73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: former520 on March 14, 2011, 10:32:12 pm
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Now that the exterior paint is on the truck, the interior is due for attention. When I was swapping in the new dash pad, one of the last mounting points of the cluster turned to dust. The cluster is held in with a bunch of pieces of duct metal and screws. I would like to replace the cluster entirely. It does not line up with the cluster cover and makes it hard to read.
It is a 87 350 TBI with a TH400and I know it uses the VSS as a ECM input. AFAIK, and thats not much, the VSS circuit is tied into the speedometer cable and cluster. I would like to build a sheet metal dash, mount a gps unit to use for a speedometer, mount a tach and get oil, water, volt gauges to round it out. Put some LEDS in for indicator lights and be done with it.
Here is the question. Can I separate the VSS circuit from the dash and keep the ECM happy? What is effect of running without the VSS circuit? I know when I was driving around without the speedometer cable hooked up it would trigger a CEL light.
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VSS tells the computer your speed....
That means the computer can tell speed and load to richen or lean out the fuel as needed.
It also tells the tranny when to lock the converter (if you have a 700 tranny).
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You need the vss input from your speedometer. The speedometer would be really easy to mount in your custom dash, just cut a hole and stick it in. A truck with only a gps for a speedometer, blah.
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check out the local pull-a-part or other junkyard. You might be pleasantly surprised and it would be a whole bunch easier!
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I understand the GPS dash is not everyones cup of tea. I have them laying around and have had digital dashes in several things. Less expensive than an I have a TH400, so no worries about lock up. The dash in the truck is turning to dust and is jammed behind the metal and is going to come out in pieces (aka dust). I am going to be stripping the interior out to complete the color change and start on the door cards. I have not found any good dashes at any of the yards. I did find my new dash pad and rear window though.
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You will never find a good dash in an valley yard bud. I went to 6 different yards from Apache Junction to junkyard row and the best I found was still not that great.
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I understand the GPS dash is not everyones cup of tea. I have them laying around and have had digital dashes in several things. Less expensive than an I have a TH400, so no worries about lock up. The dash in the truck is turning to dust and is jammed behind the metal and is going to come out in pieces (aka dust). I am going to be stripping the interior out to complete the color change and start on the door cards. I have not found any good dashes at any of the yards. I did find my new dash pad and rear window though.
With TBI your truck will always think it's doing 0mph without the vss, and will run lean....
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You will never find a good dash in an valley yard bud. I went to 6 different yards from Apache Junction to junkyard row and the best I found was still not that great.
I have had the results for clusters. My only savior has been persistence and luck. I found a dash pad that had a new overlay on it that is perfect and unfaded. I also had the luck to find a fresh re manufactured alternator one morning as well. I still need to find tail lights (there is usually plenty, just haven't grabbed any) and a better rear bumper. I also have had no luck with power steering pumps either. I may leave that broken, my shoulders are looking good!
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Not sure about anywhere else but a no speedometer would fail it for inspection in New Hampshire.
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I have had the results for clusters. My only savior has been persistence and luck. I found a dash pad that had a new overlay on it that is perfect and unfaded. I also had the luck to find a fresh re manufactured alternator one morning as well. I still need to find tail lights (there is usually plenty, just haven't grabbed any) and a better rear bumper. I also have had no luck with power steering pumps either. I may leave that broken, my shoulders are looking good!
Let me know what you come up with for a rear bumper, the one on my 74 needs to go!
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If the VSS is stock in your 87, then yes you can run it straight to the ECM.
In 87 GM called it a VSS Buffer in the dash. The Buffer split the signal for the ECM and speedo. If you don't need the speedo then you can go straight to the ECM. It is the correct signal for the ECM as is if everything is stock. I think the 87 speedos used a signal light output to operate (a short lived bad idea).
In 90 GM went with the DRAC and a higher output VSS. The DRAC could convert the signal from the VSS into different output levels for the new all electric speedo, the ECM and the cruise control.
