Transmission Indicator Displays Wrong

The dashboard transmission indicator on my 1998 Trooper is not indicating correctly. When the shift lever is in P, R, or N, the dashboard indicates R. When the lever is in D, 3, or 2, it indicates 3. When in L it indicates L.

When I test drive it, it seems to shift correctly. For example, when the lever is in D it will shift through 1,2,3 and 4th gear. When in 3 it will shift only to 3rd gear. When coasting I can shift back to 3, then 2, then 1 and the transmission follows my shift action.

The diagram shows a "Mode Switch" mounted on the transmission connected to the PCM and to a "A/T Shift Indicator Control Unit". That is turn is connected to the "A/T Shift Indicator" (the lights on the dash).

Since the transmission seems to shift correctly, I believe that means the Mode Switch connection to the PCM is getting through. Correct me if I'm wrong on that.

Since the wrong indicator lights come on on the dashboard, I think it is the Control Unit because if something was wrong with the lights, the some lever positions might not light any light as opposed to the wrong light.

I have a diagram that appears to show the Control unit mounted on the firewall but I can't tell which side, inside or under the hood. Anybody know where it is?

Also, anybody know the "encoding" of the wires from the Mode Switch to the Control unit? It appears only three wires connect between the two. Since there are 7 positions for the lever (P, R, N, D, 3, 2, L) and three wires, things must be encoded. Knowing the encoding could make finding a bad connection an easier job.

Any other ideas out there?

Thanks In Advance,

Pete

Have you looked here (http://isuzufaq.ibctech.ca/#faq59)?

Thanks. Yes, I did see that. I am thinking that the Mode Switch must be OK or else the tranny wouldn't shift up and down correctly. After all, if the lever is in D but the dashboard says 3 then it should shift up to only 3rd when accelerating. It seems to shift to 4th. Also, when coasting and I shift to 3 (even though the dashboard already says 3) the tranny shifts back a gear.

Tell me if I'm wrong on this reasoning.

My gut says the controller is wrong but, of course if I can't find it I'll start at the Mode Switch.

Thanks

Pete

:grinno: ...am thinking that the Mode Switch must be OK or else the tranny wouldn't shift up and down correctly. After all, if the lever is in D but the dashboard says 3 then it should shift up to only 3rd when accelerating. It seems to shift to 4th. Also, when coasting and I shift to 3 (even though the dashboard already says 3) the tranny shifts back a gear.

Tell me if I'm wrong on this reasoning.

ok, look, you're reasoning ain't wrong...but that's just not how it actually works...

the trans and trans computer etc don't follow what the dash lights say...the dash lights just try to match what that ATmode sensor/switch and computer are trying to tell them -- and when the switch is messed up it shows itself most often with wacky shift indicator light displays and/or harsh shifting. the system is very confused when it gets conflicting signals from the trans itself and from the the mode switch, and it tries to resolve those conflicts as best it can (that's one way to look at...)

[[[[ as an aside: want more evidence that the isuzu electronics are a little weird? what happens when the alternator starts acting up? answer: lots of things, including messed up AT operation.(((so go ahead and check out the alternator circuit for correct output if you really want to first check something else before you do the mode switch stuff.))) but even stranger, you'd think the battery icon idiot light would light when the alt isn't producing enough current, right? well it does -- along with just about every other idiot light including the brake idiot light. so go ahead, follow that on the schematic and check back in 6 months or so, 'cause there is no logic in that...but it does endear us to our isuzu's and gives us reason to visit these boards. :grinno: ]]]]

i know you are seeking as many views as you can get, but all i can tell you is the first place to start is the at mode sensor/switch.

i guarantee it. . .i.e. that's the first thing to rule out anyway :iceslolan you must(?) agree that evidence is kind of overwhelming when you read those various and varied posts i provided links to. not yet?

again, good luck, looking forward to what you find out. btw, there is no guarantee that cleaning and reinstalling will actually fix this for you...you could actually need a new switch. you will be able to tell more when you get in there....

Yeah, I'm thinking you're very right. No need to over brainiac this. Occam's razor is definitely in play. ("Select the theory that introduces the fewest assumptions")

http://en.wikipedia.org/wiki/Occam's_Razor


I was sort of relying on the fact that the Check Transmission light will light if the Mode Switch sends invalid codes. Maybe all the codes are valid, just wrong but not enough to cause the shift pattern to be wrong.

Funny, right now I will call a time out on this problem. It healed itself today. I know it will be back but now I have another problem. The oil pressure is going through the roof. I think I need to solve that problem first (I'll check the alternator output based on the rest of your post).

When I get back to this problem I'll decipher the encoding for the 7 positions to three wires and post it.

Just a note about weird electronics. Isuzu must have been hanging around GM for too long. A year ago I had a Chevy Celebrity that lost one diode in the alternator rectifier. The only notice was a buzzing sound behind the dashboard when the heater was in the off position.

Thanks again

Pete