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Potentiometer and Gas Gauge questions...


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Picked this up last week from a member on the site, and I need some help identifying where the "Potentiometer" is. The cluster in question was swapped into an 88 4.0 MJ, and before that, was in another 4.0 MJ. I know that the thing has to be on the back where the tach is, but I don't just want to go unscrewing and fiddling with things willy-nilly.

 

1. If someone could point out to me where this thing is located, and how to adjust it (if it is even there and can be adjusted) I sure would appreciate it.

 

Next question is probably simpler, but I'm just asking to make sure. Before I install the cluster correctly, will I need to make any adjustments to the new gas gauge to get it to read correctly since it's moving from the old spot in the middle to the corner spot?

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In order to adjust the potentiometer, you need to remove the tach from the cluster. There should be 4 (IIRC) screws holding the front clear plastic on, then I believe, 6 or 8 on the back holding the tach in.

 

Gas gauge needs no adjustment.

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In order to adjust the potentiometer, you need to remove the tach from the cluster. There should be 4 (IIRC) screws holding the front clear plastic on, then I believe, 6 or 8 on the back holding the tach in.

 

Gas gauge needs no adjustment.

 

So once I get the tach removed, how would I adjust it to correctly display?

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Ok, since I've done research both on the site, and google, I still have no answer to my first question.

 

The question isn't how to get the tach out, it's where the potentiometer is located, and how to adjust it. Because anytime anyone asks what to adjust to make the tach register correctly, everyone always answers the same "Just adjust the potentiometer."

 

I took some pictures to make it crystal clear to whomever decides to help me (which, I'll admit, I am frustrated, but I do appreciate the feedback, irregardless of whether it helps or not :bowdown: )

 

1. Tach out of cluster

 

 

2. Tach flipped over. (as if you were looking at it from the back of the cluster)

 

3. Side of tach.

 

4. Circuit board underneath the tach. 100% sure there is nothing here to adjust.

 

5. After studying the tachometer, common sense would dictate that the arrowed tube in the middle would be the thing to adjust. Either that or.....

 

6. Something on top where the circuit board is located. Pretty sure there wouldn't be though... :hmm:

 

 

So, with all this being said, am I correct in assuming that #5 is what I'm looking to adjust? If so, how do I adjust it correctly?

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The best way is to get a test tach and compare readings BEFORE removing it. Make sure it was in error. I understand you got the instrument cluster already removed so that is no longer an option. Yes, that pot you have circled in pic 5 is the adjustment. To calibrate you will need a pulse or square wave generator. If there is a ham radio operator or a electronic hobbyiest in the neighborhood maybe you can get him to do it. Try a TV repair shop. Be prepared to pay their shop rates, or take it to dealer. To calibrate it for a 6 cylinder engine, inject a 100hz signal into the input. Adjust to 2,000rpm. A 250Hz signal should read 5,000RPM. As your looking at the tach from the rear, apply +12volts to the left hand terminal. Apply the signal input to the middle terminal. Right hand terminal is ground. Might be an idea to put everything back together and put it back in the truck. Then check the reading. If your lucky it won't need adjusting. The odds are pretty good it came from a 6cyl. Just a idea.

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The best way is to get a test tach and compare readings BEFORE removing it. Make sure it was in error. I understand you got the instrument cluster already removed so that is no longer an option. Yes, that pot you have circled in pic 5 is the adjustment. To calibrate you will need a pulse or square wave generator. If there is a ham radio operator or a electronic hobbyiest in the neighborhood maybe you can get him to do it. Try a TV repair shop. Be prepared to pay their shop rates, or take it to dealer. To calibrate it for a 6 cylinder engine, inject a 100hz signal into the input. Adjust to 2,000rpm. A 250Hz signal should read 5,000RPM. As your looking at the tach from the rear, apply +12volts to the left hand terminal. Apply the signal input to the middle terminal. Right hand terminal is ground. Might be an idea to put everything back together and put it back in the truck. Then check the reading. If your lucky it won't need adjusting. The odds are pretty good it came from a 6cyl. Just a idea.

 

Thanks Jim, I appreciate the input. The cluster in question (4.0) is one I bought to replace my 2.5's Dummy lights (with no tach). I have the dummy light cluster still in the truck, and this one on the side. I'll probably end up waiting a while before I put it in though. Honestly, the tach really isn't a big deal to me, and I could care less whether or not it's geared to my 2.5, but it's always nice to have everything working correctly, y'know?

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I have a feeling that you are wishing just a little that I found that other cluster... thing would have been so much easier that way... I don't know where that thing got off to.. I searched the house several times last weekend and still can't find it, guess i will steal the one from my room mates Audi for Red.. LOL

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Would it be possible to use a nice digital timing gun with the built in tach feature to help set gauge? run jumper wires from the truck harness to power and give signal to the tach and then rev the motor to a certain rpm and then adjust the pod till the rpms match? Just wondering incase i need to do the same thing some day.

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No reason why it wouldn't work. All you need is to know the rpm's of the engine. Before I got myself a tach I would get engine rpm by holding my knuckle into the fan blade and count the number of times it got whacked in one minute. Divide by 4 for a 4 bladed fan. :doh:

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Isn't the potentiometer the white thing on top of the circuit board with the Phillips adjustment?

 

Rob.

 

 

Pffft. Well, I remember scouring many a thread and never seeing that. Although I did remember reading about adjusting it with a phillip's head.... :hmm:

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I have a feeling that you are wishing just a little that I found that other cluster... thing would have been so much easier that way... I don't know where that thing got off to.. I searched the house several times last weekend and still can't find it, guess i will steal the one from my room mates Audi for Red.. LOL

 

Actually, I'm glad I could get another cluster, irregardless of whether it had a tach or not. It isn't that much of a PITA actually, and I can understand the gist of how everything works. The way I look at it, I can help others out from learning from my lack of know-how atm. :rotf:

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