Erratic Tachometer
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Erratic Tachometer
My truck is a 94 F150 Ext. Cab 4x4 302, 4R70W auto trans. When the tachometer hits 4000 rpm it goes crazy as in jumping around and if I hold the throttle down the needle actually goes well past 6000 rpm which is where the numbers stop. Has anyone ever experienced this and as far as correcting it where should I start. The truck runs fine and other than a few little issues that I need to correct i love this truck.
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Joined: Aug 2004
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From: West Mansfield, Ohio
I would suspect a problem in the PSOM (Programmable speedometer/odometer module) which is mounted on the back side of your gauge cluster.
I hear about the same thing happening with the speedometer (like the one on my truck does), but never the odometer. I think the problem with the speedometer is the VSS (vehicle speed sensor) in the rear differential getting erratic signals.
Now I must ask, what in the world are you doing running your truck up to 4000 RPM? It sounds like you do it quite often, but is that necessary? I rarely floor it in my 95 with the 351, but it still won't get anywhere past 3500 or 3600 RPM before shifting. I'd say lay off the throttle and save your gas.
I also must ask do you have any of these problems at any other RPM? Or just once it gets up to 4 grand?
I hear about the same thing happening with the speedometer (like the one on my truck does), but never the odometer. I think the problem with the speedometer is the VSS (vehicle speed sensor) in the rear differential getting erratic signals.
Now I must ask, what in the world are you doing running your truck up to 4000 RPM? It sounds like you do it quite often, but is that necessary? I rarely floor it in my 95 with the 351, but it still won't get anywhere past 3500 or 3600 RPM before shifting. I'd say lay off the throttle and save your gas.
I also must ask do you have any of these problems at any other RPM? Or just once it gets up to 4 grand?
Post Fiend
Joined: Nov 2001
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From: Topsfield, MA
The PSOMs are known to go bad when the speedometer needle bounces - it can be from a erratic signal from the VSS - but the PSOMs are known trouble makers.
I've run my 351 to 5000 rpm a few times - my old 302 saw 5500 atleast once or twice a month.
While that isn't necessarily good - it's also not good to drive like grandma 99% of the time as the engine never works and builds up carbon.
I've run my 351 to 5000 rpm a few times - my old 302 saw 5500 atleast once or twice a month.
While that isn't necessarily good - it's also not good to drive like grandma 99% of the time as the engine never works and builds up carbon.
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The tach only starts bouncing at 4grand & above. I wouldn't say that I frequently go above 4 grand but on occasion I do like to blow the carbon out so to speak. I also pull my boat and sometimes to merge with traffic I do have to stand on it. I should have checked here first because I just had the dash surround off when I had to replace the clock spring and could have fixed it then. I'll probably live with it for a while until I get to spend more time troubleshooting the problem. Thanks for the input.
Last edited by brymac; Jan 9, 2006 at 08:20 PM.
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From: West Mansfield, Ohio
Lol sorry to jumpin to conclusions. I didnt mean to make it sound like it's horribly bad for your engine. I was more curious on how you got it up that high lol. I will occassionally keep it down in 1st and let it get up to about 4 grand....... but it usually starts to fall on its face around that rpm. Thats 220,000 miles talkin to ya...
But anyways, I'd say take a look at the PSOM.
But anyways, I'd say take a look at the PSOM.
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Post Fiend
Joined: Nov 2001
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From: Topsfield, MA
I don't know for sure - but I wouldn't suspect the PSOM. What's it have to do with the tach? Separate system...
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I just went through a similar problem with my 95 F 150 5.0.The psom has no connection to the tach.The tach has a 12v ignition wire,a tach signal wire,a ground for 6 cly or v8 and a wire for the tach ground.The tach signal wire comes from the neg. side of the coil(just spliced to it in the harness).I have a motors alldata at work so getting wire diagrams is not a problem.I would have a shop hook up a scanner and read your rpm and compare to your factory tach and if the scanner reads what it should. I would say it is "likely" your tach. New from Ford they are around $100 or you could have it repaired. Mine would be off 300 rpm @1000,500rpm @2000 and when I give it "H" I would send the needle past 6000 easily. Hope this helps! Jody
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Thanks for all the input but I think I solved the problem with a simple tune up. New plugs, wires, distributor cap and rotor button seemed to fix it. I couldn't get the greatest test drive but a couple times of WOT and no bouncing of the tach needle at or above 4K rpm.
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