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Speedo flutters, no Temp reading, limp mode...


windcalmer
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I couldn't find my original topic, so here I am again.

 

I have a 94 se and have several issues. First issue is when I crank the truck the speedo goes haywire. It's not everytime, but I have noticed a pattern. When it is cold outside it does it wverytime, then if the temp outside is warm it doesn't happen.

 

Second issue, the temp gauge has no reading. I.e. I can leave the truck running for an hour and the gauge reads maybe a mm above cold.

 

Third issue, when the speedo goes nuts and sometimes when it doesn't the truck is stuck in third gear. I.e. limp mode.

 

Any ideas?

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The trans going into limp mode is likely connected to the malfunctioning speedo. The auto trans uses the speed sensor signal to decide when to shift, so a failing sensor or damaged wiring might be causing both issues.

 

The temp gauge is connected to a temp sensor on the coolant neck (the one with a single wire, the two-wire one is for the ECU). Check resistance from that spade terminal to ground with a meter, I think there's a spec in the service manual for what it's supposed to be at various temperatures. If the sensor's buggered, the temp gauge could be an easy fix. When I got my '93, the terminal was loose where it was riveted to the sensor, and the gauge hardly moved at all. Swapped in a good used one (not that they're hard to find new, I just had one handy) and all is well.

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The sensor checks out good via the service manual test procedure. I need to replace the wiring and see if that fixes it. The part that is bugging me is that it varies with the outside temperature. It's like a connector is expanding and making contact or shrinking and not making contact. So annoying.

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I had this same issue in my old one. It turns out water was getting in after a recently replaced windshield. It was coming in through one of the trim fastener holes... The shop fixed it under warranty so I never looked any further. But it did the exact same thing. I would check all connectors for moisture/pull up the carpet on the edges as the wiring runs along there somewhere on both sides. May not be a perfect answer but hopefully points you in the right direction. I remember it would be fine on a hot day but on a dewy morning it would act up.

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I had not thought about that, but I did have a leak from the top corner of my windshield. I will pull the carpet up this afternoon and look for moisture/liquid.

 

Thanks for the idea! I've been pulling my hair out for almost a year trying to get this figured out.

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  • 2 weeks later...

Ok, I went through the wiring harness going into the TCU. I couldn't find anything wrong. So I went to pull the truck back around front and it didn't have 1st gear. I shut it off, started it again and the "power/auto" switch blinked 16 times. Does anyone know what that means? I thought when it had been in limp mode and tou restarted the truck it would flash 8 times.

 

Any clue?

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The flashes are how the TCU tells you what it's upset about. Basically it's running through each circuit. If the circuit is good, it makes a quick flash. If it's not, it makes a longer flash. Note which flash is longer than the rest and then decode that with the service manual (page AT-40 has the patterns and what they mean).

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Ok, so the tray is going into limo model but no codes are being stored. This is so weird. Rechecked the VSS and it is good as far as I can tell. I did find that the wire bundled going back to the O2 sensor, VSS and all has a melted spot. The best I could last night no wires are exposed, but I am going to splice the burned ones just in case.

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Good thinking, hopefully it's that easy to deal with. I think the trans computer forgets the codes after spitting them out--so you shut it off and start it again and it flashes whatever it was unhappy about last time, then forgets and moves on.

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  • 1 month later...

Just wanted to post a follow-up for anyone else having this issue. After rewiring damn near half my wiring harness, replacing the VSS, MAF, O2 sensor, battery, and a few other small things I may have finally fixed the issue with a simple alternator swap. I had the dreaded A/T temp, battery, and brake combo come on. Replaced the alternator and now no more getting stuck in 3rd gear. No more lack of power. It is like driving a brand new car.

 

Thanks everyone for the help.

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Thanks buddy! Yeah, I wish 8 had a way to test the old one to see what it was putting out. It's in bad shape though. Covered in grime and a few spots where broken. I had never even thought about checking the underside of it. The top looked clean as a whistle.

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  • 4 weeks later...

Weirdly This last Friday I had it do the 3rd gear thing. It only did it once. I had just plugged up my dash cam and when I did my radio went haywire. A station I normally pick up just fine got super fuzzy. I guess they could be related however I would think any interference wouldn't cause an issue with the TCU/ECU.

 

I have also noticed that my speedo is exactly 4pmh off from what it should be. Like speedo says 55mph and real speed is 51mph. I have double check this with two different GPS units and several of the speed signs that clock you. 4 mph no matter the speed. Wtf???

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Does the issue come and go as you plug and unplug the dash cam, or with other loads on the cigarette lighter? I don't know offhand which fuse links go where, but I'd check to see if those look alright; maybe one's corroded and the additional load of the dash cam is enough to sap power from the ECU and TCU and screw things up. Or, if it only goes haywire with the camera, and not other loads, maybe your dashcam is just a noisy SOB. :shrug:

 

Tire size can throw off the speedo but 4mph across the board sounds like it's just out of adjustment. You could probably just open up the cluster, remove the needle, and put it back 4mph lower, though of course this would be tricky to gauge correctly with the cluster out. I'd be inclined to leave it alone, until you figure out the other issue, on the off chance that fixing one clears up the other.

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