Gone_Rabid

Ive inherited this 88 Cherokee last year after it sat for about 5 years with 112k miles. Replaced quite a bit of parts along the way and I slowly work at the bugs as they show up.

the #1 issue that I have is with the full throttle shifting that id like to fix.

While at full throttle, with it in OD selector, it will shift about 3k rpms. I have pulled the TCU fuse and with it in 1st gear selector it will shift much higher like it should.

I have replaced the TPS as it didn’t have correct measurements on the ECU side. Per Cruiser54’s tips, I have replaced it and have the ECU side set appropriately.

This leaves me with getting voltage readings for the TCU side of the TPS.

Cruiser54 says pin A is positive, D is ground and B is output voltage. I am unable to get any voltage measurements that mean anything to me while back probing/piercing.

While unplugged and key on I get approx 9v or so on A. I do not have continuity on D from the battery.

I have looked at various wiring schematics to figure out if the ground is fed from the TCU or is an actual ground. I can’t find a schematic for an 88 only ones from the 90’s.

Can some shed some light on this? I’m capable but not finding the info as I don’t want to throw money and parts at it.

cruiser54

Have you bypassed your Power/Comfort switch yet?
THE HISTORY OF THE 4.0

BYPASSING THE POWER/COMFORT SWITCH

AUGUST 6, 2016 CRUISER54 47 COMMENTS EDITEven though the indicator lights up in your Power/Comfort switch, that does not mean the switch is working and putting the Transmission Control Unit into Power mode.

The switches had a high failure rate.

Remove and unplug the switch. In the harness are 3 wires. One has a stripe, one is tan, and the other wire is black.

Make a 4″ jumper wire with a male spade at each end.

Plug one end of the jumper wire in the cavity for the tan wire and the other in the cavity for the striped wire of the harness and you’ll be in permanent Power mode.

Stuff the harness back in and reinstall the switch for looks if you want.

Make sure the 7.5 amp “Trans” fuse in the fusebox is good.

If you happen to have a 91 or 92 XJ/MJ and it has the Power/Comfort switch, just jumper the Tan and Blue wires in the harness plug.

NEVER include the black wire when jumpering!!!

Gone_Rabid

I was hoping you would chime in.

Yes. I’ve already done that even after testing the switch as a just in case.

fuse is good for the TCU and I have power there.

did the refreshing of the ground at the dipstick tube and harness.

Gone_Rabid

Also cleaned all the transmission connectors under the hood but have not cleaned the ones on the TCU yet.

cruiser54

You beat me to that!!
All engine dipstick grounds are good?

TPS power and ground go through the pesky C101 connector FWIW.

Gone_Rabid

Yes. The grounds are good. I cut off the ring terminals and put new ones on just incase.

I also cleaned that c101 connector. I’ll investigate that connector some more.

cruiser54

what about the C101 connector?

Gone_Rabid

It was cleaned as well.

is there a pin out of that connector to determine what’s what?

cruiser54

In C101 connector:
C1 red is B+ supply to TPS pin D1 on TCU
C3 tan/org is TPS ground pin D3 on TCU
D2 on TCU is TPS input. I can't see on the diagram what pin it is in C101, but it is gry/tr

Gone_Rabid

thank you for that information. Where did you come up with that Info so I can save it for the future?

I have continuity on all 3 wires from the TPS plug to the plug at the TCU. They are at approx 2 ohms each. So no problem there.

I did find something interesting. The TCU model is 53008383 which from my research is for a 1991.

looked at the transmission tag and have a 30-40LE serial # 88BR 50775. Still researching the serial # but I doubt I can get info from it.

im going to guess someone threw this TCU in and I need one from an 1987-1990. Assuming the transmission is original.

cruiser54

What color is the TCU plug and what color is the harness plug? You may be on to something here.

Gone_Rabid

It’s a horrible pic but the harness has a white plug and the TCU is teal.

cruiser54

They should be the same color!!!!

lawsoncl

The TCU is not correct, and someone would have had to shave the keying off the grey connector to get it to fit. The difference between the 87-90 and the 91 TCM is the TPS input on the 91 TCU expects 0-5 volts, the same as the engine side of the TPS. The 91 and up has a single output TPS sensor, and that output goes to both the ECM and the TPS. I believe the 91 still had the power/comfort switch so that should still work.

If you don't have the correct TCM, you could modify the wiring to feed the TCM the output from the ECM side of the TPS. That would provide the 91 ECM with the 0-5 volts signal it expects. Disconnect the wire from D2 (gray I believe) at the TPS square connector and splice it onto the ECM output wire of the TPS.

cruiser54

Or get an 88 to 90 TCU.