Temp Guage

First of all my knowledge of auto mechanics is very limited. I need some help concerning my temp guage. Here's the story: Yesterday I took the Speedster out for a spin. I was less than a mile from the house and the temp gauge pegged all the way to the right. I turned around and went back home. I found the toggle switch for the oil cooler fan, tuned it on and heard it running. I let the car sit for about 30 minutes and left again. This time everything seemed fine until I got on the expressway. All of a sudden the temp needle pegged again all the way to the right. I turned on the oil cooler fan but it had no effect on the needle. I pulled off at the first exit and stopped to let the car cool off. The needle stayed pegged to the right when I started back up. The car was running fine but I took it home and parked it. The grill was hot to the touch but I was thinking that maybe the gauge was wrong. If the guage is correct, what would be causing the overheating? Any help that anyone could provide would be greatly appreciated. I'm a new Speedster owner.

Thanks,

Doug

I'm not sure exactly what is up with your particular case but I have seen mine do similarly. When I was testing my oil temp sender last week, I had the same reading when I grounded the single wire that attaches to the oil temp sendeing unit (dark green lead on Vintage Speedster harness). The sending unit completes a "ground" loop that goes back to the temp gauge which is really an "ohm" meter. More pressure, more "ground" flow back to the gauge, higher the needle goes. There are a few things I would check for: 1) Make sure the sending unit doesn't have something grounding the single prong where the wire connects; 2) make sure the wire hasn't chaffed somewhere and has become grounded (disconnect the wire from the sender and check the gauge, if it still pegs, the wire is chaffed and grounded or #3--> 3) Check the connections at the gauge itself to see if the connection for the lead from the engine has become grounded some way. If all these tests pass, the sender may be jammed/faulty or the gauge is faulty or #4--> 4) the oil pump may be a goner and no oil is circulating - in which case the oil pressure light would be on too (red light at the bottom right of the tachometer when the key is on and the engine is not running); 5) Cooling fan loose? not sure here, just guessing. I think items 1,2 and 3 are the more likely causes - hopefully too because they would be simplest to resolve. Hope this helps. I'm new to offering help on the forum but this sounded like something I've seen recently. Good Luck.

Doug; these gauges are so inaccurate, especially the latest Chinese repros of the Brazilian VDO repros on VS's and JPS's that have been installed on them for a little over a year. In my case, the needle hardly moves to the right when connected regularly but if it's grounded to the block it goes all the way to the right instantly. As the previous poster said, make sure it's not grounding out and check the sender too.

Hello Trent & Ricardo,

Thanks guys, thanks a million. I'll check the things that you suggested. It's great to be able to post issues and have guys like you reply so quickly and try to help. Thanks again.

Doug

check your oil too. Low oil level can still have enough pressure to keep the light off but not enough to cool the motor.

Doug: Get a candy or meat thermometer with a long probe and put it in the dipstick's place to read your oil temp. Don't forget to replace the dipstick after you get the reading. 180-220 is ok.

BD

Hi Mike & Bill,

Thanks so much for the additional info. All you guys at Speedsters Owners.com are just great. I'll check the oil level Mike and I'm going to get a candy/meat thermomrter Bill. Thanks again for all the info. As soon as I figure it out I'll post what I found out.

Doug

Doug:

If you can't find a long enough candy thermometer and have $50 bucks kicking around, you can buy a Porsche engine thermometer (looks just like a candy thermometer, but has silicone oil in it to prevent the needle from vibrating all around when the engine's running).

They are available from this site:

www.mainelycustombydesign.com/enginecomponents.htm#dipstick

Many of us have bought them, they're very high quality, and I pretty much leave mine in there all the time.

Gordon

Hi Gordon,

Thanks much for the additional info. I'll check out that web site. So glad I joined this club, info from previous owners just can't be beat. In the future I hope I can be as helpful as you guys have been to me. Thanks again.

Doug

Doug,

I would follow everything that was suggested just to be safe, but my thought is the Chinese gauge. I recently switched the Brazilian gauges in my personal car, to the Chinese and had the exact same thing on the first THREE I tried. I plugged in the Brazilian gauge just to check and see if I got the same results and it read normal...
Internal short?? Incompatability with the Brazilian/Swiss temp sender?? Don't know yet, haven't gotten that far...
Again, check out all of the possibilities and make sure it isn't something more serious, but my money is on the gauge.

CEH

Probably requires a totally different sender, which can only be found in Chinese auto parts stores or in an auto salvage yard from a 1973 Borgward Isabella (right hand drive, only).

Seriously, though, the resistance range of various senders (according to my Motormite catalog) is all over the place. If it requires a different range than the Brazilian gauge (or stock VDO, of which there are several ranges alone), here is how you can tell the range needed:

Get either a variable resistance box that goes up to about a Meg-ohm, or a potentiometer (radio shack) in the 0 - 500 ohm or so range. connect either device between the sender side of the gauge and ground, turn on the ignition and turn the potentiometer one way or the other to watch the gauge swing (use the middle and one side terminals). If you're using a resistance box, hook it up the same way, and then vary the resistance in steps to accomplish the same.

Now; play with resistance til you get the gauge to go just off the scale on the low side. Remove the resistance, measure it with an ohm-meter and write it down along side the word "low".

Reconnect the resistance and adjust it so the gauge reads just to the high side of the scale. Remove it and measure as before, noting the reading and writing that down beside the word "high".

The resistance range of the sender you want should be what you just noted, although it's OK to go more than the "low" end but try to hold it within the "high" range or just a bit lower. consult a Motormite or Standard Auto Parts catalog to get one with the range you need (and then let the rest o us know what it is!!)

gn