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Old 05-26-2021, 06:26 PM   #1
martijn266
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Question Power difference with broken speedo

Hello,

Whenever im driving around my speedo sometimes works and sometimes it doesn't.
Nothing strange, had this problem a long time.
Too lazy to fix

Suddenly i started noticing something strange.
The car felt faster under WOT when the speedo was working.
First i thought it was 'tussen mijn oren' or in my imagination, but i still wanted to test it somehow.

On my way to work i drive through a 6km (4mile) tunnel so thats the ideal place to test.
When im really really steady on the throttle the (stock) turbo pressure gauge and rpm gauge give the same constant reading.
When the speedo suddenly goes from nothing to the actual speed the boost gauge moves ever so slightly up and the rpm's go up by maybe 200 rpm.
A tiny difference but my theory was correct!
I tried it a couple of times and it always happens, and when the speedo stops working the boost gauge drops down a little and the car also slows down a bit.

How does the speedo input cause this 'issue'?
Does it have to do something with the IAC?
I hope that somebody can answer my questions because i've searched the internet but it seems like no one ever noticed or posted about it.
Car is a 97 940 b230ft m90 manual with lh2.4
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'97 940se. B230ft, upgraded intercooler, 16T turbo ran at 15psi, 3" exhaust, 470cc injectors and Bijlsma chiptune.
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Old 05-29-2021, 04:59 PM   #2
dl242gt
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Have you checked for codes? You can check pin 2 and pin 6 in the LH2.4 diagnostic connector. I think it is on the left strut tower but I'm not sure on those cars. The vehicle speed signal goes from the rear axle to the speedo and to the fuel injection ecu. It may be that when there is loss of the vehicle speed signal the fuel and ignition is managed a bit more conservatively.
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Old 05-29-2021, 05:04 PM   #3
2manyturbos
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^^ That's what I am thinking it could be if this is really happening. The ECU uses the speed signal along with the air flow signal from the AMM to determine load. If the ECU is losing the speed signal intermittently, it will adjust accordingly. The ECU should be triggering a code for loss of speed signal if that is what is going on.
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Old 05-29-2021, 05:47 PM   #4
Fa182
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Quote:
Originally Posted by dl242gt View Post
Have you checked for codes? You can check pin 2 and pin 6 in the LH2.4 diagnostic connector. I think it is on the left strut tower but I'm not sure on those cars.
Unfortunately late 940's until their end in 1998 got rid of the diagnostic box and have the OBD-II connector in the armrest cubby with their own weird OBD protocol.
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Old 05-29-2021, 07:15 PM   #5
martijn266
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Thanks, i'll try the code reading.
It doesn't have the underhood code reader so i'll have to make that little OBD/led code reader thing.
I'll keep you guys updated on monday.
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Old 06-01-2021, 03:06 PM   #6
martijn266
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Update: the car broke down this week... BLOWN headgasket and much more. So no more updates about this.
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Old 06-01-2021, 05:37 PM   #7
dl242gt
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Oh well, hope you get it sorted out.
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