (full engine power, tach working right) then the RPM box is to blame. Replace it, or just leave the jumper in place as I did several years ago on my truck.

You can also check by using a voltmeter to measure the voltage at the "+" terminal of the coil (with the engine running); a good number is in the 16-18 volt range. When my RPM box was acting up I only had 8-9 volts at the coil - the RPM box is intended to kill the voltage to the coil if the revs get too high, but after 40 years they start to die and often reduce the voltage significantly even though the revs are nowhere near 4500 rpm. With only half the intended coil voltage, the engine only seemed to make about half of the power it was capable of. And it was flakey, the problem would come and go, just like you are seeing.

Jimm391730,

Thank you for the information. It felt like a loss of power, I can see how lowering the power to the ignition would cause both issues. I was thinking maybe I had hit the 4500 limit, but I thought that would pretty much stop the truck.

I am going to dig out the old manuals and familiarize myself with this mysterious box and most likely cut it out of the loop. I know others have had to jumper it in the past, will read up on those post as well. I can see some risk in removing it, but you got to be pushing these trucks hard to get over 4500 rpm's anyways, so maybe time for it to go.

I can tell you, people on I-10 were not too happy with me cruising at 50 in a 75, hopefully I don't have to repeat that.

Thanks again, always a wealth of information.

Okay, read a bunch and figured out most everyone on here has has similar issues. Seemed easy enough, I just pulled the white plug out of the box, jumped the middle connectors, put a little dialectic grease in the others and covered the plug with some electrical tape. I assume you just tuck this back up into the dash?

Did not get to take it for a spin, just out in the yard to clean off some mud. When I started it, it jumped to life quicker than I think it ever has. I take that as a good sign...

Nice looking job. I just taped mine up under the dash against a sturdy section of the harness.

Has the tach smoothed out as well?

It seemed to, I'm going to take it for a spin later on to confirm.