Voltage Gauge / Regulator / ECM Issue

bucknut713

bucknut713

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After a mud filled day of wheeling my 1998 TJ 4.0 Automatic the voltage meter on the dash is reading 0 after I start it and drive a few minutes and the check gauges light comes on. I had both the battery and alternator tested twice at 2 different places with the battery and alternator in the Jeep and out of the Jeep and both tested good. One place tested and said it came up as a bad voltage regulator, which I found out is built in to the ECU/ECM/PCM. After reading up on this and seeing it might be a bad ground or loose wire I disconnected the battery and took off and cleaned every wire I could see. All battery terminals,alternator wires,frame grounds,body grounds and anything else I could disconnected and check under the hood. While I was doing this the battery was charging and I put everything back together and went for a ride and about 2 minutes in the check gauges light comes on and the volts drop to 0. I tested the volts with a multi meter while the jeep is running at the battery and it was 12 and then I put the negative probe on a frame ground and it reads between 1-5 and sometimes jumps to 12 if I move it around.
Anything else I could check out before getting a ECU or external voltage regulator?
Has anyone done the external VR before?
 
Well I certainly don’t want to jump to conclusions, but you may indeed have an ECM that is failing.

Not sure about the external voltage regulator though. I haven’t heard of anyone doing it before.
 
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If the engine is running and you are only measuring 12 volts then it sounds like the alternator. You should be reading close to 14 volts with engine running and around 12 without.
 
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What doesn't make sense is the varying voltage at frame ground. You said it changed when you moved the test lead around. Could you have been on a painted surface? If you were on bare metal the I would still lean toward the alternator. Try running with jumper cables from another vehicle. If it clears up then at least you can rule out the ECU. I'm with Chris.....ECU should be last resort.
 
VWilson said:
If the engine is running and you are only measuring 12 volts then it sounds like the alternator. You should be reading close to 14 volts with engine running and around 12 without.
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You are correct it's reading close to14 when running when I test it battery terminal to terminal. And hardly anything when I touch a ground bolt.
It does read fine when I first start it then when the check gauges light comes on and the dash volt gauge drops to 0 that's when I get a shitty reading.
 
VWilson said:
What doesn't make sense is the varying voltage at frame ground. You said it changed when you moved the test lead around. Could you have been on a painted surface? If you were on bare metal the I would still lean toward the alternator. Try running with jumper cables from another vehicle. If it clears up then at least you can rule out the ECU. I'm with Chris.....ECU should be last resort.
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So connect jumper cables, start the Jeep and let it run and see if the check gauges light comes on?
 
bucknut713 said:
You are correct it's reading close to14 when running when I test it battery terminal to terminal. And hardly anything when I touch a ground bolt.
It does read fine when I first start it then when the check gauges light comes on and the dash volt gauge drops to 0 that's when I get a shitty reading.
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So once the check gauge light comes on are you still getting 14 volts at the battery?
When checking to ground make sure you scratch into the ground bolt with the negative test lead. If still a bad reading then it just might be the voltage regulator. They do make external ones. I would give this a try before replacing the ECU. Gotta be much cheaper.
 
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VWilson said:
So once the check gauge light comes on are you still getting 14 volts at the battery?
When checking to ground make sure you scratch into the ground bolt with the negative test lead. If still a bad reading then it just might be the voltage regulator. They do make external ones. I would give this a try before replacing the ECU. Gotta be much cheaper.
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Yes I get almost 14 volts with the Jeep running with the check gauges light on at the battery.
 
VWilson said:
This tells me that the alternator is good. If you rev the engine up a little, the voltage should max out around 14.7 volts. If so then the VR is good too. Then it points back to a possible bad ground or ECU.
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I had to alternator tested when I had it out of the Jeep at 2 different places and it's good. I didn't take a reading while reving the engine. The VR is in the ECU so I guess it's not looking good.
 
bucknut713 said:
I had to alternator tested when I had it out of the Jeep at 2 different places and it's good. I didn't take a reading while reving the engine. The VR is in the ECU so I guess it's not looking good.
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If you do decide to replace the ECU then call Mark at wranglerfix.com and see if they have them for your year model. I know they have them for 05-06 and they work great. Just had to replace the one in my father-in-law 05. I've also seen pretty good reviews on this forum about FS1.
 
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bucknut713 said:
How would I find one of those alternator's with a built in VR for a TJ? And how to hook it up?
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Not sure. Google search or local auto parts store. If you have a local alternator rebuild shop they can point you in the right direction. Hooking one up is a breeze. Should come with instructions. If it were me I would try the external VR first due to the cost.
 
If you have +14 volts at the battery when the jeep is running but the instrument cluster is showing 0 volts you might just have an issue with the instrument cluster itself. You can run a self diagnostic on the cluster and it will show codes if it has any stored.
 
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