Need help determining high resistance ground
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Tuned
Joined: Aug 2007
Posts: 287
Likes: 1
Need help determining high resistance ground
1988 F250 351
Truck will now only idle and very poorly . As soon as the throttle plates open it dies. Virtually every major component is new. My thought is that a high resistance ground is not allowing clean signals from the sensors to the computer.
With the key off, measuring continuity between the black wire at the TPS, MAP or EGR and chassis ground, I get 0 to .3 ohms.
With KOEO resistance jumps to around 40 ohms on the black wire at all the locations . If I pull the ECM relay, it goes back to near 0 ohms.
With KOER resistance jumps to 275 ohms on the black wire at all the locations. Turn off the truck and near 0 ohms returns. I consistantly get 5.0 volts on all orange wires, .09 on the TPS with a smooth swing up to 4.8 volts at WOT. MAP and EGR signal returns are correct as well. All chassis grounds have been cleaned reattached.
What should I see for resistance with KOER - near 0 or 275 ohms?
(Also could a faulty ignition switch cause havok with sensor grounds? My ignition switch only energizes the starting system. I've used an auxiliary button wired the starter solenoid to start the truck for years.)
Thanks
Truck will now only idle and very poorly . As soon as the throttle plates open it dies. Virtually every major component is new. My thought is that a high resistance ground is not allowing clean signals from the sensors to the computer.
With the key off, measuring continuity between the black wire at the TPS, MAP or EGR and chassis ground, I get 0 to .3 ohms.
With KOEO resistance jumps to around 40 ohms on the black wire at all the locations . If I pull the ECM relay, it goes back to near 0 ohms.
With KOER resistance jumps to 275 ohms on the black wire at all the locations. Turn off the truck and near 0 ohms returns. I consistantly get 5.0 volts on all orange wires, .09 on the TPS with a smooth swing up to 4.8 volts at WOT. MAP and EGR signal returns are correct as well. All chassis grounds have been cleaned reattached.
What should I see for resistance with KOER - near 0 or 275 ohms?
(Also could a faulty ignition switch cause havok with sensor grounds? My ignition switch only energizes the starting system. I've used an auxiliary button wired the starter solenoid to start the truck for years.)
Thanks
Posting Guru
Joined: Jan 2006
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From: The dark carnival
I think someone with the same kind of truck (that runs) needs to take tjose same readings to compare.
If you open the throttle then it dies, it sounds like the computer is not getting good data as to the position of the throttle plates. To confirm this (and find the problem even if the problem is not TPS related) hook up a breakout box. If you don't have one, get one. It will likely come in handy over and over.
If you open the throttle then it dies, it sounds like the computer is not getting good data as to the position of the throttle plates. To confirm this (and find the problem even if the problem is not TPS related) hook up a breakout box. If you don't have one, get one. It will likely come in handy over and over.
Posting Guru
Joined: Jan 2006
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From: The dark carnival
The monitor will do resistance tests on any of the 60 wires on the harness using the INSIDE of the computer as a referance point.....something nothing else will do that I know of.
Did it come with literature? Overlays? Having the right overlay is a MUST, the literature is nice, but not *needed* once you get the hang of all it's features and general troubleshooting procedures. If you need an overlay I can probably hook you up with a copy at the very least if not a Rotunda original.
You may want to also invest in the adaptor cable.....makes life much easier and they go for about 20 bucks....well worth it.
I can walk you through using it if need be.
Did it come with literature? Overlays? Having the right overlay is a MUST, the literature is nice, but not *needed* once you get the hang of all it's features and general troubleshooting procedures. If you need an overlay I can probably hook you up with a copy at the very least if not a Rotunda original.
You may want to also invest in the adaptor cable.....makes life much easier and they go for about 20 bucks....well worth it.
I can walk you through using it if need be.
Last edited by eco; Nov 15, 2007 at 03:39 AM.
Postmaster
Joined: Oct 2006
Posts: 4,816
Likes: 1
From: Davis & Newcastle, CA
Originally Posted by eco
The monitor will do resistance tests using the ground INSIDE of the computer as a referance point.....something nothing else will do that I know of.
Did it come with literature? Overlays? Having the right overlay is a MUST, the literature is nice, but not *needed* once you get the hang of all it's features and general troubleshooting procedures. If you need an overlay I can probably hook you up with a copy at the very least if not a Rotunda original.
You may want to also invest in the adaptor cable.....makes life much easier and they go for about 20 bucks....well worth it.
I can walk you through using it if need be.
Did it come with literature? Overlays? Having the right overlay is a MUST, the literature is nice, but not *needed* once you get the hang of all it's features and general troubleshooting procedures. If you need an overlay I can probably hook you up with a copy at the very least if not a Rotunda original.
You may want to also invest in the adaptor cable.....makes life much easier and they go for about 20 bucks....well worth it.
I can walk you through using it if need be.
I knew you'd come around eco. Very helpful post.
Lead Driver
Joined: Nov 2002
Posts: 9,557
Likes: 44
From: Concord, NC
It sounds like the computer ground is making a bad connection. I'm not sure, though, that a resistance test will tell you much of anything with the engine running and with current passing through the computer ground path. At any rate, check for a voltage drop between pins 20, 40, 60 and battery negative with the engine running. There should be very little to no voltage drop between these two points. If there is, there's a bad ground connection between the computer and ground.
Elder User
Joined: May 2007
Posts: 754
Likes: 1
From: Solon, Ohio
You CAN'T......I mean CAN'T measure resistance with an ohmmeter with current going through the circuit.
