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Speedometer problem on a 90

L

lugnut

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I just bought a 90 2500 Suburban and the speedometer doesnt work. It stays at 65 MPH sitting still or moving it will some times jump over to 85 for a few seconds and then drop back to 65. The odometer and trip odometer both work. The brake warning light is also on. Could the 2 problems be related ?? If I unplug the ABS box the brake light goes out but it has no efect on the speedo. I have searched so I know the speedo is electric but I havent found anything helpful about trouble shooting it. I did see someone talking about useing a scaner to see if the VSS is sending a signal. What type of scaner do you use ?? Is it just a ODB I scaner or is it something special ??
 
It is an OBD1 scanner.

Does your cruise work? If your odometer/trip meter both work, along with and/or cruise, it's unlikely it's the VSS or the DRAC, since the VSS feeds everything speed related...ECM, Cruise, and all speedometer functions are tied into the VSS working correctly, and since there is only one VSS wire to the speedometer from the DRAC, unlikely the wiring all the way to the speedometer has a problem.

I'm assuming the speedometer is some sort of motor, I'd guess the speedometer itself is the problem.
 
Sounds like a bad spedo to me too. Have to replace it or take it to a speedo shop to fix it.

For your ABS light you have to find your data link under the dash by steering collumn.​


What you need to do is pull the codes for the ABS system. ABS Diagnostic Trouble Codes (DTC)'s can be displayed by jumping terminals "A" and "H" of the Data Link Connector and observing the flashing of the "BRAKE" warning light. This should only be done when the brake lamp is "ON".​

aldl_connector.gif

The terminals must be jumped for about 20 seconds before the code will begin to flash. Count the number of short flashes starting from the long flash. Include the long flash as a count. Sometimes the first count sequence will be short. However, subsequent flashes will be accurate. If there is more than one failure, only the first recognized code will be retained and flashed.
Note: DTC's 6, 9, and 10 will only turn the "BRAKE" lamp on while the fault exists. These are soft codes faults and can only be read with the Tech 1 scan tool. Do not jump terminals "A" to "H" if the "BRAKE" lamp is not on. Jumping the terminals with the "BRAKE" lamp off will set a false DTC 9. This will turn the "BRAKE" lamp on and disable the antilock system.​

To clear the Diagnostic Trouble Codes:
  1. Make sure the ignition switch is "OFF.
  2. Pull the Stop Lamp fuse.
  3. Wait five seconds.
  4. Reinstall the fuse.
  5. Verify no Diagnostic Trouble Code (DTC)s are stored.
The ABS DTCs are:
DTCs 1, 11 and 12 ECM Malfunction
DTC 2 Open Isolation Valve or Malfunctioning ECU
DTC 3 Open Dump Valve or Malfunctioning ECU
DTC 4 Grounded Antilock Valve Reset Switch
DTC 5 Eccessive Actuations of the Dump Valve
DTC 6 Erratic Speed Signal
DTC 7 Shorted Isolation Valve or Faulty ECU
DTC 8 Shorted Dump Valve or Faulty ECU
DTC 9 Open Circuit to Speed Signal
DTC 10 Brake Lamp Switch Circuit
DTCs 13, 14 and 15 ECU Malfunction

aldl_connector.gif
 
I have not tryed the cruise but I will and I'll also pull the ABS codes. Thanks for the help. Ok I just pulled the code and I got 1 long flash then 3 shot flashes so if I'm reading the instuctions right thats DTC4. Now where do I find the antilock valve reset switch?? I have got to get a manual for this thing mine stops at 87.
 
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No, that would be a code 13. Should have three code 12's first, then all your actual trouble codes, then back to 12's, and repeat.

As for cruise control, it's a seperate system, and a lot of times it's monkeyed with by a PO, so if it works, great, just another indication your speedometer is the real problem, but if it doesn't, it could just mean your cruise control is screwed up.
 
