Tach and A/C problems

[cummins2go]

I have an after market Tach in my truck not sure what kind. was in truck when I bought truck . Problem is when driving at lower RPM the tach goes in and out at higher RPM it seems to work a little longer before going in and out when the tach doesn't work the A/C doesn't work And having problems with down shifting from 4th to 3 at any time is there anything that links these all together? Any suggestions

Darrin

 

[Philip]

OK your showing three problems. Does your tach hook up to a plug down by the steering column? Or does it run out to the alt?

If it hooks up to a harness by the steering column. Then it is controlled by the computer.



4 to 3 down shift problems. This is controlled by the computer also.



No AC when the tach drops out.



Now let me take a few guesses here. On some of the performance cars. When the computer sees a wide open throttle setting from the TPS (throttle position sensor) it will drop the AC as long as it sees WOT position.



On most computer controlled apps. Before a down shift is allowed. It looks at the TPS and the speed reading from the transmission. So if it sees a wide open TPS setting then it will not allow a down shift if the speed is above a preset factory range.



So first thing I would check is the TPS. You will need a good volt meter. What you are going to check for is a steady voltage raise from idle to wide open setting with no dead areas. You will need to pierce the wires at the TPS to get this reading. So a set of small sharp probes are needed. Or you might be able to get a pair of paper clips down the connector where the TPS hooks into the main harness.



I am not sure on how the computer on our trucks takes a rpm reading. If there is a sensor for that. It would be the second thing I would be looking at.



You didn't say anything about speedo problems. So it sounds like it is all right. The two sensors I mentioned. If eather one of them is acting up. Could cause the computer to turn off the two items you mentioned. If the computer doesn't get all the inputs it is programed for. They tend to do weird things.

 

[bkokotovic]

Electrical Gremlins

I had a similar problem with my truck last winter. Although I don't have a tach yet, my speedo would do strange things. At the same time the power windows wouldn't go up/down and the fan on the AC would not blow. Some of the helpful folks here pointed out that in some cases, the key would not return to the ACC position after you start the truck. True enough, I would rotate the key back to the ACC position (maybe a few degrees) and presto.



I hope it's something as trivial as this.



Have fun.



B

 

[cummins2go]

Follow up

I just wanted to post a follow up I cleaned the connector for the TPS and everything is working normal again drove 150 miles so far so good Thanks for your help this is a great forum

Thanks Darrin

 

[Philip]

Glad to hear is was something minor.



Enjoy