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My grid heater, alternator, Speedo, and tachometer are all down and I’ve been reading about how the ESS will knock the other three out but not vice versa. Been especially glad to find DanG144’s very helpful posts on this. So I checked the voltage to the ESS, at the link on the front of the engine, from the PCM, and instead of the 5 volts it’s supposed to get I had just 0.59 volts. Since both positive pins read exactly the same I’m thinking it’s safe to say there’s no wire problem unless the two come together somewhere before going into the PCM. I’ll look that up in my service manual. I had the key on the run position and the battery had 12.5 volts.
Any suggestions much appreciated. I don’t have a spare PCM try on.
 

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Discussion Starter #2
My grid heater, alternator, Speedo, and tachometer are all down and I’ve been reading about how the ESS will knock the other three out but not vice versa. Been especially glad to find DanG144’s very helpful posts on this. So I checked the voltage to the ESS, at the link on the front of the engine, from the PCM, and instead of the 5 volts it’s supposed to get I had just 0.59 volts. Since both positive pins read exactly the same I’m thinking it’s safe to say there’s no wire problem unless the two come together somewhere before going into the PCM. I’ll look that up in my service manual. I had the key on the run position and the battery had 12.5 volts.
Any suggestions much appreciated. I don’t have a spare PCM try on.
BTW all the other gauges work
 

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The ESS has nothing to do with the grid heaters. That is regulated by the intake air sensor on the top of the intake plate. The speedo is not part of the ESS circuit either. I suggest you start cleaning and tightening the ground wires. For starters, the PCM ground wire.
 

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Discussion Starter #4
The ESS has nothing to do with the grid heaters. That is regulated by the intake air sensor on the top of the intake plate. The speedo is not part of the ESS circuit either. I suggest you start cleaning and tightening the ground wires. For starters, the PCM ground wire.
Thanks Games I’ll do that next. My service manual says the PCM relays may be energized before and after cranking depending on inputs from the intake air temp sensor like you said. But also from the ESS and the VSS. The ESS lets the PCM know when the starter is working, so it can turn off the heater. according to the manual. I have juice to the heater relays but never at the heater. I took the relays out and tested them. They clicked every time I gave them 12 volts but I don’t know if they get 12 from the PCM so that might not have been a good test
 

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Discussion Starter #5
Thanks Games I’ll do that next. My service manual says the PCM relays may be energized before and after cranking depending on inputs from the intake air temp sensor like you said. But also from the ESS and the VSS. The ESS lets the PCM know when the starter is working, so it can turn off the heater. according to the manual. I have juice to the heater relays but never at the heater. I took the relays out and tested them. They clicked every time I gave them 12 volts but I don’t know if they get 12 from the PCM so that might not have been a good test
I’ll test the air temp sensor too, what’s a good test for it? I might have seen a resistance test for it, can’t find one in the manual
 

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Keep reading the FSM. The relays only receive a ground signal from the PCM. They always have 12 volts. If the grid heaters don't work at all it has nothing to do with the ESS.
 

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I don't recall a test for the iat.
 

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I’ll test the air temp sensor too, what’s a good test for it? I might have seen a resistance test for it, can’t find one in the manual
My manual says the ESS, VSS, air intake temp sensor, and a couple other things all share the Circuit K4 splices that supply them ground. I don’t know what that means, if they’re on the same circuit or not but just thought I’d throw that out in case anybody wants to elaborate on it
 

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I don't recall a test for the iat.
GAmes I just ran across your post from 2009 on how to switch the grid heaters on the dash, and that sounds like the perfect solution to that problem and hopefully the alternator, tach and Speedo are a bad ground problem. The alternator only has a few hundred miles on it and was working, so it’s probably some ground. Thanks again
 

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GAmes I just ran across your post from 2009 on how to switch the grid heaters on the dash, and that sounds like the perfect solution to that problem and hopefully the alternator, tach and Speedo are a bad ground problem. The alternator only has a few hundred miles on it and was working, so it’s probably some ground. Thanks again
I got 0.6 volts at the IAT sensor wire from the PCM Same as at the Engine Speed Sensor. So it must be either the PCM or the ground on the PCM. The IAT is supposed to get 5 volts, same as ESS. I don’t see a ground wire on the PCM and my book shows two grounds from it merging with grounds from 3 other things including the starter relay which works fine. (Also to data link and engine diagnostics grounds). Then those grounds splice together and go to G119. But I don’t know how to locate the G119, don’t know where in the manual it tells locations specifically. Anybody know where the PCM ground is? Thanks
 

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Discussion Starter #11
Found the PCM ground from the nice drawing in the book. It’s on top of the head up front, 15 mm bolt with two wires. No corrosion but I wiped it clean, was greasy, and still have 0.6 volts at the ESS and the IAT instead of 5 from PCM. So all I can think of is it’s the PCM. Any suggestions would be very welcome but it looks like game over from here
 

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It is on the top/front of the engine to the left of the #1 valve cover. (right as you look at it from the front)
 

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Discussion Starter #13
It is on the top/front of the engine to the left of the #1 valve cover. (right as you look at it from the front)
Thanks, it was like new, still have 0.6 volts at ESS and same at IAT sensor so it sure seems like the PCM is the culprit. I’m going to try out my new scan tool when it gets here in a week and that’s all I can think is left to try.
 

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Good luck with that. You can pull any fault codes the PCM might have doing the key on/off exercise.
 

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Discussion Starter #16
My check engine light isn’t working not even when I turn the key on. I’ll be posting what the scan tool says in a week tho
 

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I have a theory, perhaps false, that if the PCM is faulty the scan tool won't be able to read it. There are other ground wires you know. It would not be a waste of time to remove and clean them and you might get lucky.
 

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Discussion Starter #18
Sounds good, do you know where the other grounds are? I see on the schematic that there are splices to those other three things like the alternator and was wondering if those splices were on grounds. Also there are the ground wired to the ESS and VSS and a few others that are controlled by the PCM on one circuit. If you have the service manual I could give you the page that describes those but I don’t think there’s a schematic for that, altho I have not looked. Thanks GAmes
 

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Discussion Starter #19
Page 8W-30-44. Goes into a lot of detail about the other sensors connected to the ESS and the AIT which share the same K4 grounds for their signals to the PCM. If you don’t have it I’d be glad to quote it to you. It unfortunately does not reference a schematic for those but there are pages on grounding wires I’m going to look at now
 

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If you look around the engine bay you can see several ground points. The FSM also has schematics. In the '97 manual they start at page 8W-90-10. Speaking of that, how about creating a signature now that you are over 100 posts, or are we just supposed to guess year, transmission type and 2 or 4 WD.
 
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