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Running issues, dead speedometer, phantom warning lights, help?

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Running issues, dead speedometer, phantom warning lights, help?

Post by Speeddemon448 » Tue Dec 10, 2019 4:02 am

Alright, I'm honestly stumped.
I've got a 1991 volvo 740 turbo with the auto trans. Everything on the car is as stock as it comes, save for some exhaust leaks that I've had yet to have fixed.

The issues:
I'll fire the car up after work and there will be about 6 or 7 lights very dimly lit on my dash (oil pressure, battery, bulb out, wiper fluid, service light, etc.) They are very dimly lit at first, and when I hold the rpm up to about 1500, they all go out and don't come on until i shut the car off and fire it back up.

The next issue happened when I was going to work tonight. I was driving it to work when I noticed that every time I let off the gas the RPM would drop real low and it would almost act like it was trying to stall out. After about 5 minutes of this, the speedometer dropped off to 0 and then the car ran pretty much fine.

As for the last issue, when I get on the gas pretty hard (say going up an interstate ramp) it will stumble for a moment then catch itself.

From the research I've done online about the whole phantom lights and speedometer issue, things are looking to either be the whole cluster being roached or my ECM has decided to let the smoke out.

Some more information if it helps anyone
LH 2.4 fuel injection, stock B230FT engine with the TD04H-13C turbo, AW11 trans, and the Dana 30 rear end. No fuses are blown, and I checked to make sure none of the hoses in the engine bay had come undone or split apart. I replaced the sensor in the rear diff along with running new wires to the speedometer and the feed to the ECM for the cruise control the last time I had this issue, and it had seemed to have fixed it. Only other thing worthy of note is that during the time frame of all three of these issues, my area of KY has been seeing sub freezing temperatures every night.

Any help is appreciated
Just a dude who happens to like Swedish cars with surprisingly nice seats.
2002 S80 2.9L

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Re: Running issues, dead speedometer, phantom warning lights, help?

Post by LostFoam » Tue Dec 10, 2019 11:54 am

Can't give specifics from experience, but you can visit the Wayback Machine and grab the Mitchell wiring diagrams (and some other relevant docs) for the '91 740 here:

https://web.archive.org/web/20171010122 ... olvo_docs/

Best wishes --

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Re: Running issues, dead speedometer, phantom warning lights, help?

Post by mrbrian200 » Tue Dec 10, 2019 12:53 pm

I believe the battery/charge lamp is isolated from the ECM... am I correct that this lamp is wired direct to the alternator? If that's the case you may be able to put the ECM down on the list of possible culprits for these lights being partially lit at startup.

I wonder if there's a diode to ground within the cluster that's shorted or gone bad that's normally supposed to block reverse current from lighting these bulbs under certain conditions, or some other unusual condition not seen under normal conditions (bad diode in the alternator, battery not maintaining normal voltage/close to death, etc)

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Re: Running issues, dead speedometer, phantom warning lights, help?

Post by 93Regina » Tue Dec 10, 2019 3:43 pm

Speeddemon448 wrote:
Tue Dec 10, 2019 4:02 am
hold the rpm up to about 1500, they all go out
Alternator's "feed wire" from alternator's dash-bulb is not connected to alternator. Alternator will engage without it when a higher RPM exist.

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