Speedo jumps around - PowerStrokeNation : Ford Powerstroke Diesel Forum
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post #1 of 12 (permalink) Old 08-13-2011, 03:47 PM Thread Starter
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Speedo jumps around

Any time I get over about 65mph the speedo will start to jump around. It' doesn't bounce but will drop 5-10 mph hold there for a split second then jump back up. It will just do this back and forth randomly. I've replaced the VSS and with AE hooked up it shows a steady speed which tells me that everything up to the PCM is good.

I've recently got a Lightning speedo thinking it was the speedo that was bad but the new speedo still does it. I had both speedos checked out at Tacoma Speedometer and when they test them they are rock steady. So this tells me it's still something in the truck.

The guy at Tacoma Speedometer said it could be the RABS module but I think that is between the VSS and PCM and if AE is showing steady I wouldn't think that would be it. The only thing I can think of is if I've got a chafed wire or a bad pin in a connector. Anyone else had a similar issue that they found the fix for?

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post #2 of 12 (permalink) Old 08-13-2011, 08:11 PM
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Re: Speedo jumps around

Three things cause a jumpy speedo. The VSS, the tone ring in the rear diff, and the PSOM in the instrument cluster. Since you have swapped speedos around you eliminated that.

97 F-350 XLT CC DRW 4X4 PSD, 285/75-16 Custom bumpers, Gauges-boost/egt/trans, Swamps IDM, 3.5"dp- 4" exh., 3" Body lift, ES bushings/mounts, Built trans, billet conv, B&M pan, Hayden cooler, Adrenaline, stage 2's, IC, H2E, E-fuel
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post #3 of 12 (permalink) Old 08-13-2011, 11:01 PM
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Re: Speedo jumps around

I have done checking on mine and I think I have it pin pointed to the tone ring, need to flush the diff out an get a new ring. For some reason when my tranny temp gets to 160-180 it delay shifts and shifts hard, could this be caused by a bad tone ring or am I looking at a new tranny soon?




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post #4 of 12 (permalink) Old 08-13-2011, 11:37 PM Thread Starter
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Re: Speedo jumps around

I would think being AE shows steady that the VSS and tone ring would be fine as well. It seems to be something between the PCM and Speedo. Also if it was a VSS or tone ring I wouldn't think it would drop stay there then jump back up but be more of a sharp bounce if you get what I'm saying. I'm at a bit of a loss on this one.

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post #5 of 12 (permalink) Old 08-15-2011, 04:59 PM
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Re: Speedo jumps around

I've also got the same problem - been driving it like that for about 2 years. After about 62mph the speedo will jump as you described. What I found out is that there is some type of "computer" behind the speedo in the instrument cluster. I don't remember the name but it could be the PSOM that Speeddemon pointed out. When I priced the part it was about $400 so I have let it be. It hasn't messed with my shift points or anything that's why I left mine the way it is.

'96 F250 w/ E40D
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post #6 of 12 (permalink) Old 08-16-2011, 08:49 AM
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Re: Speedo jumps around

^That is exactly what I was referring to. The PSOM is the Programmable Speedometer Module. That little bugger can cause all sorts of issues from speedo jumping around to putting the trans into limp mode(full line pressure at all times).

97 F-350 XLT CC DRW 4X4 PSD, 285/75-16 Custom bumpers, Gauges-boost/egt/trans, Swamps IDM, 3.5"dp- 4" exh., 3" Body lift, ES bushings/mounts, Built trans, billet conv, B&M pan, Hayden cooler, Adrenaline, stage 2's, IC, H2E, E-fuel
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post #7 of 12 (permalink) Old 08-18-2011, 06:19 PM
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Re: Speedo jumps around

Well, if that be the case - yesterday I called Ford just out of curiosity. Ford no longer has any PSOM's in stock - although the person I talked to said you had to buy the speedometer head - is that correct and does anyone know where you could get a speedometer head other than getting lucky in a junkyard? thanks.
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post #8 of 12 (permalink) Old 08-18-2011, 06:51 PM
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Re: Speedo jumps around

i had the same problem in my 95 f150, right befor the rear end blew up hahaha. i could have been from to many shavings in the rear end from my carrier about to grenade that it threw it off. i rebuilt the rear end and it went away.
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post #9 of 12 (permalink) Old 08-18-2011, 07:12 PM
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Re: Speedo jumps around

Pull the VSS & look for rust build up on the mounting surface ...

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post #10 of 12 (permalink) Old 08-22-2011, 04:14 PM
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Re: Speedo jumps around

Mine dose the same thing .all teeth on the tone ring look good. new vss and connector swapped speedo. still the same. is there a gound someplace that could cause it. ?

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