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Old 05-28-2012, 09:14 PM   #1
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Speedo not working, bad PROM or bad VSS?

I have a 1991 TPI 5.0 Auto with autoprom.

I was driving and then my speedo stopped working. No check engine light and everything else was fine. Then I stopped at a gas station and the car had a high idle. Went back home and on my way the speedo was comming on and off. Back home I connected my computer to datalog but there is nothing moving, everything is frozen like engine speed 100rmp, afr 6.38, voltage 0,4...

I turned the engine off and waited few minutes. I started the car again and everything was back to normal for 1 minute and then all the data crashed and frozen again like engine speed 100rmp, afr 6.38, voltage 0,4...

I uploaded a stock BIN and it does the same again. Could the autoprom be crashing? Would the VSS have anything to do with this problem?

I am not sure where to start to fix the problem.
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Old 05-28-2012, 10:04 PM   #2
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Re: Speedo not working, bad PROM or bad VSS?

It could be a number of things, some of which I am not competent to comment on, but I had a similar experience as you and in my case, it turned out to be the speedometer gear within the automatic transmission itself. It's a nylon worm-type gear that eventually wears off the steel shaft it's attached to. In my case, when the gear would catch, the speedometer would work fine, and then it would crap out again. I couldn't find any gears for sale individually without buying an entire rebuild kit, so I went to the junkyard and pulled one. I think the gear ran $9 from Pick 'n' pull.

I'm not saying that's what your issue is, but that's my experience. You might also want to check the cable that runs between the transmission and the speedometer.
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Old 05-28-2012, 10:23 PM   #3
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Re: Speedo not working, bad PROM or bad VSS?

I studied my last log and observed that the PROM ID goes from 1497 to 404 when the data freeze. Not sure what it means. Tomorrow I will put my stock chip in and hopefully the problem will be gone. I hope I did not fry the ECM.

I also noticed that the driver side fan goes on even if the engine is cold.

Last edited by Alain2; 05-28-2012 at 11:00 PM.
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Old 05-28-2012, 10:40 PM   #4
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Re: Speedo not working, bad PROM or bad VSS?

If this is a 730 ecm it has a internal buffer. With the selection of vss or magnetic or optical. odd your prom id is changing. do you have a memcal to try to run too see if its the autoprom ?
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Old 05-28-2012, 11:59 PM   #5
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Re: Speedo not working, bad PROM or bad VSS?

Could it be going in limp mode when it changes PROM ID?
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Old 05-29-2012, 06:57 AM   #6
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Re: Speedo not working, bad PROM or bad VSS?

Ok I put the stock memcal in and it started and idled normal, fan were all off. Went for a drive and the speedo was normal, after couple minutes the speedo went down. Then it goes on and off, bouncing. No check engine light. I will pull the code anyway to see. I could not manage to data log with tuner pro.

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Old 05-29-2012, 08:25 AM   #7
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Re: Speedo not working, bad PROM or bad VSS?

When the speedo works the cruise works too but when the speedo fails the cruise also turns off.

I check the code and I only get "12" so I guess there is no code and the diagnostic circuit works.
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Old 05-29-2012, 10:46 AM   #8
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Re: Speedo not working, bad PROM or bad VSS?

If the auto prom .bin fails and the car went into lhm you should have got a code 51
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Old 05-29-2012, 11:36 AM   #9
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Re: Speedo not working, bad PROM or bad VSS?

If my VSS is not working properly, would I absolutly get a code 24 for it?
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Old 05-29-2012, 11:45 AM   #10
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Re: Speedo not working, bad PROM or bad VSS?

from my understanding the code 24 would set if there is a absence of the vss signal for a period of time.
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Old 05-29-2012, 11:48 AM   #11
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Re: Speedo not working, bad PROM or bad VSS?

Quote:
Originally Posted by Alain2 View Post
If my VSS is not working properly, would I absolutly get a code 24 for it?
Not if the ECM is out to lunch. From your descriptions it sounds like the ECM is bad. Losing cruise and the speedometer together is due to the ECM sending pulse streams to both of them.

The VSS signal goes to the ECM and is buffered. The ECM uses it internally to calculate the MPH for the calibration. The hardware buffer will then create 2K PPM and 4K PPM pulse streams for the speedometer and cruise control.

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Old 05-29-2012, 11:51 AM   #12
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Re: Speedo not working, bad PROM or bad VSS?

Rbob, will the vss signal still be buffered and be passive with a bad .bin loaded ?
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Old 05-29-2012, 12:35 PM   #13
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Re: Speedo not working, bad PROM or bad VSS?

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Rbob, will the vss signal still be buffered and be passive with a bad .bin loaded ?
Hard to say, the hardware divider ratio is programmed by the code. One value will turn off the divider with the result being no pulse stream. I'm not sure what the power up value is, or if there is a power up reset in the hardware for the divider ratio.

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Old 05-29-2012, 02:32 PM   #14
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Re: Speedo not working, bad PROM or bad VSS?

I cleaned few grounds, vss connection, memcal connections and now everything seems to be back to normal. Not sure what was going on, but I think it might have been a weak connection at the ecm/memcal. Now I am scared to put the Autoprom back in, not even sure I need for now because the stock tune has no problem running my little few mods.

Anyway thanks for the help.

Last edited by Alain2; 05-29-2012 at 02:45 PM.
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Old 05-30-2012, 02:46 AM   #15
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Re: Speedo not working, bad PROM or bad VSS?

Could just be the vss gears in your transmission.
Last year in a ~185mph wot run my speedo failed.... gears were gone by then, but sometimes I still got vss readings.
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Old 05-30-2012, 07:20 AM   #16
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Re: Speedo not working, bad PROM or bad VSS?

If the problem occurs again I will change the VSS for sure. But I still don't understand why the PROM ID would change when the VSS fails.
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Old 05-31-2012, 04:44 PM   #17
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Re: Speedo not working, bad PROM or bad VSS?

The problem is back and even worse. I buying a new ecm. My tuning experience has turned into a nightmare.
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Old 05-31-2012, 05:15 PM   #18
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Re: Speedo not working, bad PROM or bad VSS?

I should have read the whole thread , sorry.
VSS gears just came into my mind when I read that the speedo is sometimes working, sometimes not and acting weird.

What program are you using for datalogging/tuning?
I encountered some similar problems (frozen datastream) with tunerpro v4 ( i'm using the aldl xtreme usb by moates.net ). Most of the time a little bit throttle helped to get the stream running again.
Now I'm running Tunerpro v5, had some other glitches (spikes in the datalog, where no value made sense anymore, this could be solved by adding a pause into the datastream definition file ).
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Old 05-31-2012, 08:26 PM   #19
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Re: Speedo not working, bad PROM or bad VSS?

I am using TunerPro RT. I do not think the datalog is problematic because when the data freezes the fan starts. When the data comes back the fan shut off. The engine is running around 186F, the fan should not come on.
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Old 06-01-2012, 03:09 PM   #20
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Re: Speedo not working, bad PROM or bad VSS?

I just want to update for reference in case get the same problem.

The problem was the ECM, I swaped it and the problem was gone.

Nothing happens for no reason, while trying to find the problem I checked pretty much everything on the car and found the fuel pressure regulator was leaking inside the plenum, fuel being sucked by the vaccum. This had nothing to do with was I was looking for, but now the car runs way better than it used to.
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Old 06-01-2012, 03:09 PM
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