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Tryed cruise today it did not work. I thought hte three 12s was an engine code that you got when jumping A+B ?? I jumped A+H and all I got was the the 1 long flash folowed by 3 short flashs. From Thunders post I took that as a 4. He said count from the long flash and include the long flash as a count. Am I completly lost ??
 
Damn I'm on a roll for misreading today. ABS codes are read differently than ECM codes, ignore me. :)

As to cruise, if you've never used it before, then pretty bad indicator of a problem. You'd need to troubleshoot it, but your speedometer problem is more pressing.

Still, with Odometer and tripmeter working, I say speedometer.
 
Cruise I can live with out and I can even get by with out the speedo for a while but that red brake light now thats another story. I have to get that fixed. Any idea where the antilock valve reset switch is??

Update I filled it up with gas today so I could check the milage and when I reset the trip odometer the speedo started working.
 
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I broke down and ordered a manual from helms and that helped some but i still have questions. Manual says to test wire 824 and ground with key switch on. I have no voltage so it says to check fuse (brake) and then check wire 824 for open. So I trace 824 and find it comes from the DRAC I locate the DRAC and check 824 to ground there and find no voltage. ( fuse was good )I OHM wire 824 it checks good so much for the open. I check wire 450 BLK/WHT ground its grounded and I check wire 50 BRN its got batt. volts. My manual does not tell how to test the DRAC so from what I have so far it seams I have a bad DRAC ?? I have batt. voltage and a ground in but nothing out on 824. Anybody know how to check the DRAC ?? Am I going to have to buy a scan tool??
 
The DRAC's job is to interpret the signal from the VSS (on the t-case) and convert them to however many pulses each component needs. ABS IIRC is 128,000 pulses per mile, while speedometer is 2000.

Since the DRAC interprets the signals from the VSS, I'd be surprised to see any meaningful signal on that wire if the truck isn't moving...perhaps I'm interpreting the manual wrong, but if you check power and ground to the DRAC, the only wires left should be the output wire, which should pulse as the vehicle moves, or so I think.

The manual tells you to ground 824 right off the bat? What is the expected result?

Better yet, which manual are you using (1990 service?) and which section is the troubleshooting? I've got the 1991 version, I can probably follow along.
 
I'm useing the electrical diagram/troubleshoting section. Its seams very incomplete for the speedo trouble shooting to me. Wire 824 should be batt volts with the key switch on. Mine was .03 so I am following the steps for no voltage. Which just says check fuse and then look for open in 824. The only thing I can find on the DRAC is in the diagram for cruise or something. It shows batt+ input, ground input,2 inputs from VSS, output for ABS, output to speedo (824) and 1 more I think to the cruise. I dont have he manual in front of me right now.
 
OK I got ya, same procedure for '91, which I expected. There is another manual that covers VSS, which is emissions. Luckily I have that one too. :)

So my test procedure would be:

Turn key to "run". Test voltage on 824, use a sheetmetal ground. Something you KNOW is a good ground, not something suspected as a good ground. A lot of times I use the continuity checker to confirm, a lot of screws or surfaces have rust or paint, and can NOT be used as-is as a ground.

If you have no voltage there, page A-10 and A-11 in my manual show the fuse block. If you've got voltage to the heater/AC fuse, then apparently 824 should have power. If not, then you shouldn't have power windows either, that fuse is fed with the same power line as the heater/AC one.

Do you have an autozone close by? It would be pretty easy to see if the DRAC was working based on a scan of the ECM while driving, to indicate if vehicle speed is being picked up. DRAC CAN have problems and the truck still run, but since the ECM speed signal is the only easily readable output of it, it would be the easiest/fastest way to test.

There is another test mentioned. First, your service manual, under 8C-3, should have a page on "diagnosis of the speedometer system" which covers the DRAC. But its kind of vague. The emissions manual has these tests:

Clear codes
back probe across ckt 1233 and 1232 at PCM connector
Vehicle wheels raised, engine running in gear
voltemeter on A/C 200 volt scale
Does voltage vary from 0 up with RPM increase?

