Hello,

I just got a new used 85. And, got it started for the first time.

The display voltmeter is showing 17.8 volts.
My radio shack voltmeter is reading 14.5 volts at the battery.

The alt is new. The car hasn't run in 3 years.

I assume the ecm is putting out a signal to the display. Where is the display grounded. Do you have to take the dash out?

Thanks in advance for any information. I cant find it in the book.

 

Faulty V gauge maybe?
Do the other gauges in the IP work correctly?

Volts displayed on IP is direct from the charging system like on old skool cars
If you hook up a scanner you can the system Volts as seen by the ECM ( taken from the fuel pump relay );
typically .5V less than voltage seen at battery as you have done

 

Hello Rod and thanks.
My meter is rock solid. I don't any doubt about it. I never got a wacky reading - I use it all the time. (Like I said, I just got the car and it had been sitting for a few years so its a total unknown.)
A scanner - ??? - darn it. No, dont have one. But, I can check the relay. I didnt know where it got the signal. But, thats weird because the car was sitting with a tank full of water. The first thing i did was charge the battery and turn the key. I didnt get any noise out of the fuel pump so i pulled it and used my trusty multimeter to check for voltage at the fuel pump connector and it read ??? something ??? cant remember ... something right at 12V.

Thanks for the "clue". I was trying to find that info on the wiring diagrams and I couldnt find it. Its really annoying because that red warning light turns on around 16 volts ...

I ran the car for 10 minutes and there is NO CHECK ENGINE LIGHT WARNING.

Thanks again
Tim

 

Voltage is important to the ECM - the less there is, the less oomph to open the injectors, so with low voltage, they say stay open a tad longer; but that's not what you're seeing on your Display, the ECM monitors voltage from a different source. The Display reading is fed from the ignition switch to a fused (Cluster) input. Drop the steering column and check voltages on both side of the ignition switch (sometimes, if not always, there's a small drop). Anyway if that matches battery, something is probably mucked up with the Display. You could, if you want to, also pop off the Dash Trim and read the voltage inputs to the Display, Pins C16 and D16. If your input/output is the same as the overvoltage on the Display, I'd say your Regulator is shot and you need a new Alternator.

 

You need to get and read a FSM before making any more assumptions about how and why this car works...

FYI ex:

when you turn the key ON fuel pump only runs 2 sec to put pretension on the fuel rails and does not run again until the dist pulses to the ECM signal that the engine is running on its own and not the starter. This prevents starter flooding and wasted fuel.

Your inj are also all bank fired. There is lots to know so you don;pt get sucked into buying unnecessary parts and wasting valuable time.

FSM are available in book form or CD....I like the older books with the color wire drawings. One book for elect and the 2nd book for general.

 

Hello Lee,
I got the manual. I went through every page and looked at the wiring diagrams for an hour. I couldnt find anything that would explain why the battery voltage is 14.5 when running and the display shows 17.8.

I've had the same 85 for 20 years and i've gone through the fuel injection many times. I get that. I got this new used 85 and i went through the fuel injection in a few hours.

Sun,
What are you saying? The ecm receives the V input from the key switch? Where did you see that? I dont think it is the Display because they call the display the display. the ecm (computer) drives the display ... doesnt it ... like a computer is driving the display. the display doesnt process anything.

I just looked at the fsm for the display. i want to get into that area to see where there connectors and grounds are. I could not understand how to disassemble all that plastic. it was incomprehensible to me. i think the problem is in a connector. the display works all of it works except for the lights on the left side of the little panel with the voltage/oil pressure/fuel avg/inst. the left side bulb is out or something. i'd like to take that part but whenever i mess with that plastic, tabs get broken.

thanks for the help.

 

I was referring to the volt gauge in IP being faulty; not your test equipment

Your '870 ECM has two permanent 12V feeds on B1 and C16;
it also has a switched 12V feed from the Ign switch on A6.
(Look at the ECM pinout diagram in the FSM )


No
On your car the only thing out of the ECM is the Trip /MPG info.
Display is standalone from it's own inputs like the temp sender , OP sender and VSS in trans or in this case a connection to the charging system

 

I was referring to the volt gauge in IP being faulty; not your test equipment

well,, i dont think of the display being a guage. its described as a display. so, it displays numbers (i think the numbers are wrong but since there is a number there, the display is working). i checked and the display is microprocessor controlled so it does division. the divider software cant be wrong unless someone put in new prom ???


Your '870 ECM has two permanent 12V feeds on B1 and C16;
it also has a switched 12V feed from the Ign switch on A6.
(Look at the ECM pinout diagram in the FSM )

ok. thanks for clarifying that. the thing doesnt use the key switch voltage as the signal for the reading. i would think it uses the key switch voltage to turn on the display. when you turn the key, the display and computer get 12v and they turn on. it turns on. i get a display. its not black so the key switch is sending 12 v down the wire all the way to the ecm and to the display. i dont thijnk that has anything to do with the display saying there is an overvoltage condition.

