An interesting one
Mil light on , unable to read codes manually or with obd 1 scan tool.
T.P.S. has 5 volt reference as long as M.A.P. sensor is disconnected , with M.A.P. sensor connected both T.P.S. and M.A.P.
sensor drop to 2.5 reference voltage .
They already replaced the M.A.P. sensor with no change , and ordered an E.C.M. for Monday.
I was asked to take a look at it if the E.C.M. replacement does not take care of it .
My plan is to first test E.C.M. power and grounds .
Any input ?
Thanks in advance.
Rod When I used to deal in high performance with a 4-foot roll dyno in the floor, I ran into a similar problem. What I finally found was someone had installed a used pigtail and connector for the sensor, but crossed two of the wires (probably color blind). Anyway, the A,B, and C were not in the right position. Hope yours is as easy as this (although it took me several hours to find that one 20…
I've never worked on one so this isn't coming from any type of experience, but I pulled up the wiring diagram from Prodemand. According to the diagram there is also a Baro sensor on the 5V ref and the MAP is at the beginning of that wire. Could it be that the Baro is what is actually pulling down the 5V? It doesn't make too much sense to me but in my shop we don't take any of these OBD1 cars in…
4.3L TBI does not have a separate BARO sensor, MAP does both with software. The 3.8L carbed engine does have BARO and VAC sensors. Your diagram shows a carb solenoid, so I think you have the incorrect diagram.
Thank you , I will know more when I inspect this vehicle personally , but I think there is no baro on this one just map
Hi Rod, What scanner are you using? I typically only trust the old Snap-On MT2500 for these older vehicles. On a special note: Terminals A and B at the ALCL can only be jumped to get flash codes with the key on engine off. If this was done while running or someone cranked the engine, the ECM could be damaged. The first thing I recommend is load test the grounds and power at the ALCL (DLC)…
Thank you for the input. One thing how ever , this is a fuel injected , you can jump a to b running to inspect closed loop.
If you jump the pins (A/B) it will only work KOEO. Running will put it into Field Service Mode. Codes can only be gotten KOEO. Since the light is on all the time, disconnect the PCM and see if it goes out…if so, the PCM is grounding the circuit. Looks as though you've an ECM issue. Good Luck on finding one with the correct and functioning PROM chip, on a 37+ year old!
I will be inspecting all these things on Monday , also I have a signal simulator that I use for sensor substitution. Ever use one of those ? they work great for diagnostic purposes.
The fault being inside the ECM is highly unlikely. A faulty sensor, or incorrect wire locations in a connector, is much more likely.
Thank you . As you can tell by the post , they are trying every thing before they have me go look at it.
Hi Rod, We all deal with information shoppers all the time. Hopefully they do not create more issues poking around. Is this a DIY type, or a tech who is just unfamiliar with these early systems?
An engine shop , this el camino was restored with expensive paint job than an engine rebuild , so they may have damage the wire harness.
Hi Rod, Ah! That fills in some of the missing information. That possibly means a ground was left off or attached to a painted surface from my experience. There was a reason a star washer was used with many ground wires. They are often left off and not thought of as important. The star washers bight in to make good contact. True world experience: Body men are not good with electrical; they are…
When I back probe the ecm pins I will know more details
Subject: All General Motors Passenger Cars, Trucks and Utilities Use of Star Washer on a Body or Chassis Ground - kw wire connector terminal starwasher star washer #PIT… Models: . The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in the PI. Condition/Concern: Use of a star washer under a ground wire connection. In the past, it has…
Carey Thanks for that document. I spent years as an Automotive research engineer/manager and one of my many obstacles was “tradition”. In other words “We’ve always done it that way”. Even when we as engineers provided data with statistical repeat/confirmation it was still uphill to get a tradition changed.
Resistance in Series produce a ratio of voltages ;looks like a good one for a circuit check of with a few quick Ohm checks since battery and PCM disconnected isolated & while in there on the heap . 1 )Continuity/Ohm 2) Circuits shorted to each other /OL. 3) Shorts to ground/OL
Agreed, very valid procedure test to ground and test signal to signal.
Hi Rod, I think the industry is divided about the use of star washers on ground connections. I have attached an article below. From my own experience on mid 90's Harley Big Twins, if the star washer was left off at the main ground to the starter, it would create issues later down the road. The only thing I recommend adding with a star washer is die electric compound or copper coat to prevent…