My 1986 F150 with 4.9L was originally equipped with the factory 6000 RPM tachometer. A typical stock 4.9L engine rarely sees above 3500 RPMs and the way I drive, rarely exceeds 2500 RPMs. So, the needle on the original tachometer barely seems to move.

A while back, fellow FTE member Ford F834, found a factory 4000 RPM tachometer from a F700 with a gas [370] engine. On the back, the tachometer is just like the factory 6000 RPM unit and has the 2nd ground for V8 application. Once installed in my truck, it worked beautifully and accurately!!! How nice to actually see the needle swing. At 70 mph [~1950 RPMs], the needle is almost straight up. In my opinion, Ford should have used this tachometer or the 4500 RPM unit [for gas engines as well] as standard equipment for 6 bangers.

Look on page #1 of this thread for a photo of the 4000 RPM tachometer as installed: https://www.ford-trucks.com/forums/1...a-chapter.html

Well, recently, the tachometer has become very optimistic in its readings. I always strive for accuracy, but cannot provide exact percentages of the error since at these relatively low RPM ranges, misreading by 50-100 RPMs [500 RPM gaps in the scale] can skew things quite a bit. But, I will try my best... normally, @ ~1000 RPMs in O.D., the speed is 35 mph and @ ~1950 RPMs, the speed is 70 mph. Another speed I drive frequently is 45 mph @ 1250 RPMs in O.D., when the tachometer is acting right, but the same speed correlates to ~1900-1950 RPMs, now.

It seems to be ~50-55% high. In my head, it figures to be acting like both grounds [6 cyl. + 8 cyl.] are being activated. So, it is like a six cylinder engine with a 14 cylinder setting on the tachometer. Crazy, huh!

I did not want to hijack another thread, so have instead, added a link to a very good thread detailing what most likely needs to be done to correct this. Look on page 2 of the link:

https://www.ford-trucks.com/forums/7...whompus-2.html

I will report back after following the steps as seen in the thread [above], but that will be several weeks. I have a rather long trip to make in a few weeks and make it a practice not to do anything which "might" lead to something else in the time just before needing to be able to rely on my trusty ride.

 

David, I hope the cleaning procedure solves the problem. I think it is likely the issue, especially since you already experienced some anomalous behavior when cold. Are you sure you want to make that trip turning all those extra revs? Your legendary mpg will tank!

 

I could disconnect the ground wire and show zero. Would that produce more MPG?