What you have to do is measure the VOLTAGE at the ground connection in refference to the ground at the negative battery terminal, or where the small cable from negative cable grounds to the chassis. All grounds should measure 0.00 with good connections.
What you have to do is measure the VOLTAGE at the ground connection in refference to the ground at the negative battery terminal, or where the small cable from negative cable grounds to the chassis. All grounds should measure 0.00 with good connections.
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Tuned
Joined: Aug 2007
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Thanks for the replies.
ECO: I remember in your previous post that you said you had the overlays. I'm not sure what will arrive with the unit. Your offer with the overlays and walk through is really very decent. Thank-you. I'll let you know. What is the adapter cable for?
EPNCSU2006: I'm all too familiar with pins 20, 40, and 60. I try the voltage drop test to ground this morning.
netscanner: The quote, "what you don't know you don't know" comes to mind. I was driving myself nuts with my logic that the high ohm reading with current on was indicative of something. Thanks for clearing that up!
I like my truck but I close to shooting the thing!
ECO: I remember in your previous post that you said you had the overlays. I'm not sure what will arrive with the unit. Your offer with the overlays and walk through is really very decent. Thank-you. I'll let you know. What is the adapter cable for?
EPNCSU2006: I'm all too familiar with pins 20, 40, and 60. I try the voltage drop test to ground this morning.
netscanner: The quote, "what you don't know you don't know" comes to mind. I was driving myself nuts with my logic that the high ohm reading with current on was indicative of something. Thanks for clearing that up!
I like my truck but I close to shooting the thing!
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Thread Starter
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Tuned
Joined: Aug 2007
Posts: 287
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Something else. One of the connectors at the ECU relay has corrosion . I fiddled around for quite a while trying to release the female side out of the brown socket with no luck. Didn't want to screw it up. Any tips on removing?
Also, I see three relays on the driver's fender. Brown, green/blue and grey. I know(assume) the brown one is the ECU. Which one is the fuel relay and what what is the other one for.
Thanks again.
Also, I see three relays on the driver's fender. Brown, green/blue and grey. I know(assume) the brown one is the ECU. Which one is the fuel relay and what what is the other one for.
Thanks again.
Fleet Owner
Joined: Aug 2005
Posts: 23,667
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From: Easton,Ks
Your grounds seem to be OK.
The ones you speak of are part of voltage divider verbal resistors.
Take the TPS for example the only voltage there that the computer cares about is the one telling it the position of the throttle plates and from your readings it seems OK.
I would say you have a fuel problem.
Put a fuel pressure gauge on the test point at the fuel manifold and watch the pressure when you open the throttle.
As far as the test equipment for the electrical goes all you need is a breakout box, DVM meter and a test light.
Do not waste your money on all of that other stuff.
The ones you speak of are part of voltage divider verbal resistors.
Take the TPS for example the only voltage there that the computer cares about is the one telling it the position of the throttle plates and from your readings it seems OK.
I would say you have a fuel problem.
Put a fuel pressure gauge on the test point at the fuel manifold and watch the pressure when you open the throttle.
As far as the test equipment for the electrical goes all you need is a breakout box, DVM meter and a test light.
Do not waste your money on all of that other stuff.
Thread Starter
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Tuned
Joined: Aug 2007
Posts: 287
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Subford: If it's fuel related it's very hard to see. I've had a pressure gauge on the rail most all the time and the pressure is 35 PSI constantly except when it stalls then it goes to above 40 PSI. Both tanks, sending units, and all pumps are new and the injectors were reconditioned 600 miles ago.
Distributor, module, rotor, wires, plugs, coil, all sensors, ECU, altenator and battery are new or reconditioned. The three relays on the driver's fender are not new.
I hope the Rotunda monitor is the silver bullet!
(What does "voltage divider verbal resistors" mean?
Distributor, module, rotor, wires, plugs, coil, all sensors, ECU, altenator and battery are new or reconditioned. The three relays on the driver's fender are not new.
I hope the Rotunda monitor is the silver bullet!
(What does "voltage divider verbal resistors" mean?
Fleet Owner
Joined: Aug 2005
Posts: 23,667
Likes: 301
From: Easton,Ks
Originally Posted by jemini
Subford:
(What does "voltage divider verbal resistors" mean?
(What does "voltage divider verbal resistors" mean?
"voltage divider variable resistors"
/
Last edited by subford; Nov 15, 2007 at 01:51 PM.
Posting Guru
Joined: Jan 2006
Posts: 1,263
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From: The dark carnival
Originally Posted by subford
As far as the test equipment for the electrical goes all you need is a breakout box, DVM meter and a test light.
Do not waste your money on all of that other stuff.
Do not waste your money on all of that other stuff.
Is a new BMW a waste of money when you could buy a new KIA for less?
Is a Jesse James custom West Coast Chopper a waste when you could buy a Honda motorcycle for less?
Is a new $100,000 Hummer a waste when you could buy older Jeep Charokee for $1500?
To someone that can't even afford to keep bread crumbs on the table luxury items may be a waste because there are other things have to come first for survival, but to the folks that can afford the nicer things without significant financial stress they are far from a waste due to what they can do compared to the lesser, cheaper alternatives.
Posting Guru
Joined: Jan 2006
Posts: 1,263
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From: The dark carnival
Originally Posted by jemini
ECO: I remember in your previous post that you said you had the overlays. I'm not sure what will arrive with the unit. Your offer with the overlays and walk through is really very decent. Thank-you. I'll let you know. What is the adapter cable for?