What are you going to do for a speedo? GPS? You could go with an electric speedo. Automer has electric ones that eliminate the need for the Buffer and DRAC if you only have the speedo and ECM. The Autometer speedo takes the feed from the VSS and has an output tab with the right signal for your ECM.
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With TBI your truck will always think it's doing 0mph without the vss, and will run lean....
I will have to disagree with this statement. The VSS has no effect on the air fuel ratio on these tbi trucks. FritzA is correct. One thing to keep in mind is the odomenter. If you plan to keep the truck forever, no big deal but if you sell it the truck will be TMU and depending on where it's being registered may cause problems.
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CODE 24
Trouble Code 24 indicates that the Vehicle Speed Sensor (VSS) is not sending the expected (based on other system parameters like LV8, TPS, RPM etc.) signal to the ECM.
Code 24 will be set if vehicle speed equals zero 0 mph when:
1 VSS indicates less than 2mph.
2 MAP is less than 30 kPa.
3 Engine speed is between 2200 and 4400 RPM
4 TPS is less than 2%.
5 Not in Park or Neutral
6 No Code 21, 22, 33, or 34.
7 All conditions met for 3 seconds
If the code was logged when the vehicle was in motion, the following should be checked:
1. VSS circuit open, shorted to ground, shorted together
2. Faulty Connections
3. Defective VSS
4. Defective ECM
5. A faulty or misadjusted Park/Neutral (P/N) switch can result in false Code 24.
Or :
OK, here is what "Chevrolet TPI & TBI Engine Swapping" says:
Regarding the VSS:
The VSS tells the ECM how fast the vehicle is going. Most people think the VSS is only used for for the lock-up torque convertor. The VSS is also used to control the EGR valve, the charcoal canister purge valve, the electric cooling fans, idle speed, and air/fuel ratio.
It must be emphasized that the VSS is used to control the idle speed when the vehicle is moving. Without the VSS the vehicle may have stalling problems under certain conditions. The reason an engine not equipped with VSS may stall is because when the ECM has the signals that indicate the engine should be idling (foot off gas, vehicle moving less than 2 MPH), idle speed is closed loop ( which is not the same as the O2 sensor running closed loop) and the ECM will try to maintain a programmed idle speed. If the vehicle is moving, the ECM opens the IAC (idle air control) a programmed amount, regardless of engine speed, which is usually a position that will make the engine idle about 50-100 RPM above the programmed idle speed. Stalling can occur when the vehicle is in the over-run condition (foot off gas- engine speed above the programmed idle speed) because the ECM will try to lower the idle speed to the stationary programmed speed. The IAC may not be able to open rapidly enough to prevent the engine from stalling.
Raising the minimum idle speed with the adjusting screw can eliminate stalling, but the engine will not run optimally without a VSS. Some Chevrolet engines are programmed to run lean under highway mode. Without VSS the ECM will not get the signals to run the engine for best fuel economy. There are a lot of other programs in the ECM which depend on the VSS.
And in '87, the speedo/odometer was still cable driven.....
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topp, I think you are mixing up TBI and TPI. TBI is speed density and fuel metering is not controlled by vss input on an 87 tbi truck.
The vehicle speed sensor (VSS) sends a pulsating voltage signal to the ECM, which the ECM converts to MPH. This sensor mainly controls the operation of the transmission Torque Converter Clutch (TCC), shift light and cruise control (if equipped).
Nevertheless In either system the vss although an important input will have very little affect on a/f ratio if the circuit is open. The ECM will easily correct the a/f ratio and will ignore the vss signal if a fault is detected therefore I will again have to disagree with your statement suggesting the truck will run lean.
Look on your scan tool at fuel integrator and block learn while driving down the road. Now unplug the vss and monitor fuel integrator and block learn driving exactly the same down the same road.
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Not at all...
It's OK. I Can utilize my manuals and Software which tells me that the VSS IS used for more than just TC Lockup on ODB1 systems...