No
On your car the only thing out of the ECM is the Trip /MPG info.

OK. thanks.

Display is standalone from it's own inputs like the temp sender , OP sender and VSS in trans or in this case a connection to the charging system.

Rod, i get it that the display is a standalone box and on the other side of the dash is the ecm which is a stand alone box. i just dug through the fsm ("when all else fails, read the book") You said down below early in this thread that the ecm got the voltage from the fuel pump and that was confusing me. Thats kinda strange because the battery is sitting about a foot from the display and about 5 ft from the ecm, so if they wanted an a more accurate measurement, they would take a reading from a source that was close ... the battery ... rather than from a source that was all the way down a wire to the very rear of the car and then send it all the way back through a bunch of push on connectors back to the display at the front of the car. if they took the voltage reading from the back of the car, the reading would always be low.

What did you say ... b1 and c16? I think thats what you said. i have to see where they come from. then it would be a straightforward thing to see which one effects the display reading.

i appreciate it rod. thanks a lot. i got to take the dash plastic off to see what the wires look like. maybe one got pinched or there is a bad connection or there is a loose ground wire. i think thats where the problem would be. i dont think its in the ecm, the fuel pump wiring, the alt or the key switch wiring or the display. that kinda narrows it down.

thanks again,

Tim

 

Don't forget to check the ALT while it's running. Even if the battery is only putting 14V out the ALT may be overcharging. I had to replace my ALT about 2 months ago for the same reason. Wasn't keeping battery at 12V and sometime over charging to 17.8V.

 

Just to clarify - the Voltage reading on the Display is via the Cluster Fuse at the Pins I gave you. That's from the ignition switch "Run" position. There are other fused Circuits powered when the key is in "Run". Follow the Power Schematic in Section 8A of the Manual, but what you see as Voltage on the Display is from this Cluster Fuse Circuit. It's not tied to the ECM or anything else which has separate circuitry. There is another voltage feed (LCD Circuit) to the Cluster which is simply for Memory.

If you drop the column and look at the Ignition Switch, you will see that it moves to different positions to feed battery voltage to different Circuits (and that Voltage is fed/comes through the Big Connector behind the Battery). Basically the positions are: Start, Run and Accessory. Run is the only the position that will light up the Display. Your other Indicators, such as Coolant Temp, are processed by the Display which sends a low voltage signal through a Sensor (which is a Thermistor) or a device that loses Resistance as it warms up. Whatever voltage makes it through the Sensor to ground results in a voltage drop and the Display uses that drop to calculate the Temperature you get on the Display. Voltage through the Cluster Fuse is also being used to generate that number along with everything else you see, including "Voltage", but it has nothing to do with the ECM Circuitry.

Your ECM feed to the Cluster is serial data or a binary stream. It's only uses what it needs to for Mileage and Range, but it's the same stream of info that you can read with a Scanner. In fact, it's spliced into the Diagnostic Terminal.

 

Hello Bro,
Good point. Yeah, i measured while it was running so you alt voltage. if its not running you are measuring the battery voltage.

Thats an interesting coincidence that we are seeing the same 17.8. Unless maybe i should just get a voltmeter and string in to the baittery and watch it all the time. i guess it is conceivable that there is an intermittant problem in the alt that causes it to jump to 17.8 and then back down to 14.5. That could be happening. You know what, i think i will do that and watch it for a while to be sure.

Thanks for that.
Tim

 

OK. I found the problem and i could actually see it with my eye. No magnification; no special lenses.

So, since Bro sent me this idea ... every time I started the car, I looked at the voltage display and then measured the battery. The voltage would fluctuate from 17 to 14.5 to 17 to 14.5 blau blau blau and then it went to 11.2. That was weird but I measured the battery with the car ruinning and it was 11.2. So, I abandoned the display wiring and took out the alternator. When I separated it and looked inside (this is a brand new alternator - delco assembled in mexico) well when i looked inside one of the brushes didnt pop out. they are spring loaded and they should pop out but it didnt. it was stuck ... so, i popped the brush out and there was chunk out of it and the chunk was causing the brush to bind. the brush holder wasnt installed right. it wasnt flat. so it flexed and it cracked . i could see it the moment i looked at it. so, i whipped it together with a new used holder and brush, slapped it together, in the car ... and 14.5volts at idle. Life is good.

When I read the thread and saw that that i wrote this:

"I just got a new used 85. And, got it started for the first time.

The display voltmeter is showing 17.8 volts.
My radio shack voltmeter is reading 14.5 volts at the battery."

Now that was reall bad luck. It went from 17 to 14 in the time that i i went from inside the car to outside the car ... and I went on a wild goose chase for 2 or 3 days.

Thanks to everybody for the help.

 

...and the SHIFT light. ;-)