Yes: Problem is intermittent or internal transmission. (I'd think that could indicate DRAC too!)

No:
Disconnect speed sensor with vehicle still in air, running, in gear
voltmeter on A/C 200
Measure voltage across sensor
Voltage vary with RPM?

No:
Replace sensor

Yes:
Problem is Poor PCM connection
Poor connection at output speed sensor
Open CKT 1233 or 1232
Shorted CKT 1233
(I'd also add that again, DRAC is between the sensor and the ECM/PCM, so it could still be faulty!
 
Thanks for all your help. From what you are saying i think I should have ordered a manual for a 91. My emishons manual says to see 8-C for the VSS and then 8-C tells about the speedo but its all about the mech. speedo not a word abot the elec. one. From searching and looking at my diagram book I have done some more testing. All these test were done at the DRAC with it pluged in back probimg the conector with the key switch on (821+822 were tested with the truck running driving down the driveway). Wires 821 and 822 input from the VSS to the DRAC show AC voltage going up as speed goes up.( sounds like the VSS and the wiring from it are good to me ) Wire 450 shows to be a good ground( good input ). Wire 50 shows 12.3 volts ( good input ) Wire 437 output to ECM shows 9.5 volts I dont know if it should be batt volts or less?? Wire 696 output to ABS controler shows 8.5 volts agin I dont know if it should be batt. volts or not. Wire 824 output to speedo is .03 volts the manual says it should be batt volts. The HTR-AC fuse feeds wire 50 ( input to the DRAC ) then the DRAC feeds 824 and I have .03 volts on 824 . From this I am assumeing the DRAC is bad. A lot of probing and checking but I still dont have a real way to test the DRAC. Everything says 824 should be batt volts with the key switch on so for grins I put a jumper from batt + to 824 to see if the speedo would go to zero. It did not move at all. I assume it should read zero with the switch on and the truck standing still so from this I beleve the speedo is also bad. :confused: Will a bad DRAC kill the speedo?? Will a ba speedo kill the DRAC??
 
Thanks for that link lotsa good info in that post and it was one I hadnt found with search. I have all the wiring figgered out I'm just unsure what somethe voltage readings should be. I'm on the search now for a good speedo and maybe a DRAC. If i can catch this thing working long ( it still works every now and then ) enough to retest all my in and outputs I will post the new readings. If I cant I will retest every thing after I get a new speedo in and working. It will be interesting to see if the other outputs change or just the speedo ooutput. The one thing i wish I had done when I was testing is to check voltage on 824 with the cluster unpluged. i took all my readings with every thing pluged in. Next time I'm messin with it I'll check 824 with te cluster unpluged and see if my voltage reading changes.
 
If it's intermittent, it sure sounds like a bad connection. Did you check all that relate to the speedometer, making sure they are clean and "springy"?

Could of course be intermittent internally, but maybe you can get lucky. :)
 
I checked all the conections making sure everything is tight and clean. Its intermittent but its not like hit a bump and it works. If i drive it for a long distance ( about 1 hour or so) it will sometimes strat working and will work until I let the truck sit and cool off. I'm thinking the motor that turns the speedo hand must warm up and start working but I dont know.
 
Checked 824 with the cluster unpluger it has .03 volts with the key switch on. I pulled the speedo out of the cluster and disasmbled it. Now I know why the odometer works. It has its own motor and I also think I know why the speedo doesnt work. There is a piece on the board that is burnt so the speedo search is on. Im also gonna see in an electronics repair shop can fix the board. Looks like it was made so you could rplace the motor for the speedo it just plugs in. The motor for the odometer and the rest of the board are sordered together but the speedo motor is plug in.

Update.............$75 for a cluster with 88,000 miles and now i have working speedo. I'm still gonna try to get the old one repaired. I'll let yall know how that goes.
 
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