We agree to disagree.
http://www.jagsthatrun.com/Pages/SpeedSensors_Speedometer.html
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You are right, we can disagree... So let’s go over a few things here:
2)The TPS is not an electronically controlled output device, it's an input not an output actuator like IAC or EGR so are you saying the TPS is controlled by speed and engine load? The TPS isn’t “controlled” it is again an input that relays information to the ECM based on driver demand ie how far the throttle plate is open or closed.
c)The EGR in this truck is controlled by the ECM and uses information from the following sensors to regulate the solenoid:
• Coolant Temperature
• Throttle Position (TPS)
• P/N switch
• Distributor (RPM Signal)
The EGR vacuum control has an EVRV solenoid that uses "pulse width modulation". This means the ECM turns the solenoid "ON" and "OFF" many times a second and varies the amount of "ON" time ("pulse width") to vary the amount of EGR.
d)IAC Idle Air Control controls idle speed (Idle speed… It means when the throttle plate is closed, it doesn’t mean the speed at which your truck is rolling down the road).
Engine idle speed is computer controlled by signals to the Idle Air Control (IAC) valve mounted on the throttle body. The IAC valve regulates idle speed by moving a pintle valve in and out, varying air flow through a passage bypassing the throttle valve. The more air that is allowed to bypass the throttle valve, the faster the idle speed. The ECM sends voltage pulses to the IAC motor windings causing the IAC motor shaft to move a given distance for each pulse, (referred to as IAC counts). The number of IAC counts can be read on a Scan tool display. 0 counts corresponds to fully closed passage, while 140 counts or more (depending on application) corresponds to full flow. The ECM monitors idle speed and increases the IAC counts if the idle speed is too slow. If the idle speed is too fast, IAC counts will be decreased.
3)Your engine load sensors here are MAP & TPS based on RPM. (Speed Density) – Air fuel ratio in this truck is targeted and adjusted by the ECM utilizing Fuel Integrator and Block Learn. Corrections to the A/F ratio are based on the primary sensors in both open and closed loop. I will list them for you:
RPM – from the distributor
TPS – Throttle Position Sensor
MAP – Manifold Absolute Pressure
ECT - Engine Coolant Temp
O2 – Oxygen Sensor (closed loop only)
Other sensors like Knock, VSS etc are not considered primary load sensors.
Nevertheless If the VSS circuit faults, the ECM will set DTC 24 and will ignore the VSS signal. So even if it did use the VSS to control the A/F ratio here how would it trick the ECM and allow the truck to run lean as you suggested?
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2-The ECM does control the IAC, and EGR and Fuel mixture based on it's program, and it USES the SPEED of the vehicle (and RPM, and MAP)to adjust the A/F ratio as you drive....If it did not, when you let off the throttle it would stall(which is why stock carbs have that little plunger- to raise idle for A/C and prevent stall when throttle blades shut rapidly) and when you were under load and accelerating heavily it would bog. That is just the fact, and is STATED in MANY CHEVY SERVICE MANUALS regarding TBI. FACT. It is also listed in the Snap-On Mitchells Shop Software. It also controls the converter lock up. Fact. Not assumptions or guesses... It also contributes (hence is USED and NEEDED) to adjust the EGR under the same conditions.
IAC changes postition as you drive (based on RPM, SPEED(VSS), and Load...
what does the IAC do when you go on the throttle???? It is SIGNALED by the ECM from the VSS and RPM that you are not at idle and are MOVING above 2MPH and it CLOSES. If it does not, you are Leaning out the mixture (and according to GM, the ECM will run in LEAN mode at Highway speeds as a default, and the VSS CORRECTS THAT by SIGNALING the ECM that you are moving at over 2MPH and RICHENS the mixture slightly.
4-MAP and RPM are not the only thing that the ECM needs, and yes, if VSS fails, the ECM ignores it and goes into default mode. Yet, if you did NOT need it, you would have no code and it would not be there to begin with.
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The original poster was given information to which I believe needed clarification. There were no personal attacks on my behalf here and if it was taken that way by topp I offer my apologies. You are correct in that there is no point in discussing this anymore as there is plenty of information here for the original poster to determine if in fact losing his vss signal would make his truck run lean. To anyone who wants to use the information posted here and do their own fact finding please go right ahead and choose to draw your own conclusions.